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The New Genetics

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The New Genetics is available online at: NATIONAL INSTITUTES OF HEALTH http://publications.nigms.nih.gov/ NATIONAL INSTITUTE OF GENERAL MEDICAL SCIENCES OFFICE OF COMMUNICATIONS AND PUBLIC LIAISON thenewgenetics. 45 CENTER DR RM 3AN.32 MSC 6200 BETHESDA MD 20814-9692 Produced by the Office of Communications and Public Liaison National Institute of General Medical Sciences National Institutes of Health U.S. Department of Health and Human Services The New Genetics U.S. DEPARTMENT OF HEALTH AND HUMAN SERVICES NIH Publication No. 07-662 National Institutes of Health Revised October 2006 National Institute of General Medical Sciences http://www.nigms.nih.gov Contents FOREWORD 2 CHAPTER 1: HOW GENES WORK 4 Beautiful DNA 5 Copycat 8 Let’s Call It Even 9 Getting the Message 11 Nature’s Cut-and-Paste Job 14 All Together Now 16 Genetics and You: Nursery Genetics 17 Found in Translation 18 RNA Surprises 19 An Interesting Development 20 The Tools of Genetics: Mighty Microarrays 22 CHAPTER 2: RNA AND DNA REVEALED: NEW ROLES, NEW RULES 24 RNA World 25 Molecular Editor 26 Healthy Interference 29 Dynamic DNA 30 Secret Code 30 Genetics and You: The Genetics of Anticipation 32 Battle of the Sexes 33 Starting at the End 34 The Other Human Genome 36 The Tools of Genetics: Recombinant DNA and Cloning 38 CHAPTER 3: LIFE’S GENETIC TREE 40 Everything Evolves 40 Selective Study 42 Clues from Variation 43 Living Laboratories 46 The Genome Zoo 52 Genes Meet Environment 53 Genetics and You: You’ve Got Rhythm! 56 Animals Helping People 58 My Collaborator Is a Computer 58 The Tools of Genetics: Unlimited DNA 60 CHAPTER 4: GENES ARE US 62 Individualized Prescriptions 64 The Healing Power of DNA 65 Cause and Effect 67 Us vs. Them 68 Genetics and You: Eat Less, Live Longer? 69 Gang Warfare 70 The Tools of Genetics: Mathematics and Medicine 72 CHAPTER 5: 21ST- CENTURY GENETICS 74 No Lab? No Problem! 76 Hard Questions 78 Good Advice 80 Genetics and You: Crime-Fighting DNA 81 Genetics, Business, and the Law 82 Careers in Genetics 85 The Tools of Genetics: Informatics and Databases 86 GLOSSARY 88 Foreword And every living thing Consider just three of Earth’s inhabitants: does one thing the same a bright yellow daffodil that greets the way: To make more of itself, it first copies its spring, the single-celled creature called molecular instruction manual—its genes—and then passes this infor- Thermococcus that lives in boiling hot mation on to its offspring. This cycle has been springs, and you. Even a science-fiction repeated for three and a half billion years. But how did we and our very distant rela- writer inventing a story set on a distant tives come to look so different and develop so many different ways of getting along in the planet could hardly imagine three more dif- world? A century ago, researchers began to answer ferent forms of life. Yet you, Thermococcus, that question with the help of a science called genetics. Get a refresher course on the basics in and the daffodil are related! Indeed, all of Chapter 1, “How Genes Work.” It’s likely that when you think of heredity the Earth’s billions of living things are kin you think first of DNA, but in the past few years, to each other. researchers have made surprising findings about The New Genetics I Foreword 3 another molecular actor that plays a starring role. Can DNA and RNA help doctors predict Check out the modern view of RNA in Chapter 2, whether we’ll get diseases like cancer, diabetes, or “RNA and DNA Revealed: New Roles, New Rules.” asthma? What other mysteries are locked within When genetics first started, scientists didn’t the 6 feet of DNA inside nearly every cell in our have the tools they have today. They could only bodies? Chapter 4, “Genes Are Us,” explains what look at one gene, or a few genes, at a time. Now, researchers know, and what they are still learning, researchers can examine all of the genes in a liv- about the role of genes in health and disease. ing organism—its genome—at once. They are Finally, in Chapter 5, “21st-Century doing this for organisms on every branch of the Genetics,” see a preview of things to come. Learn tree of life and finding that the genomes of mice, how medicine and science are changing in big frogs, fish, and a slew of other creatures have ways, and how these changes influence society. many genes similar to our own. From metabolism to medicines to agriculture, So why doesn’t your brother look like your the science of genetics affects us every day. It is dog or the fish in your aquarium? It’s because of part of life … part of your life! evolution. In Chapter 3, “Life’s Genetic Tree,” find out how evolution works and how it relates to genetics and medical research. CHAPTER 1 How Genes Work P eople have known for many years that living things inherit traits from their parents. That common-sense observation led to agricul- Proteins do many other things, too. They provide the body’s main building materials, forming the cell’s architecture and structural ture, the purposeful breeding and cultivation of components. But one thing proteins can’t do is animals and plants for desirable characteristics. make copies of themselves. When a cell needs Firming up the details took quite some time, more proteins, it uses the manufacturing instruc- though. Researchers did not understand exactly tions coded in DNA. how traits were passed to the next generation The DNA code of a gene—the sequence of until the middle of the 20th century. its individual DNA building blocks, labeled A Now it is clear that genes are what carry our (adenine), T (thymine), C (cytosine), and G traits through generations and that genes are (guanine) and collectively called nucleotides— made of deoxyribonucleic acid (DNA). But spells out the exact order of a protein’s building genes themselves don’t do the actual work. blocks, amino acids. Rather, they serve as instruction books for mak- Occasionally, there is a kind of typographical ing functional molecules such as ribonucleic error in a gene’s DNA sequence. This mistake— acid (RNA) and proteins, which perform the which can be a change, gap, or duplication—is chemical reactions in our bodies. called a mutation. Genetics in the Garden In 1900, three European scientists inde-  The monk Gregor pendently discovered an obscure research Mendel first described paper that had been published nearly 35 how traits are inherited from one generation to years before. Written by Gregor Mendel, the next. an Austrian monk who was also a scien- tist, the report described a series of offspring and learned that these characteristics breeding experiments performed with pea were passed on to the next generation in orderly, plants growing in his abbey garden. predictable ratios. Mendel had studied how pea plants When he cross-bred purple-flowered pea plants inherited the two variant forms of easy-to-see with white-flowered ones, the next generation had traits. These included flower color (white or purple) only purple flowers. But directions for making white and the texture of the peas (smooth or wrinkled). flowers were hidden somewhere in the peas of that Mendel counted many generations of pea plant generation, because when those purple-flowered The New Genetics I How Genes Work 5 A mutation can cause a gene to encode a Beautiful DNA protein that works incorrectly or that doesn’t Up until the 1950s, scientists knew a good deal work at all. Sometimes, the error means that no about heredity, but they didn’t have a clue what protein is made. DNA looked like. In order to learn more about But not all DNA changes are harmful. Some DNA and its structure, some scientists experi- mutations have no effect, and others produce mented with using X rays as a form of molecular new versions of proteins that may give a survival photography. advantage to the organisms that have them. Over Rosalind Franklin, a physical chemist work- time, mutations supply the raw material from ing with Maurice Wilkins at King’s College in which new life forms evolve (see Chapter 3, London, was among the first to use this method “Life’s Genetic Tree”). to analyze genetic material. Her experiments plants were bred to each other, some of their off- factors, whatever they were, must be physical spring had white flowers. What’s more, the material because they passed from parent to second-generation plants displayed the colors in a offspring in a mathematically orderly way. It wasn’t predictable pattern. On average, 75 percent of the until many years later, when the other scientists second-generation plants had purple flowers and unearthed Mendel’s report, that the factors were 25 percent of the plants had white flowers. Those named genes. same ratios persisted, and were reproduced when Early geneticists quickly discovered that the experiment was repeated many times over. Mendel’s mathematical rules of inheritance applied Trying to solve the mystery of the missing color not just to peas, but also to all plants, animals, and blooms, Mendel imagined that the reproductive people. The discovery of a quantitative rule for cells of his pea plants might contain discrete inheritance was momentous. It revealed that that a “factors,” each of which specified a particular trait, common, general principle governed the growth such as white flowers. Mendel reasoned that the and development of all life on Earth. 6 National Institute of General Medical Sciences produced what were referred to at the time as “the most beautiful X-ray photographs of any COLD SPRING HARBOR LABORATORY ARCHIVES substance ever taken.” Other scientists, including zoologist James Watson and physicist Francis Crick, both work- ing at Cambridge University in the United Kingdom, were trying to determine the shape of DNA too. Ultimately, this line of research  In 1953, Watson and Crick created their historic model of the shape of DNA: the double helix. revealed one of the most profound scientific dis- coveries of the 20th century: that DNA exists as handrails—were complementary to each other, a double helix. and this unlocked the secret of how genetic The 1962 Nobel Prize in physiology or medi- information is stored, transferred, and copied. cine was awarded to Watson, Crick, and Wilkins In genetics, complementary means that if for this work. Although Franklin did not earn a you know the sequence of nucleotide building share of the prize due to her untimely death at age blocks on one strand, you know the sequence of 38, she is widely recognized as having played a sig- nucleotide building blocks on the other strand: nificant role in the discovery. A always matches up with T and C always links The spiral staircase-shaped double to G (see drawing, page 7). helix has attained global status as Long strings of nucleotides form genes, the symbol for DNA. But what and groups of genes are packaged tightly into is so beautiful about the dis- structures called chromosomes. Every cell in your covery of the twisting body except for eggs, sperm, and red blood cells ladder structure isn’t just contains a full set of chromosomes in its nucleus. its good looks. Rather, the If the chromosomes in one of your cells were structure of DNA taught uncoiled and placed end to end, the DNA would researchers a fundamental be about 6 feet long. If all the DNA in your body lesson about genetics. It taught were connected in this way, it would stretch them that the two connected approximately 67 billion miles! That’s nearly strands—winding together like parallel 150,000 round trips to the Moon.  Rosalind Franklin’s original X-ray diffraction photo revealed the physical structure of DNA. OREGON STATE UNIVERSITY LIBRARIES SPECIAL COLLECTIONS The New Genetics I How Genes Work 7  The long, stringy DNA that makes up genes is spooled within chromosomes inside the nucleus of a cell. (Note that a gene would actually be a much Chromosome longer stretch of DNA than what is shown here.) Nucleus G C Bases Cell C G A T G C DNA Guanine G C Cytosine A T C G Thymine T A Adenine Gene A T Sugar- G C Phosphate Backbone C G A T  DNA consists of two long, twisted chains made up of nucleotides. Each nucleotide contains one base, one phosphate molecule, and the sugar molecule deoxyribose. The bases in DNA nucleotides are adenine, thymine, cytosine, and guanine. P Nucleotide S C 8 National Institute of General Medical Sciences Copycat It’s astounding to think that your body consists of trillions of cells. But what’s most amazing is that it all starts with one cell. How does this massive expansion take place? As an embryo progresses  Humans have 23 pairs of chromosomes. Male DNA (pictured here) through development, its cells contains an X and a Y chromosome, whereas female DNA contains two X chromosomes. must reproduce. But before a cell divides into two new, CYTOGENETICS LABORATORY, BRIGHAM AND WOMEN’S HOSPITAL nearly identical cells, it must copy its DNA so there will be a complete set of the complementary new strand. The process, genes to pass on to each of the new cells. called replication, is astonishingly fast and To make a copy of itself, the twisted, com- accurate, although occasional mistakes, such as pacted double helix of DNA has to unwind and deletions or duplications, occur. Fortunately, a separate its two strands. Each strand becomes a cellular spell-checker catches and corrects nearly pattern, or template, for making a new strand, so all of these errors. the two new DNA molecules have one new strand Mistakes that are not corrected can lead to and one old strand. diseases such as cancer and certain genetic disor- The copy is courtesy of a cellular protein ders. Some of these include Fanconi anemia, early machine called DNA polymerase, which reads aging diseases, and other conditions in which the template DNA strand and stitches together people are extremely sensitive to sunlight and some chemicals. DNA copying is not the only time when DNA damage can happen. Prolonged, unprotected sun exposure can cause DNA changes that lead to skin cancer, and toxins in cigarette smoke can cause lung cancer.  When DNA polymerase makes an error while copying a gene’s DNA sequence, the mistake is called a mutation. In this example, the nucleotide G has been changed to an A. The New Genetics I How Genes Work 9 C G A T C G It may seem ironic, then, that many drugs A T used to treat cancer work by attacking DNA. That’s T A because these chemotherapy drugs disrupt the DNA copying process, which goes on much faster C G in rapidly dividing cancer cells than in other T A cells of the body. The trouble is that most of these G C T A drugs do affect normal cells that grow and T A divide frequently, such as cells of the immune system and hair cells. G C A A T Understanding DNA replication better could T A be a key to limiting a drug’s action to cancer G C G C cells only. A T A T T G C Let’s Call It Even G C After copying its DNA, a cell’s next challenge is New Strand getting just the right amount of genetic material G C G C A T into each of its two offspring. A T G C Most of your cells are called diploid G C A T (“di” means two, and “ploid” refers to sets of A T chromosomes) because they have two sets of chromosomes (23 pairs). Eggs and sperm are A T A T different; these are known as haploid cells. Each G C C G G C haploid cell has only one set of 23 chromosomes C G so that at fertilization the math will work out: A T A T A haploid egg cell will combine with a haploid sperm cell to form a diploid cell with the right A T number of chromosomes: 46. A T Chromosomes are numbered 1 to 22, according to size, with 1 being the largest chromosome. The 23rd pair, known as the sex  During DNA replication, each strand of the original molecule acts as a template for chromosomes, are called X and Y. In humans, the synthesis of a new, complementary DNA strand. abnormalities of chromosome number usually occur during meiosis, the time when a cell 10 National Institute of General Medical Sciences Meiosis Chromosomes Cell Nucleus from Parents  During meiosis, chromosomes Chromosomes from both parents are copied replicate. and paired to exchange portions of DNA. Like chromosomes pair up.  This creates a mix of new genetic Chromosomes swap material in the offspring’s cells. sections of DNA. Nucleus divides into Chromosome pairs divide. daughter nuclei. Daughter nuclei Chromosomes divide. divide again. Daughter nuclei have single chromosomes and a new mix of genetic material. The New Genetics I How Genes Work 11 reduces its chromosomes from diploid to haploid Amon has made major progress in under- in creating eggs or sperm. standing the details of meiosis. Her research shows What happens if an egg or a sperm cell gets how, in healthy cells, gluelike protein complexes the wrong number of chromosomes, and how called cohesins release pairs of chromosomes at often does this happen? exactly the right time. This allows the chromo- Molecular biologist Angelika Amon of somes to separate properly. the Massachusetts Institute of Technology in These findings have important implications Cambridge says that mistakes in dividing DNA for understanding and treating infertility, birth between daughter cells during meiosis are the defects, and cancer. leading cause of human birth defects and mis- Getting the Message carriages. Current estimates are that 10 percent So, we’ve described DNA—its basic properties of all embryos have an incorrect chromosome and how our bodies make more of it. But how number. Most of these don’t go to full term and does DNA serve as the language of life? How do are miscarried. you get a protein from a gene? In women, the likelihood that chromosomes won’t be apportioned properly increases with age. One of every 18 babies born to women over 45 has three copies of chromosome 13, 18, or 21 instead of the normal two, and this improper balancing can cause trouble. For example, three copies of chromosome 21 lead to Down syndrome. To make her work easier, Amon—like many other basic scientists—studies yeast cells, which separate their chromosomes almost exactly the same way human cells do, except that yeast do it much faster. A yeast cell copies its DNA and produces daughter cells in about 11/2 hours, compared to a whole day for human cells.  Trisomy, the hallmark of Down syndrome, results when a baby is born with three copies of chromo- The yeast cells she uses are the same kind some 21 instead of the usual two. bakeries use to make bread and breweries use to make beer! 12 National Institute of General Medical Sciences There are two major steps in making a pro- You’d think that for a process so essential to tein. The first is transcription, where the life, researchers would know a lot about how information coded in DNA is copied into RNA. transcription works. While it’s true that the The RNA nucleotides are complementary to basics are clear—biologists have been studying those on the DNA: a C on the RNA strand gene transcribing by RNA polymerases since matches a G on the DNA strand. these proteins were first discovered in 1960— The only difference is that RNA pairs a some of the details are actually still murky. nucleotide called uracil (U), instead of a T, with an A on the DNA. A protein machine called RNA polymerase 1 reads the DNA and makes the RNA copy. This copy is called messenger RNA, or mRNA, because A T C G it delivers the gene’s message to the protein- A T T A producing machinery. At this point you may be wondering why all of the cells in the human body aren’t exactly alike, since they all contain the same DNA. What makes a liver cell different from a brain cell? How do the cells in the heart make the organ contract, but those in skin allow us to sweat? Cells can look and act differently, and do entirely different jobs, because each cell “turns on,” or expresses, only the genes appropriate for what it needs to do. That’s because RNA polymerase does not work alone, but rather functions with the aid of many helper proteins. While the core part of RNA polymerase is the same in all cells, the helpers vary in different cell types throughout the body. DNA  RNA polymerase transcribes DNA to make messenger RNA (mRNA). The New Genetics I How Genes Work 13 The biggest obstacle to learning more has But our understanding is improving fast, been a lack of tools. Until recently, researchers thanks to spectacular technological advances. were unable to get a picture at the atomic level We have new X-ray pictures that are far more of the giant RNA polymerase protein assemblies sophisticated than those that revealed the structure inside cells to understand how the many pieces of DNA. Roger Kornberg of Stanford University in of this amazing, living machine do what they do, California used such methods to determine the and do it so well. structure of RNA polymerase. This work earned 2 3 4 Threonine Arginine Amino Acids DNA Strand Tyrosine Threonine RNA Strand  Amino acids link up to make a protein. A A T tRNA C C G A A T Ribosome T U A G G C C C G T U A A A T T UA G C G A C G U A U C G U A C A C G C A T A Codon 1 Codon 2 Codon 3 Codon 4 mRNA  The mRNA sequence (dark red strand) is com-  On ribosomes, transfer RNA (tRNA) helps plementary to the DNA sequence (blue strand). convert mRNA into protein. 14 National Institute of General Medical Sciences him the 2006 Nobel Nature’s Cut-and-Paste Job Prize in chemistry. In Several types of RNA play key roles in making addition, very powerful a protein. The gene transcript (the mRNA) microscopes and other transfers information from DNA in the nucleus to tools that allow us to the ribosomes that make protein. Ribosomal RNA watch one molecule forms about 60 percent of the ribosomes. Lastly, at a time provide a transfer RNA carries amino acids to the ribo- new look at RNA poly- somes. As you can see, all three types of cellular merase while it’s at work RNAs come together to produce new proteins. reading DNA and pro- But the journey from gene to protein isn’t ducing RNA. quite as simple as we’ve just made it out to be. For example, Steven After transcription, several things need to hap- Block, also of Stanford, pen to mRNA before a protein can be made. For has used a physics tech- example, the genetic material of humans and nique called optical other eukaryotes (organisms that have a trapping to track RNA nucleus) includes a lot of DNA that doesn’t polymerase as it inches encode proteins. Some of this DNA is stuck right z RNA polymerase (green) and one end of a DNA strand (blue) are attached to clear beads pinned along DNA. Block and in the middle of genes. down in two optical traps. As RNA polymerase moves along the DNA, it creates an RNA copy of his team performed To distinguish the two types of DNA, scien- a gene, shown here as a pink strand. this work by designing tists call the coding sequences of genes exons and STEVEN BLOCK a specialized microscope the pieces in between introns (for intervening sensitive enough to watch the real-time motion of sequences). a single polymerase traveling down a gene on If RNA polymerase were to transcribe DNA one chromosome. from the start of an intron-containing gene to The researchers discovered that molecules of the end, the RNA would be complementary to RNA polymerase behave like battery-powered the introns as well as the exons. spiders as they crawl along the DNA ladder, To get an mRNA molecule that yields a work- adding nucleotides one at a time to the growing ing protein, the cell needs to trim out the intron RNA strand. The enzyme works much like a sections and then stitch only the exon pieces motor, Block believes, powered by energy released together (see drawing, page 15). This process is during the chemical synthesis of DNA. called RNA splicing. The New Genetics I How Genes Work 15 Gene DNA Exon 1 Intron 1 Exon 2 Intron 2 Exon 3  Genes are often interrupted by stretches of DNA (introns, blue) that do not contain instructions for making a protein. The DNA Transcription segments that do contain (RNA Synthesis) protein-making instructions are known as exons (green). Nuclear RNA Exon 1 Intron 1 Exon 2 Intron 2 Exon 3 RNA Splicing Messenger RNA Exon 1 Exon 2 Exon 3 Translation (Protein Synthesis) Protein Gene  Arranging exons in different DNA Exon 1 Exon 2 Exon 3 Exon 4 patterns, called alternative splicing, enables cells to make different proteins from a single gene. Exon 1 Exon 2 Exon 3 Exon 4 Alternative Splicing Exon 1 Exon 2 Exon 3 Exon 1 Exon 2 Exon 4 Translation Protein A Protein B 16 National Institute of General Medical Sciences Splicing has to be extremely accurate. An By cutting and pasting the exons in different error in the splicing process, even one that results patterns, which scientists call alternative splicing, in the deletion of just one nucleotide in an exon a cell can create different proteins from a single or the addition of just one nucleotide in an gene. Alternative splicing is one of the reasons intron, will throw the whole sequence out of why human cells, which have about 25,000 alignment. The result is usually an abnormal genes, can make hundreds of thousands of protein—or no protein at all. One form of different proteins. Alzheimer’s disease, for example, is caused by All Together Now this kind of splicing error. Until recently, researchers looked at genes, and Molecular biologist Christine Guthrie of the the proteins they encode, one at a time. Now, they University of California, San Francisco, wants can look at how large numbers of genes and pro- to understand more fully the mechanism for teins act, as well as how they interact. This gives removing intron RNA and find out how it stays them a much better picture of what goes on in a so accurate. living organism. She uses yeast cells for these experiments. Already, scientists can identify all of the genes Just like human DNA, yeast DNA has introns, that are transcribed in a cell—or in an organ, like but they are fewer and simpler in structure and the heart. And although researchers can’t tell you, are therefore easier to study. Guthrie can identify right now, what’s going on in every cell of your which genes are required for splicing by finding body while you read a book or walk down the abnormal yeast cells that mangle splicing. street, they can do this sort of “whole-body” scan So why do introns exist, if they’re just going to for simpler, single-celled organisms like yeast. be chopped out? Without introns, cells wouldn’t Using a new technique called genome-wide need to go through the splicing process and keep location analysis, Richard Young of the monitoring it to be sure it’s working right. Massachusetts Institute of Technology unraveled As it turns out, splicing also makes it possible a “regulatory code” of living yeast cells, which for cells to create more proteins. have more than 6,000 genes in their genome. Think about all the exons in a gene. If a cell Young’s technique enabled him to determine the stitches together exons 1, 2, and 4, leaving out exact places where RNA polymerase’s helper pro- exon 3, the mRNA will specify the production teins sit on DNA and tell RNA polymerase to of a particular protein. But instead, if the cell begin transcribing a gene. stitches together exons 1, 2, and 3, this time leav- Since he did the experiment with the yeast ing out exon 4, then the mRNA will be translated exposed to a variety of different conditions, into a different protein (see drawing, page 15). The New Genetics I How Genes Work 17 GENETICS AND YOU: Nursery Genetics hile most genetic research Newborn screening is governed by W uses lab organisms, test tubes, and petri dishes, the results have real consequences for individual states. That means that the state in which a baby is born determines the people. Your first encounter with genetic conditions for genetic analysis probably happened which he or she will be shortly after you were born, when a screened. Currently, doctor or nurse took a drop of blood some states test for from the heel of your tiny foot. fewer than 10 conditions, whereas Lab tests performed with that single others test for more than 30. All states drop of blood can diagnose certain rare test for PKU. genetic disorders as well as metabolic Although expanded screening for problems like phenylketonuria (PKU). genetic diseases in newborns is advo- Screening newborns in this way cated by some, others question the began in the 1960s in Massachusetts value of screening for conditions that with testing for PKU, a disease affecting are currently untreatable. Another 1 in 14,000 people. PKU is caused by an issue is that some children with mild enzyme that doesn’t work properly due versions of certain genetic diseases to a genetic muta- may be treated needlessly. tion. Those born Based on recommendations with this disorder released in 2005 by the American cannot metabolize College of Medical Genetics, gov- the amino acid ernment policymakers are now phenylalanine, considering a proposal that would which is present establish a standard, national set in many foods. Left untreated, PKU can of newborn tests for 29 conditions, lead to mental retardation and neurolog- ranging from relatively common ical damage, but a special diet can hearing problems to very rare prevent these outcomes. Testing for this metabolic diseases. condition has made a huge difference in many lives. 18 National Institute of General Medical Sciences Young was able to figure out how transcription method to scan the entire human genome in patterns differ when the yeast cell is under stress small samples of cells taken from the pancreases (say, in a dry environment) or thriving in a sugary- and livers of people with type 2 diabetes. He rich nutrient solution. Done one gene at a time, used the results to identify genes that aren’t tran- using methods considered state-of-the-art just a scribed correctly in people with the disease. few years ago, this kind of analysis would have This information provides researchers with taken hundreds of years. an important tool for understanding how dia- After demonstrating that his technique betes and other diseases are influenced by worked in yeast, Young then took his research defective genes. By building models to predict a step forward. He used a variation of the yeast how genes respond in diverse situations, researchers may be able to learn how to stop or jump-start genes on demand, change the course of a disease, or prevent it from ever happening. Found in Translation After a gene has been read by RNA polymerase and the RNA is spliced, what happens next in the journey from gene to protein? The next step is reading the RNA information and fitting the building blocks of a protein together. This is called translation, and its principal actors are the ribosome and amino acids. Ribosomes are among the biggest and most intricate structures in the cell. The ribosomes of bacteria contain not only huge amounts of RNA, but also more than 50 different proteins. Human ribosomes have even more RNA and between 70 and 80 different proteins! Harry Noller of the University of California,  A ribosome consists of large and small protein subunits with transfer RNAs Santa Cruz, has found that a ribosome performs nestled in the middle. several key jobs when it translates the genetic RIBOSOME STRUCTURE COURTESY OF JAMIE CATE, MARAT YUSUPOV, code of mRNA. As the messenger RNA threads GULNARA YUSUPOVA, THOMAS EARNEST, AND HARRY NOLLER. GRAPHIC COURTESY OF ALBION BAUCOM, UNIVERSITY OF CALIFORNIA, SANTA CRUZ. through the ribosome protein machine, the The New Genetics I How Genes Work 19 ribosome reads the mRNA sequence and helps recognize and recruit the correct amino acid- carrying transfer RNA to match the mRNA code. The ribosome also links each additional amino acid into a growing protein chain (see drawing, page 13). For many years, researchers believed that even though RNAs formed a part of the ribosome, the protein portion of the ribosome did all of the work. Noller thought, instead, that maybe RNA,  Some first-aid ointments contain the antibiotic neomycin, not proteins, performed the ribosome’s job. His which treats infections by attacking ribosomes in bacteria. idea was not popular at first, because at that time it was thought that RNA could not perform such RNA Surprises complex functions. But which ribosomal RNAs are doing the work? Some time later, however, the consensus Most scientists assumed that RNA nucleotides changed. Sidney Altman of Yale University in buried deep within the ribosome complex—the New Haven, Connecticut, and Thomas Cech, ones that have the same sequence in every species who was then at the University of Colorado in from bacteria to people—were the important Boulder, each discovered that RNA can perform ones for piecing the growing protein together. work as complex as that done by protein enzymes. However, recent research by Rachel Green, Their “RNA-as-an-enzyme” discovery turned the who worked with Noller before moving research world on its head and earned Cech and to Johns Hopkins University in Baltimore, Altman the 1989 Nobel Prize in chemistry. Maryland, showed that this is not the case. Noller and other researchers have continued Green discovered that those RNA nucleotides the painstaking work of understanding ribo- are not needed for assembling a protein. Instead, somes. In 1999, he showed how different parts she found, the nucleotides do something else of a bacterial ribosome interact with one entirely: They help the growing protein slip off another and how the ribosome interacts with the ribosome once it’s finished. molecules involved in protein synthesis. Noller, Green, and hundreds of other scien- These studies provided near proof that the tists work with the ribosomes of bacteria. Why fundamental mechanism of translation is should you care about how bacteria create performed by RNA, not by the proteins of proteins from their genes? the ribosome. 20 National Institute of General Medical Sciences One reason is that this knowledge is impor- An Interesting Development tant for learning how to disrupt the actions of In the human body, one of the most important disease-causing microorganisms. For example, jobs for proteins is to control how embryos antibiotics like erythromycin and neomycin work develop. Scientists discovered a hugely important by attacking the ribosomes of bacteria, which are set of proteins involved in development by study- different enough from human ribosomes that our ing mutations that cause bizarre malformations cells are not affected by these drugs. in fruit flies. As researchers gain new information about The most famous such abnormality is a fruit bacterial translation, the knowledge may lead to fly with a leg, rather than the usual antenna, more antibiotics for people. growing out of its head (see page 21). According New antibiotics are urgently needed because to Thomas C. Kaufman of Indiana University many bacteria have developed resistance to the in Bloomington, the leg is perfectly normal—it’s current arsenal. This resistance is sometimes the just growing in the wrong place. result of changes in the bacteria’s ribosomal RNA. In this type of mutation and many others, It can be difficult to find those small, but critical, something goes wrong with the genetic program changes that may lead to resistance, so it is that directs some of the cells in an embryo to important to find completely new ways to block follow developmental pathways, which are a bacterial translation. series of chemical reactions that occur in a spe- Green is working on that problem too. Her cific order. In the antenna-into-leg problem, it strategy is to make random mutations to the is as if the cells growing from the fly’s head, genes in a bacterium that affect its ribosomes. which normally would become an antenna, mis- But what if the mutation disables the ribosome takenly believe that they are in the fly’s thorax, so much that it can’t make proteins? Then the and therefore ought to grow into a leg. And so bacterium won’t grow, and Green wouldn’t find it. they do. Using clever molecular tricks, Green figured Thinking about this odd situation taught sci- out a way to rescue some of the bacteria with entists an important lesson—that the proteins defective ribosomes so they could grow. While made by some genes can act as switches. Switch some of the rescued bacteria have changes in genes are master controllers that provide each their ribosomal RNA that make them resistant body part with a kind of identification card. If a to certain antibiotics (and thus would not make protein that normally instructs cells to become good antibiotic targets) other RNA changes that an antenna is disrupted, cells can receive new don’t affect resistance may point to promising instructions to become a leg instead. ideas for new antibiotics. The New Genetics I How Genes Work 21 FLYBASE; R. TURNER  Normal fruit fly head.  Fruit fly head showing the effects of the Antennapedia gene. This fly has legs where its antennae should be. Scientists determined that several different genes of different organisms, it’s a good clue genes, each with a common sequence, provide that these genes do something so important and these anatomical identification card instructions. useful that evolution uses the same sequence Kaufman isolated and described one of these over and over and permits very few changes in genes, which became known as Antennapedia, its structure as new species evolve. a word that means “antenna feet.” Researchers quickly discovered nearly Kaufman then began looking a lot more identical versions of homeobox DNA in almost closely at the molecular structure of the every non-bacterial cell they examined—from Antennapedia gene. In the early 1980s, he and yeast to plants, frogs, worms, beetles, chickens, other researchers made a discovery that has been mice, and people. fundamental to understanding evolution as well Hundreds of homeobox-containing genes as developmental biology. have been identified, and the proteins they The scientists found a short sequence of DNA, make turn out to be involved in the early stages now called the homeobox, that is present not only of development of many species. For example, in Antennapedia but in the several genes next to researchers have found that abnormalities in it and in genes in many other organisms. When the homeobox genes can lead to extra fingers or geneticists find very similar DNA sequences in the toes in humans. 22 National Institute of General Medical Sciences The Tools of Genetics: Mighty Microarrays We now have the ability to attach a piece of every but teachers and students are using them, too. gene in a genome (all of an organism’s genes) to The Genome Consortium for Active Teaching a postage stamp-sized glass microscope slide. program (www.bio.davidson.edu/GCAT) pro- This ordered series of DNA spots is called a DNA vides resources and instructions for high school microarray, a gene chip, or a DNA chip. and college students to do gene-chip experiments Whichever name you prefer, the chip could in class. also be called revolutionary. This technology has Microarrays are used to get clues about changed the way many geneticists do their work which genes are expressed to control cell, tissue, by making it possible to observe the activity of or organ function. By measuring the level of RNA thousands of genes at once. production for every gene at the same time, In recent years, microarrays have become researchers can learn the genetic programming standard equipment for modern biologists, that makes cell types different and diseased cells different from healthy ones. The chips consist of large numbers of DNA fragments distributed in rows in a very small space. The arrays are laid out by robots that can DNA Fragments  DNA fragments are attached to  The resulting pattern of fluorescence indicates glass or plastic, then fluorescently which genes are active. tagged molecules are washed over the fragments. Complementary mRNA  Some molecules (green) bind to their complementary sequence. These mol- ecules can be identified because they glow under fluorescent light. Got It? Why are some infections hard to treat with antibiotics? What are some things researchers might do to solve this public health problem? How does DNA work as a form position DNA fragments so precisely that In December 2004, the U.S. Food and of information storage? more than 20,000 of them can fit on one micro- Drug Administration approved the first scope slide. gene chip for medical use. The Amplichip Scientists isolate mRNA from cells grown CYP450™, made by Roche Molecular Systems How can 25,000 human genes under two conditions and tag the two sources Inc. of Pleasanton, California, analyzes varia- provide the instructions for of RNA with different colors of fluorescent mole- tions in two genes that play a major role in making hundreds of thousands cules. The two colors of RNA are then placed the body’s processing of many widely pre- of different proteins? on the chip, where they attach to complementary scribed drugs. This information can help DNA fragments anchored to the chip’s surface. doctors choose the proper dose of certain Next, a scanner measures the amount of medicines for an individual patient. What newborn tests does your fluorescence at each spot on the chip, revealing area hospital routinely do? how active each gene was (how much mRNA each gene produced). A computer analyzes the patterns of gene activity, providing a snapshot of a genome under two conditions (e.g., healthy or diseased). CHAPTER 2 RNA and DNA Revealed: New Roles, New Rules F or many years, when scientists thought about heredity, DNA was the first thing to come to mind. It’s true that DNA is the basic ingredient of our genes and, as such, it often steals the limelight from RNA, the other form C G of genetic material inside our cells. U C Sugar- But, while they are both types of genetic U Phosphate material, RNA and DNA are rather different. Backbone C The chemical units of RNA are like those of G A DNA, except that RNA has the nucleotide uracil C U (U) instead of thymine (T). Unlike double- C stranded DNA, RNA usually comes as only a single G strand. And the nucleotides in RNA contain ribose G A sugar molecules in place of deoxyribose. U RNA is quite flexible—unlike DNA, which is U Base a rigid, spiral-staircase molecule that is very stable. G C RNA can twist itself into a variety of complicated, U three-dimensional shapes. RNA is also unstable in C that cells constantly break it down and must con- C tinually make it fresh, while DNA is not broken A down often. RNA’s instability lets cells change G their patterns of protein synthesis very quickly C A in response to what’s going on around them. C Many textbooks still portray RNA as a passive A U molecule, simply a “middle step” in the cell’s gene-reading activities. But that view is no longer  Ribonucleic acid (RNA) has the bases adenine (A), accurate. Each year, researchers unlock new cytosine (C), guanine (G), and uracil (U). secrets about RNA. These discoveries reveal that it is truly a remarkable molecule and a multi- talented actor in heredity. RNA RNA RNA RNA RNA RNA RNA RNA RNA RNA RNA RNA RNA RNA RNA RNA RNA RNA RNA RNA RNA RNA RNA RNA RNA RNA RNA The New Genetics I RNA and DNA Revealed: New Roles, New Rules 25  Riboswitches are RNA sequences that control gene activity. The riboswitch shown here bends into a special shape when it grips RONALD BREAKER tightly onto a molecule called a metabolite (colored balls) that bacteria need to survive. Today, many scientists believe that RNA because of its ability to lead a double life: to store evolved on the Earth long before DNA did. information and to conduct chemical reactions. Researchers hypothesize — obviously, no one In other words, in this world, RNA served the was around to write this down — that RNA was functions of both DNA and proteins. a major participant in the chemical reactions What does any of this have to do with human that ultimately spawned the first signs of life health? Plenty, it turns out. on the planet. Today’s researchers are harnessing some of RNA’s flexibility and power. For example, through RNA World a strategy he calls directed evolution, molecular At least two basic requirements exist for making engineer Ronald R. Breaker of Yale University is a cell: the ability to hook molecules together and developing ways to create entirely new forms of break them apart, and the ability to replicate, or RNA and DNA that both work as enzymes. copy itself, from existing information. Recently, Breaker and others have also uncov- RNA probably helped to form the first cell. ered a hidden world of RNAs that play a major The first organic molecules, meaning molecules role in controlling gene activity, a job once containing carbon, most likely arose out of random thought to be performed exclusively by proteins. collisions of gases in the Earth’s primitive atmos- These RNAs, which the scientists named phere, energy from the Sun, and heat from naturally riboswitches, are found in a wide variety of occurring radioactivity. Some scientists think that bacteria and other organisms. in this primitive world, RNA was a critical molecule RNA RNA RNA RNA RNA RNA RNA RNA RNA RNA RNA RNA RNA RNA RNA RNA RNA RNA RNA RNA RNA RNA RNA RNA RNA RNA RNA 26 National Institute of General Medical Sciences This discovery has led Breaker to speculate that new kinds of antibiotic medicines could be developed to target bacterial riboswitches. Molecular Editor Scientists are learning of another way to cus- tomize proteins: by RNA editing. Although DNA sequences spell out instructions for producing v RNA comes in a variety of different shapes (above RNA and proteins, these instructions aren’t and right). always followed precisely. Editing a gene’s mRNA, even by a single chemical letter, can radically change c Double-stranded DNA the resulting protein’s function. (left) is a staircase-like molecule. Nature likely evolved the RNA editing function as a way to get more proteins out of the same number of Small But Powerful Recently, molecules called microRNAs have been found in organisms as diverse as plants, worms, Larger RNA and people. The molecules are truly “micro,” con- sisting of only a few dozen nucleotides, compared to typical human mRNAs that are a few thousand nucleotides long. Dicer Enzyme What’s particularly interesting about microRNAs is that many of them arise from DNA that used to be considered merely filler material (see MicroRNA page 14). How do these small but important RNA mole- cules do their work? They start out much bigger but get trimmed by cellular enzymes, including mRNA one aptly named Dicer. Like tiny pieces of Near-perfect complementarity to target mRNA c The enzyme Dicer generates microRNAs by No Translation chopping larger RNA molecules into tiny Velcro®-like pieces. MicroRNAs stick to mRNA molecules and prevent the mRNAs from being No Protein made into proteins. RNA RNA RNA RNA RNA RNA RNA RNA RNA RNA RNA RNA RNA RNA RNA RNA RNA RNA RNA RNA RNA RNA RNA RNA RNA RNA RNA The New Genetics I RNA and DNA Revealed: New Roles, New Rules 27 genes. For example, researchers have found that the RNA sequence, which in turn changes the the mRNAs for certain proteins important for the protein that gets made. proper functioning of the nervous system are Bass’ experiments show that RNA editing particularly prone to editing. It may be that RNA occurs in a variety of organisms, including peo- editing gives certain brain cells the capacity to ple. Another interesting aspect of editing is that react quickly to a changing environment. certain disease-causing microorganisms, such as Which molecules serve as the editor and how some forms of parasites, use RNA editing to gain does this happen? Brenda Bass of the University of a survival edge when living in a human host. Utah School of Medicine in Salt Lake City studies Understanding the details of this process is an one particular class of editors called adenosine important area of medical research. deaminases. These enzymes “retype” RNA letters at various places within an mRNA transcript. They do their job by searching for characteris- tic RNA shapes. Telltale twists and bends in folded RNA molecules signal these enzymes to change AMY PASQUINELLI Velcro®, microRNAs stick to certain mRNA mole- cules and stop them from passing on their protein-making instructions. z Worms with a mutated form of the microRNA let-7 (right) have severe growth problems, rupturing as First discovered in a roundworm model system they develop. (see Living Laboratories, page 49), some microRNAs help determine the organism’s body plan. In their absence, very bad things can happen. For example, worms engineered to lack a microRNA called let-7 MicroRNA molecules also have been linked to develop so abnormally that they often rupture and cancer. For example, Gregory Hannon of the Cold practically break in half as the worm grows. Spring Harbor Laboratory on Long Island, New Perhaps it is not surprising that since microRNAs York, found that certain microRNAs are associ- help specify the timing of an organism’s develop- ated with the severity of the blood cancer B-cell mental plan, the appearance of the microRNAs lymphoma in mice. themselves is carefully timed inside a developing Since the discovery of microRNAs in the organism. Biologists, including Amy Pasquinelli first years of the 21st century, scientists have of the University of California, San Diego, are cur- identified hundreds of them that likely exist as rently figuring out how microRNAs are made part of a large family with similar nucleotide and cut to size, as well as how they are produced sequences. New roles for these molecules are at the proper time during development. still being found. RNA RNA RNA RNA RNA RNA RNA RNA RNA RNA RNA RNA RNA RNA RNA RNA RNA RNA RNA RNA RNA RNA RNA RNA RNA RNA RNA 28 National Institute of General Medical Sciences RNA Interference (RNAi) Dicer Enzyme dsRNA  Double-stranded RNA (dsRNA) is chopped into short interfering RNAs (siRNAs) by the enzyme Dicer. Short Interfering RNAs (siRNAs) A G U G U C A C RISC A G U G U C A C  The RNA-Induced Silencing U C A C Complex (RISC) enzyme A G U G attaches to siRNA. mRNA  The siRNA-RISC complex attaches to target mRNA and chops the mRNA into small pieces. Chopped mRNA (no longer functional) RNA RNA RNA RNA RNA RNA RNA RNA RNA RNA RNA RNA RNA RNA RNA RNA RNA RNA RNA RNA RNA RNA RNA RNA RNA RNA RNA The New Genetics I RNA and DNA Revealed: New Roles, New Rules 29 Healthy Interference RNA controls genes in a way that was only discov- ered recently: a process called RNA interference, or RNAi. Although scientists identified RNAi less than 10 years ago, they now know that organisms have been using this trick for millions of years. Researchers believe that RNAi arose as a way to of genes that affect cell growth and tissue reduce the production of a gene’s encoded protein formation in roundworms, using a molecular tool for purposes of fine-tuning growth or self-defense. called antisense RNA. When viruses infect cells, for example, they com- To their surprise, Mello and Fire found mand their host to produce specialized RNAs that that their antisense RNA tool wasn’t doing allow the virus to survive and make copies of itself. much at all. Rather, they determined, a double- Researchers believe that RNAi eliminates stranded contaminant produced during the unwanted viral RNA, and some speculate that it synthesis of the single-stranded antisense RNA may even play a role in human immunity. interfered with gene expression. Mello and Oddly enough, scientists discovered RNAi Fire named the process RNAi, and in 2006 were from a failed experiment! Researchers investi- awarded the Nobel Prize in physiology or gating genes involved in plant growth noticed medicine for their discovery. something strange: When they tried to turn Further experiments revealed that the double- petunia flowers purple by adding an extra stranded RNA gets chopped up inside the cell “purple” gene, the flowers bloomed white instead. into much smaller pieces that stick to mRNA and This result fascinated researchers, who could block its action, much like the microRNA pieces not understand how adding genetic material of Velcro discussed above (see drawing, page 28). could somehow get rid of an inherited trait. The Today, scientists are taking a cue from nature mystery remained unsolved until, a few years and using RNAi to explore biology. They have later, two geneticists studying development saw learned, for example, that the process is not limited a similar thing happening in lab animals. to worms and plants, but operates in humans too. The researchers, Andrew Z. Fire, then of the Medical researchers are currently testing new Carnegie Institution of Washington in Baltimore types of RNAi-based drugs for treating condi- and now at Stanford University, and Craig Mello tions such as macular degeneration, the leading of the University of Massachusetts Medical School cause of blindness, and various infections, includ- in Worcester, were trying to block the expression ing those caused by HIV and herpes virus. RNA RNA RNA RNA RNA RNA RNA RNA RNA RNA RNA RNA RNA RNA RNA RNA RNA RNA RNA RNA RNA RNA RNA RNA RNA RNA RNA 30 National Institute of General Medical Sciences DNA  Histone proteins loop together with double-stranded DNA to Histones form a structure that resembles beads on a string. Chromatin Dynamic DNA without changing its sequence. These changes A good part of who we are is “written in our make genes either more or less likely to be genes,” inherited from Mom and Dad. Many expressed (see drawing, page 31). traits, like red or brown hair, body shape, and Currently, scientists are following an intrigu- even some personality quirks, are passed on from ing course of discovery to identify epigenetic parent to offspring. factors that, along with diet and other environ- But genes are not the whole story. Where we mental influences, affect who we are and what live, how much we exercise, what we eat: These type of illnesses we might get. and many other environmental factors can all Secret Code affect how our genes get expressed. DNA is spooled up compactly inside cells in an You know that changes in DNA and RNA can arrangement called chromatin. This packaging produce changes in proteins. But additional con- is critical for DNA to do its work. Chromatin trol happens at the level of DNA, even though consists of long strings of DNA spooled around these changes do not alter DNA directly. Inherited a compact assembly of proteins called histones. factors that do not change the DNA sequence of One of the key functions of chromatin is to nucleotides are called epigenetic changes, and they control access to genes, since not all genes are too help make each of us unique. turned on at the same time. Improper expression Epigenetic means, literally, “upon” or “over” of growth-promoting genes, for example, can lead genetics. It describes a type of chemical reaction to cancer, birth defects, or other health concerns. that can alter the physical properties of DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA The New Genetics I RNA and DNA Revealed: New Roles, New Rules 31 DNA Many years after the structure of DNA was determined, researchers used a powerful device known as an electron microscope to take pictures of chromatin fibers. Upon viewing chromatin up close, the researchers described it as “beads on a string,” an image still used today. The beads were the histone balls, and the string was DNA wrapped around the histones and connecting one bead to the next. Decades of study eventually revealed that histones have special chemical tags that act like switches to control access to the DNA. Flipping these switches, called epigenetic markings, unwinds the spooled DNA so the Histone Tails genes can be transcribed. The observation that a cell’s gene-reading machinery tracks epigenetic markings led Histones C. David Allis, who was then at the University of Virginia Health Sciences Center in Charlottesville and now works at the Rockefeller University in New York City, Chromosome to coin a new phrase, the “histone code.” He and others believe that the histone code plays a major role in determining which proteins get made in a cell. Flaws in the histone code have been associated with several types of cancer, and  The “epigenetic code” controls gene activity with researchers are actively pursuing the develop- chemical tags that mark DNA (purple diamonds) and the “tails” of histone proteins (purple triangles). ment of medicines to correct such errors. These markings help determine whether genes will be transcribed by RNA polymerase. Genes hidden from access to RNA polymerase are not expressed. DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA 32 National Institute of General Medical Sciences GENETICS AND YOU: The Genetics of Anticipation ccasionally, unusual factors The number of triplet repeats seems O influence whether or not a child will be born with a genetic disease. to increase as the chromosome is passed down through several genera- tions. Thus, the grandsons of a man An example is the molecular error with a fragile X chromosome, who is that causes Fragile X syndrome, a rare not himself affected, have a 40 percent condition associated with mental retar- risk of retardation if they inherit the dation. The mutation leading to a fragile repeat-containing chromosome. The X chromosome is not a typical DNA typ- risk for great-grandsons is even higher: ing mistake, in which nucleotides are 50 percent. switched around or dropped, or one of Intrigued by the evidence that triplet them is switched for repeats can cause genetic disease, scien- another nucleotide. tists have searched for other examples Instead, it is a kind of disorders associated with the DNA of stutter by the DNA expansions. To date, more than a dozen polymerase enzyme such disorders have been found, and all that copies DNA. of them affect the nervous system. This stutter creates a string of repeats of Analysis of the rare families in a DNA sequence that is composed of which such diseases are common has just three nucleotides, CGG. revealed that expansion of the triplet Some people have only one repeat repeats is linked to something called of the CGG nucleotide triplet. Thus, they genetic anticipation, when a disease’s have two copies of the repeat in a gene, symptoms appear earlier and more and the extra sequence reads CGGCGG. severely in each successive generation. Others have more than a thousand copies of the repeat. These people are the most severely affected. DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA The New Genetics I RNA and DNA Revealed: New Roles, New Rules 33 Normal Igf2 Gene Variant  Igf2 is an imprinted gene. A (expressed) single copy of the abnormal, or mutant, form of the Igf2 Paternal gene (red) causes growth defects, but only if the abnormal gene variant is Maternal inherited from the father. Mutant Igf2 Gene Variant (not expressed) Normal Size Mouse Mutant Igf2 Gene Variant (expressed) Paternal Maternal Normal Igf2 Gene Variant Dwarf Mouse (not expressed) Battle of the Sexes father’s copy of Igf2 is expressed, and the mother’s A process called imprinting, which occurs natu- copy remains silent (is not expressed) throughout rally in our cells, provides another example of the life of the offspring. how epigenetics affects gene activity. Scientists have discovered that this selective With most genes, the two copies work exactly silencing of Igf2 and many other imprinted genes the same way. For some mammalian genes, how- occurs in all placental mammals (all except the ever, only the mother’s or the father’s copy is platypus, echidna, and marsupials) examined switched on regardless of the child’s gender. This so far, but not in birds. is because the genes are chemically marked, or Why would nature tolerate a process that puts imprinted, during the process that generates eggs an organism at risk because only one of two and sperm. copies of a gene is working? The likely reason, As a result, the embryo that emerges from the many researchers believe, is that mothers and joining of egg and sperm can tell whether a gene fathers have competing interests, and the battle- copy came from Mom or Dad, so it knows which field is DNA! copy of the gene to shut off. The scenario goes like this: It is in a father’s One example of an imprinted gene is insulin- interest for his embryos to get bigger faster, like growth factor 2 (Igf2), a gene that helps a because that will improve his offspring’s chances mammalian fetus grow. In this case, only the of survival after birth. The better an individual’s DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA 34 National Institute of General Medical Sciences chance of surviving infancy, the better its chance Starting at the End of becoming an adult, mating, and passing its When we think of DNA, we think of genes. genes on to the next generation. However, some DNA sequences are different: Of course mothers want strong babies, but They don’t encode RNAs or proteins. Introns, unlike fathers, mothers provide physical resources described in Chapter 1, are in this category. to embryos during pregnancy. Over her lifetime, Another example is telomeres—the ends of a female is likely to be pregnant several times, so chromosomes. There are no genes in telomeres, she needs to divide her resources among a num- but they serve an essential function. Like ber of embryos in different pregnancies. shoelaces without their tips, chromosomes with- Researchers have discovered over 200 imprinted out telomeres unravel and fray. And without genes in mammals since the first one was identified telomeres, chromosomes stick to each other and in 1991. We now know that imprinting controls cause cells to undergo harmful changes like divid- some of the genes that have an important role in ing abnormally. regulating embryonic and fetal growth and allocat- Researchers know a good deal about telo- ing maternal resources. Not surprisingly, mutations meres, dating back to experiments performed in these genes cause serious growth disorders. in the 1970s by Elizabeth Blackburn, a basic Marisa Bartolomei of the University of researcher who was curious about some of the Pennsylvania School of Medicine in Philadelphia fundamental events that take place within cells. is trying to figure out how Igf2 and other genes become imprinted and stay silent throughout the life of an individual. She has already identified sequences within genes that are essential for imprinting. Bartolomei and other researchers have shown that these sequences, called insula- tors, serve as “landing sites” for a protein that keeps the imprinted gene from being transcribed. HESED PADILLA-NASH AND THOMAS RIED  Telomeres, repeated nucleotide sequences at the tips of chromosomes, appear white in this photo. DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA The New Genetics I RNA and DNA Revealed: New Roles, New Rules 35 At the time, Blackburn, now at the University of California, San Francisco, was working with Joseph Gall at Yale University. For her experi- mental system, she chose a single-celled, pond-dwelling organism named Tetrahymena. CAROL GREIDER These tiny, pear-shaped creatures are covered with hairlike cilia that they use to propel them- selves through the water as they devour bacteria  Molecular biologist Carol Greider discovered the enzyme telomerase. This license plate, which was and fungi. on her car when she worked at Cold Spring Harbor Tetrahymena was a good organism for Laboratory on Long Island, New York, advertises her research interest! Blackburn’s experiments because it has a large number of chromosomes—which means it has a lot of telomeres! Her research was also perfectly timed, because within a single cell over time. Blackburn reasoned methods for sequencing DNA were just being that the repeat number might vary if cells had developed. Blackburn found that Tetrahymena’s an enzyme that added copies of the repeated telomeres had an unusual nucleotide sequence: sequence to the telomeres of some but not all TTGGGG, repeated about 50 times per telomere. chromosomes. Since then, scientists have discovered that the With her then-graduate student Carol telomeres of almost all organisms have repeated Greider, now at Johns Hopkins University, sequences of DNA with lots of Ts and Gs. In Blackburn hunted for the enzyme. The team human and mouse telomeres, for example, the found it and Greider named it telomerase. repeated sequence is TTAGGG. The telomerase enzyme, it turns out, con- The number of telomere repeats varies enor- sisted of a protein and an RNA component, mously, not just from organism to organism but which the enzyme uses as a template for copying in different cells of the same organism and even the repeated DNA sequence. DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA 36 National Institute of General Medical Sciences What is the natural function of telomerase? The Other Human Genome As cells divide again and again, their telomeres Before you think everything’s been said about get shorter. Most normal cells stop dividing when DNA, there’s one little thing we didn’t mention: their telomeres wear down to a certain point, and Some of the DNA in every cell is quite different eventually the cells die. Telomerase can counter- from the DNA that we’ve been talking about up act the shortening. By adding DNA to telomeres, to this point. This special DNA isn’t in chromo- telomerase rebuilds the telomere and resets the somes—it isn’t even inside the cell’s nucleus cell’s molecular clock. where all the chromosomes are! The discovery of telomerase triggered new So where is this special DNA? It’s inside mito- ideas and literally thousands of new studies. chondria, the organelles in our cells that produce Many researchers thought that the enzyme the energy-rich molecule adenosine triphosphate, might play important roles in cancer and aging. or ATP. Mendel knew nothing of mitochondria, Researchers were hoping to find ways to turn since they weren’t discovered until late in the telomerase on so that cells would continue to 19th century. And it wasn’t until the 1960s that divide (to grow extra cells for burn patients, researchers discovered the mitochondrial genome, for example), or off so that cells would stop which is circular like the genomes of bacteria. dividing (to stop cancer, for instance). In human cells, mitochondrial DNA makes So far, they have been unsuccessful. Although up less than 1 percent of the total DNA in each it is clear that telomerase and cellular aging are of our cells. The mitochondrial genome is very related, researchers do not know whether telo- small—containing only about three dozen genes. merase plays a role in the normal cellular aging These encode a few of the proteins that are in the process or in diseases like cancer. mitochondrion, plus a set of ribosomal RNAs Recently, however, Blackburn and a team of used for synthesizing proteins for the organelle. other scientists discovered that chronic stress and Mitochondria need many more proteins the perception that life is stressful affect telomere though, and most of these are encoded by genes length and telomerase activity in the cells of in the nucleus. Thus, the energy-producing capa- healthy women. Blackburn and her coworkers bilities of human mitochondria—a vital part of are currently conducting a long-term, follow-up any cell’s everyday health—depend on coordi- study to confirm these intriguing results. nated teamwork among hundreds of genes in two cellular neighborhoods: the nucleus and the mitochondrion. DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA The New Genetics I RNA and DNA Revealed: New Roles, New Rules 37 Mitochondrial DNA gets transcribed and  Mitochondria (labeled with a red dye) are the RNA is translated by enzymes that are very scattered throughout the cytoplasm of this different from those that perform this job for human cancer cell. genes in our chromosomes. Mitochondrial enzymes look and act much more like those from bacteria, which is not surprising because mitochondria are thought to have descended from free-living bacteria that were engulfed by another cell over a billion years ago. Scientists have linked mitochondrial DNA defects with a wide range of age-related diseases  The cell has also been treated with a dye that including neurodegenerative disorders, some colors the mitochondrial DNA green. forms of heart disease, diabetes, and various cancers. It is still unclear, though, whether dam- aged mitochondria are a symptom or a cause of these health conditions. Scientists have studied mitochondrial DNA for another reason: to understand the history of the human race. Unlike our chromosomal DNA, which we inherit from both parents, we get all of our mitochondrial DNA from our mothers.  A computerized overlay of these two images of Thus, it is possible to deduce who our mater- the same cell shows that mitochondria and its nal ancestors were by tracking the inheritance of DNA appear together mutations in mitochondrial DNA. For reasons (yellow regions). that are still not well understood, mutations accumulate in mitochondrial DNA very slowly compared to chromosomal DNA. So, it’s possible to trace your maternal ancestry way back beyond ALISON DAVIS any relatives you may know by name—all the way back to “African Eve,” the ancestor of us all! DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA DNA 38 National Institute of General Medical Sciences The Tools of Genetics: Recombinant DNA and Cloning E. coli bacteria, taken from human intestine Nucleus Human Cell Plasmid E. coli Strand of DNA from human cell Chromosome Plasmid removed Human DNA cut into pieces from E. coli by restriction enzyme Plasmid cut open by restriction enzyme at a specific site  Recombinant DNA. To splice a human gene (in this case, the one for insulin) into a plasmid, scientists take the plas- Human Insulin Gene mid out of an E. coli bacterium, cut the plasmid with a restriction enzyme, and splice in insulin-making human DNA. The resulting hybrid plasmid can be inserted into another E. coli bacterium, where it multiplies along with the bac- terium. There, it can produce large quantities of insulin. Two pieces spliced together Recombinant DNA Human Insulin Gene (hybrid plasmid) Human plasmid inserted into E. coli cell Bacteria with hybrid plasmid replicate, creating clone capable of producing human insulin ROSLIN INSTITUTE, EDINBURGH  Scientists in Scotland were the first to clone an animal, this sheep named Dolly. She later gave birth to Bonnie, the lamb next to her. In the early 1970s, scientists sequence. Most restriction endo- discovered that they could nucleases make slightly staggered change an organism’s genetic incisions, resulting in “sticky ends,” Got It? traits by putting genetic out of which one strand protrudes. material from another organ- The next step in this example is ism into its cells. This discovery, which caused to splice, or paste, the human insulin gene into Besides the sequence of quite a stir, paved the way for many extraordinary a circle of bacterial DNA called a plasmid. nucleotides in genes, what accomplishments in medical research that have Attaching the cut ends together is done with are some other changes to occurred over the past 35 years. a different enzyme (obtained from a virus), DNA and RNA that can How do scientists move genes from one called DNA ligase. The sticky ends join back affect our health and who organism to another? The cutting and pasting together kind of like jigsaw puzzle pieces. The we are? gets done with chemical scissors: enzymes, to be result: a cut-and-pasted mixture of human specific. Take insulin, for example. Let’s say a sci- and bacterial DNA. entist wants to make large quantities of this The last step is putting the new, recombi- Can you imagine treat- protein to treat diabetes. She decides to transfer nant DNA back into E. coli and letting the ments—other than the human gene for insulin into a bacterium, bacteria reproduce in a petri dish. Now, the vaccines and current Escherichia coli, or E. coli, which is commonly scientist has a great tool: a version of E. coli medicines—crafted from used for genetic research (see Living Laboratories, that produces lots of human insulin that can genetic information and page 46). That’s because E. coli reproduces really be used for treating people with diabetes. new molecular tools? fast, so after one bacterium gets the human So, what is cloning? Strictly speaking, it’s insulin gene, it doesn’t take much time to grow making many copies of a gene—in the millions of bacteria that contain the gene. example above, E. coli is doing the cloning. How is cloning a gene The first step is to cut the insulin gene out of However, the term cloning is more generally different from cloning an a copied, or “cloned,” version of the human DNA used to refer to the entire process of isolating animal or a person? How using a special bacterial enzyme from bacteria and manipulating a gene. Dolly the cloned do researchers use gene called a restriction endonuclease. (The normal role sheep contained the identical genetic material cloning to study health of these enzymes in bacteria is to chew up the of another sheep. Thus, researchers refer and disease? DNA of viruses and other invaders.) Each restric- to Dolly as a clone. tion enzyme recognizes and cuts at a different Do you have any recurring nucleotide sequence, so it’s possible to be very pre- illnesses in your extended cise about DNA cutting by selecting one of several family? hundred of these enzymes that cuts at the desired Today CHAPTER 3 Life’s Genetic Tree I n all of biology, there is one thing that always stays the same. That thing, believe it or not, is change itself! The millions of different living things on Earth—plants, bacteria, insects, chimps, people, and everything else—all came to be because of a process called biological evolution, in which organisms change over time. Time Because of biological evolution, early humans gained the ability to walk on two feet. Because of evolution, air-breathing whales can live in the ocean despite being mammals like us. Because of evolution, some bacteria can live in scalding water, others can survive in solid ice, and still others can live deep in the Earth eating only rocks! Evolution happens every day, and it affects every species—including us. It changes entire populations, not individuals. And it has a big impact on medical research. Everything Evolves To understand evolution, let’s go back in time a century and a half to 1854, when the British naturalist Charles Darwin published The Origin First Living Species The New Genetics I Life’s Genetic Tree 41  Charles Darwin described evolution in his classic text, The Origin of Species. of Species, a book that proposed an explanation for how evolution works. The main concept in evolution is that all living things share a common ancestor. The very earliest ancestor of all life forms on Earth lived about 4 billion years ago. From that early organ- within a given generation will survive long ism, millions of types of creatures—some living enough to reproduce. and some now extinct—have evolved. As an example, consider houseflies, each of Evolution requires diversity. You can tell that which lays thousands of eggs every year. Why living things are diverse just by walking down the haven’t they taken over the world? Because street and looking around you. Individual people almost all of the baby houseflies die. The flies that are very different from one another. Chihuahuas survive are the ones that can find something to are different from Great Danes, and Siamese cats eat and drink … the ones that avoid being eaten, are different from tabbies. stepped on, or swatted … and the ones that don’t Evolution also depends on inheritance. Many freeze, drown, or land on a bug zapper. of our unique characteristics are inherited—they The flies that survive all these ways to die have are passed from parent to offspring. This is easy what it takes to outlive most of their brothers and to see: Dalmatian puppies look like Dalmatians, sisters. These inherited traits give an organism a not Chihuahuas. Petunias grow differently from survival edge. Those who survive will mate with pansies. Evolution works only on traits that are each other and will pass on to the next generation inherited. some of their DNA that encoded these advanta- Finally, as you probably already know, geous traits. evolution favors the “fittest.” Through a process Of course, not all aspects of survival are called natural selection, only some offspring determined by genes. Whether a fly gets swatted 42 National Institute of General Medical Sciences depends on genes that affect its reflexes—whether discovered a rare genetic variant that protects it’s fast enough to avoid the swatter— but also people from getting AIDS. A genetic variant is a on the environment. If there’s no human around different version of a gene, one that has a slightly waving the swatter, the fly is quite likely to sur- different sequence of nucleotides. vive, regardless of its reflexes. Scientists think that the rare variant of a gene Evolution often takes a long time to make a called CCR5 originally may have been selected difference. But it can also happen very quickly, during evolution because it made people resistant especially in organisms with short lifespans. For to an organism unrelated to HIV. example, as you read earlier, some bacteria have Montgomery Slatkin of the University of molecular features that let them survive in the California, Berkeley, has used mathematical presence of antibiotics. When you take an modeling techniques to show that natural selec- antibiotic medicine, antibiotic-resistant bacteria tion over time could explain the frequency of the flourish while antibiotic-sensitive bacteria die. CCR5 variant in human populations. The work Because antibiotic resistance is a growing indicates that the CCR5 gene variant’s ability to public health threat, it’s important to take the protect against AIDS may contribute to keeping it whole dosage of antibiotic medicine, not stop in the human gene pool. when you feel better. And you should take antibi- So, through evolution, living things change. otics only when they’re needed, not for colds or Sometimes, that’s good for us, as when humans other viral infections, which antibiotics can’t understand HIV resistance in hopes of preventing treat. Viruses must simply run their course. AIDS. But sometimes the changes aren’t so great Selective Study —from a human perspective, anyway—as when Scientists doing medical research are very inter- bacteria become resistant to antibiotics. ested in genetic variants that have been selected Whether the consequences of evolutionary by evolution. For example, researchers recently change are good or bad, understanding the  Different nucleotides T C G A T A A T G C A T G C A T A One Person’s DNA (in this example, A or G) can appear in the DNA sequence of the same chromosome T C G A T A G T G C A T G C A T A Another Person’s DNA from two different individuals, creating a single-nucleotide polymorphism (SNP). The New Genetics I Life’s Genetic Tree 43 Original Haplotype  Haplotypes are combina- tions of gene variants, or on Chromosome T A T C A T SNPs, that are likely to be inherited together within 10,000 nucleotides the same chromosomal region. In this example, an original haplotype (top) evolved over time to create Haplotype 1 three newer haplotypes C A T C A T that each differ by a few nucleotides (red). Haplotype 2 T A T C A A Haplotype 3 T A T C C A Haplotype 4 C G T C A T process can help us develop new strategies for polymorphisms (abbreviated SNPs and pro- fighting disease. nounced “snips”). For example, let’s say that a certain nucleotide Clues from Variation in one of your genes is A. In your uncle, however, Scientists know quite a bit about how cells the nucleotide in the same place on the same reshuffle genetic information to create each per- gene might be G. You and your uncle have slightly son’s unique genome. But many details are different versions of that gene. Scientists call the missing about how this genetic variation con- different gene versions alleles. tributes to disease, making for a very active area If two genes sit right next to each other on a of research. chromosome, the SNPs in those genes tend to be What scientists do know is that most of the inherited together. This set of neighboring SNPs human genome is the same in all of us. A little is called a haplotype (see drawing above). bit of genetic variation—differences that Most chromosome regions have only a few, account for much less than 1 percent of our common haplotypes among all humans. As it DNA—gives each of us a unique personality, turns out, these few haplotypes—in different appearance, and health profile. combinations in each person—appear to account The parts of the human genome where the for most of the variation from person to person DNA sequences of many individuals vary by a in a population. single nucleotide are known as single-nucleotide 44 National Institute of General Medical Sciences Scientists can use haplotype information to compare the genes of people affected by a disease with those of unaffected people. For example, this approach revealed a genetic varia- tion that substantially increases the risk of age-related macular degeneration, the leading cause of severe vision loss in the elderly. Scientists discovered that a single SNP—one nucleotide in the 3 billion-nucleotide human genome—makes some people more likely to get this eye disease. The discovery paves the way for better diagnostic tests and treatments. What about other diseases? In October 2005, an international scientific team published a environments. He’s also curious about whether catalog of human haplotypes. Researchers are it can create problems for some individuals. looking through the catalog in an effort to iden- You might be surprised to learn that tify genes that determine susceptibility to many Rieseberg’s principal research subject is the sun- common diseases, including asthma, diabetes, flower. Although many plants produce only one cancer, and heart disease. generation a year, plants like sunflowers can be But not all SNPs are in genes. Scientists study- very useful tools for researchers asking funda- ing genetic variation have also found SNPs in mental questions about genetics. Because their DNA that doesn’t encode proteins. Nonetheless, genetic material is more malleable than that of some of these SNPs appear to affect gene activity. many animals, plants are excellent models for Some researchers suspect that the “cryptic” studying how evolution works. (hidden) variation associated with SNPs in non- Wild sunflowers appealed to Rieseberg coding DNA plays an important role in because there are several species that live in dif- determining the physical characteristics and ferent habitats. Two ancient species of wild behaviors of an organism. sunflowers grow in moderate climates and are Loren Rieseberg of Indiana University in broadly distributed throughout the central and Bloomington is one scientist who would love to western United States. take the mystery out of cryptic variation. He Three recently evolved sunflower species live wants to know how this non-coding genetic in more specialized environments: One of the variation can help organisms adapt to new new species grows on sand dunes, another grows The New Genetics I Life’s Genetic Tree 45 in dry desert soil, and the third species grows in a But when Rieseberg looked at the genomes salt marsh. of his hybrid sunflowers, he was surprised to To see how quickly new plant species could find that they were just cut-and-pasted versions evolve, Rieseberg forced the two ancient sunflow- of the ancient sunflower species’ genomes: ers to interbreed with each other, something large chunks had been plants but not other organisms can do. Among moved rather than many the hybrid progeny were sunflowers that were just new SNPs created. like the three recently evolved species! What that Rieseberg reasons means is that Rieseberg had stimulated evolution that plants stash away in his lab, similar to what actually happened in unused genetic material, nature some 60,000 to 200,000 years ago, when giving them a ready supply of the newer species first arose. ingredients they can use to adapt That Rieseberg could do this is pretty amaz- quickly to a new environment. It may be that ing, but the really interesting part is how it human genomes can recycle unused genetic happened. Scientists generally assume that, for a material to confront new challenges, as well. new species with very different characteristics to evolve, a lot of new mutations have to occur.  Plants like these sunflowers make great models for study- ing how evolution works. ALISON DAVIS 46 National Institute of General Medical Sciences 1 2 Living REX L. CHISHOLM Laboratories Like most people, you probably think of fruit flies advance human health. Genome sequencing proj- as kitchen nuisances. But did you know that sci- ects for all of these organisms are either already entists use these organisms for medical research? done or well under way. This means that a Fruit flies and other model organisms—as genetic process discovered in a tiny, see-through different as mice, plants, and zebrafish—permit worm can be found, and studied, in people, too. scientists to investigate questions that would not 1 Escherichia coli: Bacterium be possible to study in any other way. These “Once we understand the biology of Escherichia living systems are, relatively speaking, simple, coli, we will understand the biology of an ele- inexpensive, and easy to work with. phant.” So said Jacques Monod, a French scientist Model organisms are indispensable to science who won the 1965 Nobel Prize in physiology or because creatures that appear very different from medicine for his work on gene regulation. Monod us and from each other actually have a lot in was an early proponent of the value of experi- common when it comes to body chemistry. Even menting with simple organisms like bacteria. Are organisms that don’t have a body—mold and all bacteria bad? If all you’ve ever heard about E. yeast, for example—can give scientists clues to coli is its notorious link to tainted hamburger the workings of the tissues and organs of people. meat, you may not realize that non-disease-causing This is because all living things process the strains of the bacterium live in the intestinal tracts nutrients they consume into the same chemicals, of humans and other animals, helping them in a more or less. The genes for the enzymes involved variety of ways. For one thing, these bacteria are in metabolism are similar in all organisms. a main source of vitamin K and B-complex Below is a sampling of the wide variety of living laboratories that scientists are using to The New Genetics I Life’s Genetic Tree 47 3 NAMBOORI B. RAJU vitamins. They also aid digestion and protect Dicty normally grows as separate, independent against infection by harmful bacteria. cells. However, when food is limited, neighboring Scientists all over the world have banded cells pile on top of each other to create a large, together to sequence different versions of the multicelled structure containing up to 100,000 E. coli genome. Among other things, these studies cells. This blob ambles along like a slug, leaving will help distinguish the genetic differences a trail of slime behind. After migrating to a more between bacteria in a healthy human gut and suitable environment, the blob firms up into a those that cause food poisoning. towerlike structure that disperses spores, each capable of generating a new amoeba. Because of 2 Dictyostelium discoideum: Amoeba its unusual properties and ability to live alone or This microscopic amoeba—100,000 of them in a group, Dicty intrigues researchers who study form a mound as big as a grain of sand—is an cell division, movement, and various aspects of important tool for health studies. Scientists have organ and tissue development. determined that Dictyostelium discoideum (Dicty) has somewhere between 8,000 and 10,000 genes, 3 Neurospora crassa: Bread Mold many of which are close copies of those in people Chances are you don’t think of a moldy bread and animals but are missing in another single- crust as a potential science experiment, but celled organism, yeast. Dicty was first discovered thousands of researchers around the world do! in the 1930s in a North Carolina forest and has Neurospora crassa (Neurospora), which is since been found in many similar habitats around a species of mold that thrives on bread, is a widely the world. used model organism for genetic research. 48 National Institute of General Medical Sciences 4 GARY DITTA 5 ALAN WHEALS Biologists like to use Neurospora because like yeast because it grows fast, is cheap to feed it is simple to grow and has features that make and safe to handle, and its genes are easy to work it very suitable for answering questions about with. We know a lot about mammalian genes how species arise and adapt, as well as how cells because scientists can easily insert them into yeast and tissues change their shape in different and then study how they work and what happens environments. Since Neurospora produces spores when they don’t work. on a 24-hour cycle, the organism is also useful 5 Arabidopsis thaliana: Mustard Plant for studying the biological clocks that govern Researchers who study plant growth often use sleep, wakefulness, and other rhythms of life. Arabidopsis thaliana (Arabidopsis), a small, 4 Saccharomyces cerevisiae: Yeast flowering plant related to cabbage and mustard. There are hundreds of different kinds of yeast, but This organism is appealing to biologists because Saccharomyces cerevisiae, the one scientists study Arabidopsis has almost all of the same genes as most often, is an important part of human life other flowering plants and has relatively little outside the lab, too. It is the yeast that bakers use DNA that does not encode proteins, simplifying to make bread and brewers use for beer. the study of its genes. Like people and yeast, Like Neurospora, yeast is actually a fungus— plants are also eukaryotes. Arabidopsis grows not a plant, not an animal, but related to both. quickly, going from seed to mature plant in only It is also a eukaryote (as is Neurospora)—a 6 weeks—another plus for researchers who study “higher” organism with an organized, protective how genes affect biology. nucleus that holds its chromosomes. Researchers The New Genetics I Life’s Genetic Tree 49 6 7 What do you have in common with a mustard has a lot of genes—more than 19,000 (humans plant? Plant cells, and parts of plant cells, com- have about 25,000). Decoding the C. elegans municate with each other in much the same way genome was a huge milestone for biology, since that human cells do. For that reason, plants are it was the first animal genome to be sequenced good models for genetic diseases that affect completely. Scientists quickly learned that a vast cell communication. number of genes in C. elegans are very similar to genes in other organisms, including people. 6 Caenorhabditis elegans: Roundworm Caenorhabditis elegans (C. elegans) is a creature 7 Drosophila melanogaster: Fruit Fly that is a lot smaller than its name! Several of The fruit fly species most commonly used for these harmless roundworms would fit on the research is named Drosophila melanogaster head of a pin, although their usual habitat is (Drosophila). A geneticist’s fruit fly is pretty dirt. In the lab, they live in petri dishes and eat much the same as the ones that fly around your bacteria. C. elegans contains just 959 cells, overripe bananas. In the lab, though, some of almost a third of them forming its nervous the flies are exposed to damaging chemicals or system. Researchers know the fate of every one radiation, which changes the sequence of their of these cells! DNA. Researchers allow the flies to mate, then This worm is particularly prized by biologists search among the offspring for flies with because it is transparent, so what goes on in its abnormalities. The mutant flies are then mated tiny body is in plain view under a microscope. to produce more offspring with the abnormality, But for such a small, simple animal, C. elegans enabling researchers to close in on the defective genes involved. 50 National Institute of General Medical Sciences 9 MONTE WESTERFIELD 8 Fruit flies have been a favorite experimental Many researchers are drawn to zebrafish organism among geneticists since early in the because their eggs and embryos are transparent, 20th century. Hundreds of them can live in a making it possible to watch development unfold. pint-sized milk bottle or even a small vial, and In a span of 2 to 4 days, zebrafish cells split and they reproduce so quickly that keeping track of a form different parts of the baby fish’s body: eyes, particular gene as it passes through a couple of heart, liver, stomach, and so on. Sometimes, Drosophila generations takes only about a researchers will move a cell to another spot to see month. It’s also relatively easy to create flies with if it will still go on to form the same part of the mutations in many genes, enabling scientists to body or if it will do something different. This study how the genes work together. research has taught scientists about a range of health-related matters in people, including birth 8 Danio rerio: Zebrafish defects and the proper development of blood, the Zebrafish were originally found in slow streams, heart, and the inner ear. rice paddies, and the Ganges River in East India and Burma. They can also be found in most pet 9 Mus musculus: Mouse stores and are a home aquarium favorite. The branches of life’s genetic tree that led eventu- Although the fish have been used by some ally to mice and to human beings split off from geneticists for research since the early 1970s, in each other 75 million years ago, back in the recent years they have become an especially dinosaur age. But we are both mammals, and we popular model organism. share 85 percent of our genes. Because some mouse diseases are very similar—sometimes The New Genetics I Life’s Genetic Tree 51 10 identical—to human diseases, mice are Although rats are mammals just like mice, exceptionally valuable for research. they differ in important ways. Rats are much Since the late 1980s, researchers have been bigger than mice, making it easier for scientists able to engineer mice with missing genes. to do experiments that involve the brain. For Scientists make these “knockout” mice to learn example, rats have taught scientists a lot about what goes wrong when a particular gene is substance abuse and addiction, learning, memory, removed. This gives them valuable clues about and certain neurological diseases. Rats are also the gene’s normal function. Identifying these much better models than mice for studying genes in humans has helped define the molecular asthma and lung injury. And since, in people, the basis for many illnesses. disease arthritis is more common in women, studying rats makes more sense because female 10 Rattus norvegicus: Rat rats appear to be more susceptible to than male The Norway rat, or lab rat, was the first animal rats. The opposite is true with mice. domesticated for use in scientific research. Currently, they make up about one-fourth of all research animals in the United States. Lab rats have been used for many decades for testing drugs, and much of what we know about cancer-causing This Living Laboratories section is molecules was learned in basic research with rats. available as a poster. To order a free copy, visit http://www.nigms.nih.gov/classroom 52 National Institute of General Medical Sciences The Genome Zoo longer ago than the ancestor of humans and Scientists often use an image of a tree to depict chimpanzees, yet we still share hundreds of genes how all organisms, living and extinct, are related with bacteria. to a common ancestor. In this “tree of life,” each Scientists use the term comparative genomics branch represents a species, and the forks between to describe what they’re doing when they com- branches show when the species represented by pare the genomes of different species to see how those branches became different from one another. similar (or how different!) the species’ DNA For example, researchers estimate that the com- sequences are. Sequences that the species have in mon ancestor of humans and chimpanzees lived common are the molecular footprints of an about 6 million years ago. ancestor of those species. While it is obvious just by looking that people Why are “old” DNA sequences still in our have a lot in common with our closest living rela- genomes? It turns out that nature is quite eco- tives, chimpanzees, what about more distant nomical, so DNA sequences that are responsible species? If you look at an evolutionary tree, you’ll for something as complicated and important as see that humans are related to mice, worms, and controlling gene activity may stay intact for even bacteria. The ancestral species that gave rise millions of years. to both humans and bacteria was alive a lot Comparative genomic studies also have med- ical implications. What would you do if you wanted to develop new methods of preventing, diagnosing, or treating a human disease that animals don’t get? Starting All Over Again Stem cells—what embryos just what it is that gives stem cells their ability are made up of just days to change into anything, upon the body’s after an egg is fertilized by a request, but stay in the “I can do anything” sperm—have the amazing state until asked. ability to grow up into any Some basic researchers are trying to figure out kind of cell: skin, heart, how stem cells work by using a unique model muscle, nerve, and every- system: tiny, freshwater worms called planarians. thing else. How do they These worms are like stem cells in the sense that do it? they can regenerate. You can cut up planarians PHILLIP NEWMARK Intrigued by the potential into hundreds of pieces, and each piece will of these masterful cells, regenerate into an intact worm that looks the researchers want to know same as all the others. The New Genetics I Life’s Genetic Tree 53 If people have a gene that influences their risk cytochrome P450 family, abbreviated 3A4 and for a disease, and mice have the gene too, you 3A5, encode proteins that process more than half could study some aspect of the disease in mice, of all of the medicines that are sold today. even though they don’t ever have the symptoms Since the chemicals to which people are of the disease. You could even study the disease exposed vary so widely, a scientist might in yeast, if it has the gene, as well. predict that there would be different variants of cytochrome P450 genes in different human Genes Meet Environment populations. Using comparative genomics, If toxins from the environment get into our researchers such as Anna Di Rienzo of the bodies, they don’t always make us sick. That’s University of Chicago have shown that this is because liver enzymes come to our rescue to indeed the case. Di Rienzo has found many make the chemicals less harmful. The genes that sequence differences within these genes in people encode those enzymes are under constant evolu- living throughout the world. tionary pressure to adapt quickly to new toxins. It turns out that one variant of the gene that For example, certain liver enzymes called encodes the cytochrome P450 3A5 protein makes cytochrome P450 proteins metabolize, or break this enzyme very efficient at breaking down down, hormones that our bodies make as well as cortisol, a hormone that raises salt levels in the many of the foreign substances that we encounter. kidneys and helps the body retain water. Di Rienzo These include harmful molecules like cancer- compared the DNA sequences of the 3A5 gene in causing agents as well as beneficial ones, like DNA samples taken from more than 1,000 people medicines. In fact, just two genes within the representing over 50 populations worldwide. She Planarians’ resemblance to stem cells isn’t just and regenerative ability. Interestingly, 16 percent of coincidence. Scientists have discovered that these looked very much like genes that had been planarians can perform the amazing act of regen- linked to human disease! eration due to the presence of, yes, specialized Sánchez Alvarado and his team hope to figure stem cells in their bodies. out how regeneration genes allow the specialized Developmental biologist Alejandro Sánchez stem cells within the worm to travel to a wounded Alvarado of the University of Utah School of site and “turn into” any of the 30 or so cell types Medicine in Salt Lake City used the gene-silenc- needed to recreate a mature worm. ing technique RNAi (see page 28) to search for Although humans are only distantly related to planarian genes that were essential for regenera- planarians, we have many of the same genes, so tion. He found 240 genes that, when silenced, these findings have the potential to reveal strategies caused a physical defect in the worm’s growth for regenerating injured body parts in people, too. 54 National Institute of General Medical Sciences since retaining salt helps ward off dehydration caused by intense heat. However, there seems to be a cost associated with that benefit—the 3A5 gene variant raises the risk for some types of high blood pressure. That means that in environments in which retaining salt is not beneficial, evolution selects against this gene variant. Another scientist who studies interactions between genes and the environment is Serrine Lau of the University of Arizona in Tucson. She studies a class of harmful molecules called polyphenols, present in cigarette smoke and car exhaust, that cause kidney cancer in rats, and perhaps, in people. Lau discovered that rats and humans who are more sensitive to some of the breakdown products of polyphenols have an unusual DNA sequence — a genetic signature—that increases their risk of developing cancer. She suspects that  Scientists have discovered that some African popu- lations near the equator have evolved a genetic the gene that is affected encodes a tumor sup- variant that helps the body conserve water. pressor: a protein that prevents cancer from developing. In people and rats with the genetic was amazed to find a striking link between the signature, she reasons, the tumor suppressor existence of the gene variant and the geographic doesn’t work right, so tumors grow. locale of the people who have it. Taking this logic one step further, it may be Di Rienzo discovered that African populations that certain people’s genetic make-up makes living very close to the equator were more likely them unusually susceptible to DNA damage than other populations to have the salt-saving caused by exposure to carcinogens. If doctors version of the 3A5 gene. She suggests that this is could identify those at risk, Lau says, such people because this gene variant provides a health could be forewarned to avoid contact with spe- advantage for people living in a very hot climate, cific chemicals to protect their health. The New Genetics I Life’s Genetic Tree 55 However, think about this scenario: Who should make those decisions? For example, would it be ethical for an employer to refuse to hire somebody because the person has a genetic signature that makes him or her more likely to  The liver and kidneys are susceptible to damage from get cancer if exposed to a chemical used in the toxins since these body workplace? Tough question. organs process chemicals. Liver Kidneys 56 National Institute of General Medical Sciences GENETICS AND YOU: You’ve Got Rhythm! hat do waking, sleeping, The human body keeps time with W eating, reproducing, and birds flying south for the winter have in common? These are all a master clock called the suprachiasmatic nucleus or SCN. Situated inside the brain, it’s a tiny sliver of tissue about the size of a examples of nature’s amazing sense of grain of rice, located behind the eyes. It sits rhythm. All living things are equipped quite close to the optic nerve, which con- with molecular timepieces that set the trols vision, and this means that the SCN pulse of life. “clock” can keep track of day and night. If you’ve ever crossed the country or Given enough time, your SCN can reset an ocean by plane, you know about the itself after you fly in an airplane from one importance of these clocks. You probably time zone to another. experienced that traveler’s misery called The SCN helps control sleep by coordi- jet lag, where the body is forced to adapt nating the actions of billions of miniature quickly to a new time zone. “clocks” throughout the body. These aren’t But did you know that certain forms actually clocks, but rather are ensembles of of insomnia and manic-depressive illness genes inside clusters of cells that switch on are associated with biological clocks and off in a regular, 24-hour cycle—our not working properly? And biological physiological day. rhythms may be the reason why some Scientists call this 24-hour oscillation medicines and surgical treatments a circadian rhythm. (“Circadian” comes from appear to work best at certain times the Latin words meaning “approximately of day. a day.”) Researchers have discovered that all living things—plants, animals, and bac- Output Rhythms: Light teria—have circadian rhythms. Many Physiology Behavior researchers working with insect and other model systems have identified genes that are critical for keeping biological time. Understanding circadian rhythms will help scientists better understand sleep disorders. If we have the opportunity, most of us sleep 7 or 8 hours at night, and if we don’t get Suprachiasmatic Nucleus (SCN) enough rest we may have a hard time getting things done the next day. Some people, The New Genetics I Life’s Genetic Tree 57 however, routinely get by with only 3 to Although the shaker flies don’t 4 hours of sleep. Researchers have noted appear sleep-deprived, Cirelli found that that this trait seems to run in families, they have a different problem: They suggesting a genetic link. don’t live as long as flies without the As it turns out, fruit flies need even mutation. She is now studying this new more sleep than people. Neuroscientist connection between sleep and lifespan. Chiara Cirelli of the University of Her work may also pave the way for Wisconsin-Madison did a genetic search improved sleep aids and effective for fruit fly mutants that don’t sleep remedies for jet lag. much. She discovered that flies with a variant of a gene called shaker sleep only 3 to 4 hours per night. 58 National Institute of General Medical Sciences Animals Helping People melanogaster, a fruit fly species widely used Using technology that grew out of the Human in genetic research (see Living Laboratories, Genome Project, scientists have read the page 49). sequences of the genomes of hundreds of organ- Currently, Bier and other scientists are using isms: dogs, mice, rats, chickens, honeybees, fruit experimental flies to investigate a wide range of flies, sea urchins, puffer fish, sea squirts, round- genes involved in conditions such as blindness, worms, several fungi, and many disease-causing deafness, mental retardation, heart disease, and bacteria. Next in line are dozens of additional the way in which bacterial toxins cause illness. species including marmosets, skates, tree shrews, By reading the DNA sequences of many other sloths, and zebra finches! species, researchers hope to find model systems What effect will all this gene sequence infor- that are even better than fruit flies for studying mation have on medical research? We’ve already some aspects of human disease. talked about the fact that people share many of Sometimes, the genes that we don’t have in their genes with other species. This means that common with other species are as important as when scientists read the sequence of another the genes we share. For example, consider the fact species’ genome, they’re likely to discover that the that humans and chimpanzees have remarkably organism has many of the genes that, in humans, different abilities and physical features. But the cause disease or raise disease risk when mutated. chimpanzee genome is 99 percent identical to Take fruit flies as one example. According to our own. biologist Ethan Bier of the University of California, And did you know that chimpanzees don’t San Diego, 30 percent of the currently identified get malaria or AIDS? human disease genes most likely have functional So about 1 percent of our genome determines counterparts in none other than Drosophila whether we look and behave like a person or a chimp, and whether we are susceptible to malaria or AIDS. My Collaborator Is a Computer We’ve made the case that comparing genomes can offer fresh insight on the basic genetic ingre- dients for health and the causes of disease. But what does a scientist actually do when he or she compares gene sequences? Does this mean staring at thousands of pages of genetic letters, looking for those that are the same or different? The New Genetics I Life’s Genetic Tree 59  Computers are an essential tool for scientists who store and analyze huge amounts of genomic data. Yes and no. Comparative genomics does things, the programs can figure out where in involve looking for similarities and differences, the DNA sequences a gene starts and stops: but it isn’t something that scientists do by hand. its “boundaries.” Certainly not for thousands of genes at a time. Other researchers who work in the field of Rather, the gigantic task of comparing the bioinformatics mine genomic information hid- nucleotides that make up the genomes of two or den in the masses of data. They are looking for more species is the perfect job for a computer, a scientific treasure in the form of new biological natural multitasker. If you consider that the knowledge. These experiments can zero in on pre- human genome contains 3 billion nucleotides, viously hidden patterns and reveal links between you can easily see why this is work well suited to different fields of research. a machine (with a human operator, of course). Bioinformaticists and computational biolo- Researchers called computational biologists gists are in high demand because they play a very help analyze genomic data. These scientists important role in 21st-century medical science. develop software programs that enable computers These scientists must be fluent in both computer to perform genome comparisons. Among other science and biology. 60 National Institute of General Medical Sciences The Tools of Genetics: Unlimited DNA You might be amazed to learn that a microbe that essential to a laboratory technique called the lives in a boiling hot spring in Yellowstone polymerase chain reaction, or PCR. And PCR National Park is the essential ingredient for one is essential to lots of things that life scientists of the most important biological research tools do—and to many other fields, too. PCR’s inven- ever invented. tor, Kary Mullis, won the 1993 Nobel Prize in Thermus aquaticus is a bacterium that makes chemistry. a heat-resistant enzyme, which is why it can thrive PCR is a quick, easy method for generating in hot springs. The enzyme, Taq polymerase, is unlimited copies of tiny amounts of DNA. Words  A microbe that lives in hot springs, like this one in Yellowstone National Park, is home to the enzyme that makes the polymerase chain reaction, or PCR, possible.  PCR machine. Got It? APPLIED BIOSYSTEMS Discuss reasons why research like “revolu- PCR is a key element of “genetic finger- studies with identical twins tionary” and printing,” which has helped free prisoners can provide valuable informa- “breakthrough” are who relied on it to prove that they were inno- tion about health and disease. not an exaggeration of its impact. cent of the crimes that got them locked up. PCR is at the heart of modern DNA sequenc- Conversely, it has provided scientific evidence Humans and mice share over ing methods. It is essential for pinpointing that helped convict criminals. 80 percent of the same mutations in genes, so it is the basis for much of PCR has even revolutionized archaeology genetic material: for chimps the research discussed in this booklet. PCR has by helping to analyze badly damaged ancient and humans, it’s more than 99 done for genetic material what the invention of DNA—sometimes thousands of years percent. Why are people and the printing press did for written material. It old—which can reveal new information animals so different, if their makes copying easy, inexpensive, and widely about past people and cultures. genes are so similar? available. Scientists predict that future uses of PCR PCR underlies many diagnostic techniques, technology will enhance medical treatment, like testing individuals for genes that cause enabling better diagnosis and more accurate You are a scientist and you breast cancer. It can also help diagnose diseases subtyping of disease. want to learn more about how other than cancer, such as infections by HIV humans age. Is there a way and hepatitis C. you can address your research question without spending many decades studying people? Can you think of an experi- ment using fruit flies that could help researchers better understand jet lag? CHAPTER 4 Genes Are Us F or science, the sequencing of the human genome was a groundbreaking achievement, one that made a lot of news. But what does it changes create words with new meanings— genes that code for different proteins. Other spelling changes appear to have no effect actually mean? Will any of this information make whatsoever, at least not ones that today’s scien- a difference in your life? tists know how to measure. A genome is all of the genetic material that an Researchers are beginning to use knowledge individual (or a species) has. The human genome learned from genome sequencing research to differs from the gorilla genome, which differs figure out how being healthy and being sick are from the rice genome, and so on. And while every different at the level of molecules. And doctors person has a “human genome,” it is not exactly are starting to use genetic information to make the same in all people. Sequence variations treatment choices. within your genes makes your DNA different For example, a diagnostic test can search for from that of your mother, your cousin, or a differences in the level of expression of a particu- complete stranger. lar gene in breast cancer cells and predict whether Think of the human genome as a long story a person will respond to a drug called Herceptin®. that contains roughly 25,000 words (the genes). The cancerous cells of some people who have With few exceptions, each person has the same breast cancer make an abundance of “HER2” number of words, but certain words have slightly proteins that are targeted by Herceptin. For those different spellings. In some cases, the spelling people, Herceptin is a miracle drug because it Reading the Book of Human Genes In April 2003, researchers across the world cele- brated a milestone and an anniversary. Almost 50 years to the day after James Watson, Francis Crick, L. BARRY HETHERINGTON and Maurice Wilkins unveiled their Nobel Prize- winning description of the DNA double helix, scientists completed the sequencing of the human genome, a momentous achievement in biology. The day was long in coming. In the 1980s,  Many DNA sequencing centers joined efforts to geneticists realized that they had both the need form the Human Genome Project, completed in and the ability to learn the complete layout of the 2003. Now the centers, like this one at the Broad human genome. They wanted to map the location Institute of MIT and Harvard University in of every gene within chromosomes and decipher Cambridge, Massachusetts, are working to better understand the human genome and to sequence the complete, letter-by-letter sequence of the the genomes of other organisms. genome’s 3 billion nucleotides. The New Genetics I Genes Are Us 63 reduces the risk that their breast cancer will come though, so it shouldn’t be prescribed. Research is back, and it also decreases their odds of dying proceeding quickly to develop other genetic tests from the disease. that may help diagnose and treat a wide range of For cancer patients whose tumor genes do health problems beyond cancer. not express HER2, Herceptin won’t do a thing, With that information in hand, scientists Another is the inter- reasoned, it would eventually be possible to national HapMap Project, learn exactly what job each gene performs as the first stage of which well as how genes contribute to human health was completed in October and disease. 2005. Short for Haplotype Soon, thousands of scientists in labs all over the Map, this project’s goal is world got into the act. Critical to their success to describe the common were new tools and technologies that made the patterns of genetic varia- work go faster and helped the researchers man- tion in humans. The plan age and analyze the flood of data. is to figure out which dif- Although the Human Genome Project is done, ferences may be linked to disease risk and other related gene sequencing efforts continue. One health-related traits, such as individual reactions involves sequencing the genomes of many other to medicines and environmental chemicals. species (see page 58). 64 National Institute of General Medical Sciences Individualized Prescriptions Because each person’s set of genes is a little One way variations in our genes make a differ- different, the proteins that the genes encode are ence in our health is by affecting how our bodies also slightly different. These changes can affect react to medicines. The unsettling truth is that how the cytochrome P450 proteins (and many medicines work as expected in fewer than half of other types of proteins) work on drugs. the people who take them. Doctors first realized this in the 1950s, While environmental and lifestyle factors when some patients had bad—sometimes fatal— can explain some of this, a good part of the reactions to an anesthetic medicine used in surgery. individual variability in response to medicines Experiments revealed that those who reacted can be attributed to variants in the genes that poorly had a genetic variation in the enzyme that make cytochrome P450 proteins (see page 53). breaks down and disposes of the anesthetic after These proteins process many of the drugs it’s been in the body for a while. we take. People whose genes encode the variant enzyme had no trouble at all until they needed surgery that required general anesthesia. In the operating room, a normal human genetic variation suddenly led to a medical crisis! Fortunately, this type of serious reaction to an anesthetic is very rare. But many reactions to medicines aren’t so unusual. Researchers know that genetic variations can cause some common medicines to have dangerous side effects. For example, some people who take the colon cancer drug Camptosar® (also known as irinotecan) can develop diarrhea and a life-threatening infection if they have a variant form of the gene for the protein that metabolizes Camptosar. Genetic variations can also cause drugs to have little effect at all. For example, in some people, pain medicines containing codeine, like Tylenol®  Did you know that medicines work like they’re supposed to in fewer than half of the people who with Codeine Elixir, offer no relief because their take them? Genetic differences among people are one reason. bodies break it down in an unusual way. The New Genetics I Genes Are Us 65 The use of genetic information to predict how people will respond to medicines is called pharmacogenetics. The ultimate goal of this field of study is to customize treatments based on an individual’s genes. With this kind of approach, every patient won’t be treated the same, because doctors will have the molecular tools to know ahead of time which drug, and how much of it, to prescribe— or whether to prescribe it at all. The Healing Power of DNA  Pharmacogenetic researchers have discovered Pharmacogenetics is advancing quickly since sci- that a gene test can predict which children with acute lymphoblastic leukemia will be cured by entists have a lot of new information from the chemotherapy. Human Genome Project and new computer tools that help them analyze the information. One dis- ease for which progress has been rapid is cancer. Consider the fact that cancer is often treated with a chemotherapy “cocktail,” a combination common childhood cancer. The remaining 20 of several different medicines. Each of the drugs percent are at risk of the cancer coming back. in the mixture interacts with different proteins Mary Relling, a research clinical pharmacist that control how well that particular drug works at St. Jude Children’s Research Hospital in and how quickly it is metabolized in the body. Memphis, Tennessee, discovered that variations What’s more, each drug may have its own set of in two genes can predict which patients with unpleasant—even potentially life-threatening— acute lymphoblastic leukemia are likely to be side effects. cured by chemotherapy. Her research team also For these reasons, individually targeted, gene- identified more than 100 genes expressed only in based prescriptions for chemotherapy may offer cancer cells that can be used to predict resistance a real benefit to people with cancer. to chemotherapy drugs. Currently, chemotherapy cures about 80 per- By taking patient and cancer cell genetic pro- cent of the children who have been diagnosed files into account, Relling says, researchers can with acute lymphoblastic leukemia, the most develop more effective treatments for the disease. 66 National Institute of General Medical Sciences  Genetic variation Other pharmaco- who are taking the same dose. Giving the right produces different individual responses to genetic scientists are dose is essential, because too much Coumadin the blood-thinning drug studying the effects of can cause excessive bleeding, while too little can Coumadin®. A genetic test could lead to more gene variants on patients’ responses to drugs used allow blood clots to form. accurate doses. to treat AIDS, allergies, infections, asthma, heart Allan Rettie, a medicinal chemist at the disorders, and many other conditions. University of Washington in Seattle, discovered For example, researchers recently identified that genetic variation among people influences two different genetic variants that play a central the activity of a protein in the blood that is role in determining the body’s response to Coumadin’s molecular target. He and other sci- Coumadin® (also known as warfarin), a widely entists are now trying to translate these findings prescribed medicine given to people who are at into a genetic test that could help doctors predict risk for blood clots or heart attacks. Although what dose of Coumadin is appropriate based on 2 million Americans take this blood-thinning each patient’s DNA profile. drug every day, it is very difficult to administer, since its effects vary widely in different people Genes Can Do That? Did you know that, in addition to This characteristic makes them a perfect species traits you can see like hair color in which to study the genetics of behavior. and physique, genes also con- What’s particularly interesting about bees is ZACHARY HUANG, HTTP://CYBERBEE.MSU.EDU tribute to how we behave? It may that rather than being stuck in a particular job, come as a surprise that many they change jobs according to the hive’s needs. researchers are answering basic Robinson has identified certain genes whose questions about the genetics of activity changes during a job shift, suggesting behavior by studying insects. that the insects’ environment helps to shape their For example, Gene Robinson, gene expression. an entomologist at the University Researchers who are beginning to understand of Illinois at Urbana-Champaign, these connections are working in a brand-new  Honeybees are social works with honeybees. Robinson says that if you field of investigation named by Robinson himself: animals and they work look at honeybees in their natural hive environ- sociogenomics. together to keep their ment, you’ll quickly see that they are very What does all of this mean for humans, you hive healthy. The forager outgoing. In fact, according to Robinson, honey- wonder? It underscores the fact that, far from bee (on the left) is about a month old and hunts bees can’t survive without the social structure of being set in stone, our genomes are influenced for food. The 14-day-old their community within the hive. by both heredity and environment, fine-tuned and undertaker bee (on the sculpted by our social life and the things we do right) removes dead bees every day. from the hive. The New Genetics I Genes Are Us 67 Cause and Effect What more do we need to know about how genes shape who we are and what we become? “A lot,” says Harvard’s Richard Lewontin, who warned against oversimplifying the role of genes in health in his 2001 book, The Triple Helix. Lewontin’s main point is that context plays an enormous role in determining how organisms grow and develop, and what diseases they get. A unique combination of genetic and environmen- disease, diabetes, or particular types of cancer  Knowing about diseases that run in your family can tal factors, which interact in a way that is very hard “run in your family,” especially if a lot of your help you guard against illness in the future. to predict, determines what each person is like. relatives get the condition when they are fairly Very few, if any, scientists would argue with young, you may want to talk with your doctor this. Whether a gene is expressed, and even about your own risk for developing the disease. whether the mRNA transcript gets translated In 2005, the U.S. Surgeon General developed into a protein, depends on the environment. a Web-based tool for organizing family health Few diseases—most of which are very rare—are information. Called “My Family Health Portrait” caused completely by a mutated gene. (see http://www.hhs.gov/familyhistory/), this tool In most cases, getting or avoiding a disease arranges information into a printout that you can depends not just on genes but on things within carry to the doctor’s office. The information can your control, such as diet, exercise, and whether help you and your doctor determine your risks or not you smoke. for various conditions. It will be many years before scientists clearly If you do discover that you are at higher-than- understand the detailed meaning of our DNA usual risk for a disease like breast cancer or heart language and how it interacts with the environ- disease, you may be able to prevent the disease, or ment in which we live. Still, it’s a great idea to delay its onset, by altering your diet, exercising find out as much as you can about your family’s more, or making other lifestyle changes. You may health history. Did any of your relatives have dia- also be able to take advantage of screening tests betes? Do people in your family tree have cancer like mammograms (breast X rays that detect signs or heart disease? of cancer), colonoscopies (imaging tests for colon Keep in mind that diseases such as these are cancer), or blood sugar tests for diabetes. relatively common, so it’s pretty likely that at least Screening tests can catch diseases early, when one relative will have one of them. But if heart treatment is most successful. 68 National Institute of General Medical Sciences  Resistance to antimalarial drugs like chloroquine is widespread throughout much of Africa and other parts of the developing world where malaria transmission is high. WHO/UNICEF. WORLD MALARIA REPORT 2005, GENEVA, MAY 3, 2005 Areas where malaria transmission occurs (high/low) Chloroquine resistance Us vs. Them So why don’t they kill human cells, too? The Many scientists focus on human genes, most of answer is that human and bacterial ribosomes are which have counterparts in the genomes of different. Genome sequencing is a powerful tool model organisms. However, in the case of infec- for identifying differences that might be promis- tions caused by microorganisms, understanding ing targets for new drugs. how the genomes of bacteria, viruses, and para- Comparing genetic sequences in organisms sites differ from ours is a very important area of that are resistant and non-resistant to drugs can health research. reveal new approaches to fighting resistance. Most of the medicines we take to treat infec- Drug resistance is a worldwide problem for a tions by bacteria and viruses have come from number of diseases, including malaria. scientists’ search for molecular weak points in Although researchers have developed several these tiny organisms. As mentioned in Chapter 1, different types of medicines to treat this for example, some antibiotics kill bacteria by disease—caused by parasites carried by mosqui- disarming their protein-making ribosomes. toes, not by a bacterium or a virus—malaria is rampant, especially in the developing world. The New Genetics I Genes Are Us 69 GENETICS AND YOU: Eat Less, Live Longer? ould you consume an ex- that by restricting the W tremely low-calorie diet if it meant you would live longer? The kind of diet we’re talking about formation of extra DNA, sirtuins keep the yeast young. isn’t just cutting back here and there. It Not so fast, say other involves severely reducing calorie intake scientists like geneticist to about 60 percent of what we nor- Stanley Fields of the mally eat, enough to make most people University of Washington. His experiments ravenously hungry. have turned up other, unrelated genes A 19th-century French doctor, linked to lifespan in yeast. He argues that Maurice Gueniot, thought the tradeoff while calorie restriction is the only inter- would be worth it. Throughout his adult vention that has been shown to extend life, he ate very little. He died at the ripe lifespan in a wide range of organisms, old age of 102! including mammals, the accumulation of Later, in the 1930s, researchers extra DNA does not always appear to play followed up on this observation by a role in this process. showing that rats on a diet containing What’s the final answer, you ask? It’s 20 percent indigestible fiber—calories probably a bit of both. that can’t be used—lived much longer Molecules like sirtuins, which are than their normally fed peers. involved in cellular metabolism, may pro- Intrigued by the health connection, tect cells against the harmful effects of scientists are continuing to investigate stress, extending lifespan. Other mole- potential links between diet and aging, cules that affect different aspects of cell and genetic studies are starting to turn health may be just as important. up some clues. Lifespan in complex, multicellular For example, geneticist David Sinclair organisms like people is affected by many of Harvard Medical School has found that different factors, most of which we know proteins known as sirtuins may be able to very little about. For sure, understanding stall aging. As yeast cells age, they accu- more about these mystery molecules mulate extra DNA, which eventually kills could have a considerable benefit— them. Sinclair discovered that sirtuins perhaps providing you a chance to add become more active in yeast cells that years to your life without starving! are on a low-nutrient “diet.” He reasons 70 National Institute of General Medical Sciences This is partly Gang Warfare because not all people Did you know that scientists are using genetics to have access to treat- break up gangs … of microbes, that is? These ment, or to simple gangs, known as biofilms, are layers of slime that preventive measures like develop naturally when bacteria congregate on bed nets, which protect surfaces like stone, metal, and wood. Or on your CDC/ JAMES GATHANY sleeping people from teeth: yuck! mosquito bites. But Biofilms grow in all sorts of conditions. For another problem is the example, one biofilm known as “desert varnish”  Mosquitoes spread malaria parasite itself, which has rapidly evolved thrives on rocks, canyon walls, or, sometimes, malaria by picking up parasites from blood and ways to avoid the effects of antimalarial drugs. entire mountain ranges, leaving a reddish or spreading them to the next person they bite. Scientists are trying to counter this process by other-colored stain. It is thought that petroglyphs Resistance spreads this studying microbial genetic information. In the left on boulders and cave walls by early desert way, too. case of malaria, geneticists like Dyann Wirth of dwellers were often formed by scraping through the Harvard School of Public Health compare the the coating of desert varnish formations with a genomes of drug-resistant parasites and those hard object. that can still be killed by antimalarial medicines. Sometimes, biofilms perform helpful func- Wirth’s research suggests that it should be tions. One of the best examples of the use of possible to develop a simple, inexpensive genetic biofilms to solve an important problem is in the test that could be given to people with malaria, cleaning of wastewater. anywhere in the world. This test would identify drugs that are likely to be most effective and help decrease the rate at which parasites become resistant to the anti- malarial medicines we already have.  Biofilms, like the one shown in this P. SINGH AND E. PETER GREENBERG fluorescent microscopic photo, are bacterial communities. The New Genetics I Genes Are Us 71  Bonnie Bassler (right) uses glow-in-the dark bacteria to study the genetics of biofilms. DENISE APPLEWHITE But biofilms can be quite harmful, con- goal of being able to use this knowledge to break tributing to a wide range of serious health up bacterial “gang meetings.” problems including cholera, tuberculosis, cystic Bassler’s research subjects have a definite fibrosis, and food poisoning. They also underlie visual appeal. They glow in the dark, but only many conditions that are not life-threatening when they are part of a group. The biolumines- but are nonetheless troublesome, like tooth cence, as the glow is called, arises from chemical decay and ear infections. reactions taking place within the biofilm. It pro- Bacteria form biofilms as a survival measure. vides a way for the bacteria to talk to each other, By living in big groups rather than in isolation, estimate the population size of their community, the organisms are able to share nutrients and and distinguish themselves from other types of conserve energy. How do they do it? microorganisms. A biofilm is not just a loose clump of cells— Through her studies, Bassler has identified a it’s a highly sophisticated structure. As in any set of molecules that biofilm-forming microor- community, the individuals in biofilms commu- ganisms use to pass messages to each other. By nicate with each other. devising genetically based methods to cut off Beyond that, many aspects of biofilms are the chatter, Bassler reasons, she may be able to poorly understood. Bacterial geneticist Bonnie cause bacterial communities to fall apart. This Bassler of Princeton University in New Jersey is approach would provide a whole new way to treat working to understand biofilms better, with the health problems linked to harmful biofilms. 72 National Institute of General Medical Sciences The Tools of Genetics: Mathematics and Medicine What if public health officials had a script for In 2005, the Models of Infectious Disease what to do in the face of an infectious disease Agent Study (MIDAS), a team of biologists, outbreak that had never been seen before? One computer scientists, statisticians, mathematicians, thing that would help them prepare for this sort social scientists, and others, set out to model an of scenario is the ability to know, ahead of time, epidemic of avian influenza, or “bird flu.” This how an epidemic develops and spreads. disease occurs naturally among wild birds, which Toward this goal, some scientists are using carry the viruses in their intestines but don’t mathematical tools to create simulations, or always get sick from them. models, of infectious disease outbreaks. They However, health experts worldwide became can then use the models to test the effects of worried as cases of human infection with bird various intervention strategies. Part of the work flu viruses began to appear. While the bird virus involves plugging in genetic information about isn’t transmitted easily from one person to how infectious organisms evolve over time and another, there’s a risk that the virus’ genetic mate- how fast they change as they interact with human rial could mutate inside the body of an infected populations. person. These changes could make it much easier for the virus to pass between humans, a key ingredient for an influenza pandemic—a huge, global epidemic. The MIDAS team, whose leaders include computational biologist Neil M. Ferguson of Imperial College in London and biostatistician Ira M. Longini, Jr., of the University of Washington, simulated what might happen if the original, deadly strain of bird flu virus mutated and started to spread from person to person.  Computer simulations are helping scientists understand how infectious diseases spread. Got It? Discuss how mathematics can help scientists ask ques- tions about human health. Would you contribute a sample of your DNA for genetic research on common The scientists first created hypothetical The U.S. models showed that outcomes diseases like heart disease, communities in Southeast Asia, where human varied depending on the contagiousness of depression, or cancer— cases of bird flu first appeared, and then wrote the virus. When the simulated virus was less even if you didn’t have any computer programs that took into account the contagious, three effective single measures of these health problems? potential infectiousness of the bird flu virus, pop- included antiviral treatment, school closures, Why or why not? ulation densities, and the locations of schools, and vaccinating 10 million people per week hospitals, and other community structures. The with one dose of a poorly matched vaccine. models indicated that to successfully contain an When the virus was highly contagious, how- Drugs work like they’re epidemic, health officials would need to find the ever, the models predicted that all single- supposed to in only half the first flu cases early and implement a combination intervention strategies would leave nearly half people who take them, so of public health measures very fast. the population infected, but that combination scientists are trying to make Next, using data from the 2000 U.S. Census approaches could slow the spread. “personalized medicines” that and the U.S. Department of Transportation, work very well in an individ- the researchers developed a model of pandemic ual because they match his flu spread based on the demographics and travel or her genetic make-up. Are patterns of 281 million people living in the there economic, social, or United States. other issues that the develop- ment of such medicines might raise? CHAPTER 5 21st-Century Genetics M edicine has evolved tremendously since the earliest human civilizations, when the diagnosis and treatment of disease were far practices like opening the vein of a sick person and draining off quarts of precious blood! Later, in the Renaissance period of the 15th from scientific. Medieval medicine, for example, and 16th centuries, scholars centered on anatomy. relied heavily on supernatural beliefs. Limited One of them, the Italian artist-inventor Leonardo scientific knowledge led to seemingly bizarre da Vinci, created beautiful and accurate RARE BOOK AND SPECIAL COLLECTIONS DIVISION, LIBRARY OF CONGRESS  By the end of the 16th century, anatomy was a common focus for scien- tific scholars. The New Genetics I 21st-Century Genetics 75 illustrations of the human body. His work  19th-century scientists discovered that bacteria and that of other scientists of his day can cause disease. Bacillus anthracis (left) causes anthrax focused on the practice of dissection, and Vibrio cholerae (below) causes cholera. providing never-before-seen details of PAUL KEIM (ANTHRAX), the body’s architecture of limbs, joints, CDC/ WILLIAM A. CLARK (CHOLERA) muscles, nerves, and vessels. Modern medicine got its real start during the 19th century, after the micro- scope was invented. Medical school subjects like physiology, pathology, and microbiology were born. During this time, scientists discovered that bacteria—not evil spirits or other imaginary One of today’s challenges is to map the entities—caused human diseases like cholera, actions and interactions of all these molecules, anthrax, and tuberculosis. a focus of the new field called systems The birth of modern genetics, which biology. Genetic and genomic occurred in the 20th century, accelerated the research is helping scien- study of all these areas of science. Now, at tists tackle many the start of the 21st century, opportunities questions in this have never been greater for turning scientific area. By building knowledge into better health for all. models of cells, We often take for granted the amazing tissues, and complexity of the human body. Without even organs in action, thinking, we sweat to maintain body tempera- scientists hope to ture, get hungry when we need energy, and feel learn how these tired when we need to sleep. complex systems work. These seemingly simple actions require a Researchers need to sophisticated coordination of many different know these basics in order to organs and the millions of molecules that work understand how the systems fail, when together inside them. Thousands of networks disease strikes. An essential tool in this research of interacting genes underlie these actions in is the computer. our bodies. 76 National Institute of General Medical Sciences No Lab? No Problem! Those who work at the intersection of computer science and biology often combine and analyze data from many different sources, look- ing for informative patterns. Andrey Rzhetsky of Columbia University in New York City is one of these people. Through an approach known as knowledge engineering, Rzhetsky and his team write computer programs that scan the contents The program first scans scientific papers of thousands of published scientific papers. using pre-set search terms, much like a Google™ The “knowledge mining” tool they use, called search of the Web. Next, it evaluates the search GeneWays, focuses mainly on research literature results and makes sure they don’t overlap. For about changes in genes and proteins. example, if a molecule has 16 different names in different papers, the program simplifies it to just one. Green Fluorescent Protein Here’s an interesting the millions of proteins inside cells. news flash: “Glow- Called green fluorescent protein, or GFP, this in-the-dark jellyfish natural protein is found in specific parts of the revolutionizes genetic jellyfish. Those parts glow because the protein CATHERINE FERNANDEZ AND JERRY COYNE research!” absorbs energy from light in the environment Although it may and then produces a different color of light. sound bizarre, the claim Scientists don’t really know how and why jelly- is true. A jellyfish pro- fish use their glow. They do know that jellyfish tein is essential to don’t flash at each other in the dark, nor do they modern cell biology glow continuously. And the glow is rarely seen in experiments that track undisturbed animals. the movements, quanti- Taken out of the jellyfish, GFP has played a  Fruit fly sperm cells glow bright green when they express the gene for green fluorescent protein. ties, and interactions of major role in advancing the study of genes and The New Genetics I 21st-Century Genetics 77 Finally, after applying specific rules, sort of like “biological grammar,” the computer program identifies associations, which are possible links between molecules. The information then goes to a database that Rzhetsky and other scientists use to build large networks of molecular interactions. Rzhetsky and his team used GeneWays to iden- tify risk genes for Alzheimer’s disease, a complex condition thought to be caused by many factors. In analyzing the data, Rzhetsky found important “nodes,” molecules that play key roles in the dis- ease gene network that GeneWays modeled. These predicted molecular interactions were ANDREY RZHETSKY AND KEVIN P. WHITE later confirmed by other researchers working in a lab, underscoring the value of computer model- ing as a way to learn more about the molecular basis of disease.  Andrey Rzhetsky uses the computer program GeneWays to locate important “hubs” of activity (large spheres) within massive gene networks. This particular network represents embryonic developmental pathways in a fruit fly. the proteins they encode. The story of how GFP cells growing in a lab dish. Recently, scientists became a research tool began in 1992, when used the GFP gene to create green-glowing Martin Chalfie of Columbia University showed zebrafish. Although the fish were created for that the gene that makes GFP produced a fluores- the purpose of scientific research, they’ve cent protein when it was removed from the also become an “exotic” species for home MARTIN CHALFIE jellyfish genome and transferred to the cells of aquariums. other organisms (see page 38). Chalfie, a devel- Thanks to GFP and related technologies, opmental biologist, first put the gene into scientists can now view living cells and their bacteria and roundworms, creating glowing constantly moving contents. GFP is also used  Scientists engineered versions of these animals. in diagnostic tests for drugs, foods, herbicides, this experimental worm to express green fluo- Since then, researchers have transferred the and hazardous chemicals. rescent protein in two GFP gene into many other organisms, including of its nerve cells (bright fruit flies, mice, and rabbits—and even human green spots). 78 National Institute of General Medical Sciences This law protects your genetic and other personal health information from being used or shared without your knowledge. It’s important to realize that, in most cases, genetic information cannot offer definitive proof that a disease will occur. But if you have a very strong family history of breast cancer, for exam- ple, there may be a faulty gene in your family that increases your risk of getting the disease. Hard Questions Doctors can now test for two known gene While the task of sorting through large volumes variants associated with inherited forms of breast of genomic data remains a central challenge in cancer, BRCA1 and BRCA2. If you carry either of modern biology and medicine, one of the knotti- these gene variants, your lifetime risk of getting est dilemmas to emerge from this research is a breast cancer is significantly higher than it would social and ethical one. That is, how should people be for someone without either variant. But some make use of information about their own genes? people who have BRCA gene variants never get Because genetic information is both powerful breast cancer. and incredibly personal, there are deep societal Only about 5 percent of all breast cancer concerns regarding its use. These concerns can be traced to a known, inherited gene include the potential for discrimination from variant. Since so many breast cancers are not health insurance companies or employers who linked to BRCA1 or BRCA2, genetic testing for learn about a person’s risk of disease, or suscepti- these variants is irrelevant for the vast majority bility to toxicity from an environmental chemical. of people who do not have a family history of Some laws are already in place to protect breast cancer. individuals from the misuse of their genetic But let’s say you do have a relative who tested information. When you visit a new doctor, nurse positive for BRCA1 or 2. Should you get tested, too? practitioner, or dentist, you’ll be asked to read A difficult question, for sure, but consider and sign a form that outlines your medical this: Knowing about this risk ahead of time privacy rights under the Health Insurance might save your life. For example, you might Portability and Accountability Act, or HIPAA. want to begin getting mammograms or other The New Genetics I 21st-Century Genetics 79 screening tests at an early age. If cancer is found this gene can cause the disease, and those are very early, it is usually more treatable, and the just the ones researchers know about! odds for a cure are much higher. How can there be 30 different variants of Currently, diagnostic laboratories across the one gene? Remember that a gene is a long United States offer genetic tests for more than DNA sequence, consisting of hundreds of 1,000 disorders. Some of these tests detect nucleotides. A change in one of those problems with entire chromosomes, not just nucleotides produces one variant, a change in individual genes. Perhaps the most well-known another produces another variant, and so on. example of a chromosome problem is Down Because there are so many possibilities, it’s syndrome, in which cells have an extra copy of hard to tell whether a person has a variant chromosome 21 (see page 11). form of the cystic fibrosis gene. So the standard Most genetic diseases aren’t caused by a genetic screening test for this disease scans for chromosome abnormality, or even by one gene all of the more than 30 variants known to cause variant. Cystic fibrosis, for example, is due to a cystic fibrosis. faulty gene, but more than 30 different variants of  Scientists are developing genetic tests that will help doctors diagnose and treat diseases. 80 National Institute of General Medical Sciences Doctors usually order a genetic test only if a As a teen or young adult, would you want to person has a strong family history of a disease. know that you’d get a serious, perhaps incurable, But even so, deciding to have such a test is not a disease later in life? simple choice. Think about what you would do Patients and doctors face these tough issues with the information. every day. Even years from now, when One thing you might consider is whether you researchers know more about the molecular could do something with what you learn from a roots of disease, genetic tests will rarely provide genetic test. easy answers. In most cases, they won’t even pro- You’ve already read about what you could vide “yes” or “no” answers. do if you discovered that you were at high risk Rather, much like a cholesterol test, they will for developing breast cancer. But what about a predict whether a person’s risk of getting a disease condition that shows up in middle-aged or older is relatively high, low, or somewhere in between. people—or one for which there is currently This is because many factors besides genes, includ- no cure? ing lifestyle choices such as diet and exercise, also play a role in determining your health. Good Advice Since the story of genes and health is so complicated and is likely to stay that way for a while, it is very important to consider genetic information in context. Health care professionals known as genetic counselors can be a big help to people who are thinking about getting a genetic test. As a profession, genetic counseling has been around since the mid-1900s. However, only a few specialty clinics offered counseling at that time. Now, genetic counseling is much more widely available. The New Genetics I 21st-Century Genetics 81 GENETICS AND YOU: Crime-Fighting DNA ike your thumbprint, your genes blood, or skin cells), DNA forensic tech- L are unique, unless you have an identical twin. As such, DNA “fingerprinting” has become a powerful nology can identify victims in a natural disaster, such as the December 2004 tsunami that ravaged Indonesia and crime-fighting tool. DNA forensics is other Asian countries. DNA fingerprint- a fast-growing specialty that has appli- ing can also match a transplant patient cations beyond putting criminals to an organ donor or establish paternity behind bars. and other family relationships. In addition to identifying suspects Genetic fingerprinting is not limited who leave traces at the scene of a crime to people. It can find small but poten- (for example, strands of hair, drops of tially deadly traces of disease-causing bacteria in food or water, determine whether an expensive horse was sired by a Kentucky Derby winner, or figure out whether a puppy’s parents were first cousins. DNA fingerprinting techniques work by looking for differences among gene sequences that are known to vary between people (or between individuals from any species). Scientists read the sequence in a dozen or so places to create a molecular profile. The chances of a molecular fingerprint being the same in two people or two organisms are vanishingly small. 82 National Institute of General Medical Sciences Today’s genetic counselors have gone through Genetics, Business, and the Law a rigorous training process in which they earn Can a scientist claim rights to a gene that he dis- a master’s degree and learn genetics, medicine, covered in worms and that has a nearly identical laboratory procedures, counseling, social work, counterpart in humans? and ethics. Genetic counselors do their work Is a person who gave a blood or tissue sample in many different settings, including hospitals, entitled to profits from a company that develops private clinics, government agencies, and uni- a drug based on genetic information in her sam- versity laboratories. ple, or to a lifetime supply of the drug? An interesting aspect of the job is that genetic Can a blood or tissue sample that was donated counselors address the needs of entire families, for one purpose be used for an entirely different rather than just individual patients. To evaluate study several years later, without asking the donor genetic risk and its potential consequences, these if that’s OK? professionals gather a family medical history These and other issues are hotly debated covering generations. in ethics and legal circles. Many of the most Field Study The word most often used to refer to This usually involves transferring genetic mate- applications of genetic research, espe- rial from one kind of organism into another. Using cially those leading to products for the same techniques that were developed for put- human use, is biotechnology. It ting genes into animals for research purposes, involves techniques that use living scientists can create crop plants with desirable organisms—or substances derived traits, such as improved flavor or better resistance from those organisms—for various to insect pests. Transferring specific genes is practical purposes, such as making a faster and more efficient than traditional breeding biological product. approaches. One major application of biotechnol- The United States is home to far more geneti- ogy is in agriculture. Actually, this is cally modified crops than anywhere else in the hardly new: Humanity has engaged in world. In 2005, 52 percent of the country’s corn, agricultural biotechnology for 10,000 79 percent of its cotton, and 87 percent of its soy- years or more. Many traditional farming beans were cultivated from seeds genetically practices, from plant breeding to animal modified to resist viruses and other plant pests. husbandry, are really forms of biotech- Many believe that agricultural biotechnology is nology. an important driver for improving world health. But in today’s agricultural industry, They say that genetic modifications may be the biotechnology generally means the use only hope for pest-ravaged crops, such as of molecular biology, recombinant DNA bananas, that are essential to the economies of technology, cloning, and other recent poor countries. The creation of edible plants that scientific approaches to produce plants contain medicine, serve as a form of vaccination, and animals with new traits. or deliver extra nutrients—such as the recently The New Genetics I 21st-Century Genetics 83 controversial topics have to do with the idea of patenting life forms. Traditionally, when an inventor comes up with a new idea and wants to sell it—whether it’s a radio-controlled toy boat or a customized laboratory chemical—he or she submits an appli- cation to the U.S. Patent and Trademark Office. By issuing patents, the Federal Government gives an inventor ownership of his or her cre- ation. Patents give inventors time to optimize their products and control how their inventions are used, allowing them to make money from their creativity. developed rice that makes vitamin A—could also contribute in major ways to global health. But opposition from farmers and consumers within and outside the United States has clouded agricultural biotechnology’s future. Some object to the development of plants that are naturally resistant to herbicides, partly out of concern that the trait might jump to weeds, making them impossible to destroy. Environmental advocacy groups worry that genetically modified plants may impact the future biodiversity of our planet by harming beneficial insects and possibly other organisms. However, the U.S. Environmental Protection Agency has stated that there is no evidence to date that indicates that biotech crops have any adverse effects on non-targeted wildlife, plants, or benefi- cial insects. Of course, careful field tests of newly created, z Biotechnology helps agricultural scientists create genetically modified plants and animals are crops with desired traits. The majority of cotton essential to be sure that they cause no harm to and soybeans in the United States are grown with other organisms or to the environment. genetically modified seeds that resist viruses and other plant pests. 84 National Institute of General Medical Sciences However, nobody invented a gene, a naturally Patents are great for business, but they have occurring chemical, or a protein, so why should a the potential to slow research because patent- person or a company be able to own it and con- holders control how information related to the trol its destiny in the marketplace? patent is used. For example, researchers who wish Patent laws in the United States and Europe to study patented genetic information may need prohibit anyone from patenting a gene as it exists to acquire a license first. This can be time- in the human body. But for many years, scientists, consuming and expensive. universities, and companies have earned patents Concerned about possible negative effects of for methods of isolating genes or for specific patenting genes, the U.S. National Institutes of medical uses of genetic information. The breast Health has worked with the U.S. Patent and cancer gene test is one example. Trademark Office to establish guidelines for what kind of genetic information can be patented. Since this area of medical research is an ever- moving target, government scientists and policymakers continue to clarify patent and licensing issues in the hope of keeping data that is valuable for research in the public domain. The New Genetics I 21st-Century Genetics 85 Careers in Genetics Opportunities to be part of genetic and genomic research have never been greater or more exciting. In addition to studying human genes, scientists are gathering information about the genes of many other living things, from microbes that cause disease to model organisms like mice and Drosophila, livestock, and crop plants. Although computers do some of the work, this avalanche of information has to be analyzed by thousands and thousands of human brains. In addition to identifying genes, scientists must generated by life scientists, is especially short figure out what the genes do and—even more of qualified workers. As a result, bioinformatics complicated—how they do it. scientists are in high demand. We need laboratory scientists, doctors to do Many careers in genetics and genomics clinical research and treat patients, genetic coun- require advanced degrees such as a Ph.D. or M.D. selors to help people understand the information But people with master’s or bachelor’s degrees are in their genes, and lawyers and ethical specialists also needed to fill thousands of rewarding jobs as who can address legal and policy concerns about genetic counselors, research assistants, and lab the use of genetic information. technicians. In especially high demand are people with For more career information, see expertise in mathematics, engineering, computer http://www.ornl.gov/sci/techresources/ science, and physics. The field of bioinformatics, Human_genome/education/careers.shtml or which develops hardware and software to store http://science.education.nih.gov/LifeWorks. and analyze the huge amounts of data being 86 National Institute of General Medical Sciences The Tools of Genetics: Informatics and Databases For most of its history, biology managed to when you remember that DNA is itself a form of amass its data mostly with the help of plain old information storage. arithmetic. Gregor Mendel did genetic analysis Where are genetic and genomic data stored? by simply counting the different kinds of off- One of the first biological databases was created spring produced by his peas. By contrast, today’s to store the huge volume of data from experi- genetic research creates too much data for one ments with the fruit fly Drosophila melanogaster. person, or even a scientific team, to understand. Called FlyBase, it has grown into a huge, New technologies are needed to manage this comprehensive, international electronic reposi- huge amount of data. tory for information on Drosophila genetics and Consider this: Gene-sequencing machines molecular biology, run by scientists for scientists. can read hundreds of thousands of nucleotides a The information includes nearly a century’s day. Gene chips are even faster. The information worth of published scientific literature on fruit in GenBank®, a widely used database of DNA flies and data on the fruit fly’s genome sequence sequences, nearly doubles every year. It is said that a laboratory doing genetic research can gener- ate hundreds of gigabytes of data a day, every day— more than 20,000 times the contents of the com- plete works of Shakespeare or J.S. Bach. How can anyone make sense of all this informa- tion? The only way is to enlist the aid of computers and software that can store the data and make it possible for researchers to organize, search, and analyze it. In fact, many IMAGE ON COMPUTER SCREEN COURTESY OF TOM SLEZAK, of today’s challenges in biology, from gene LAWRENCE LIVERMORE NATIONAL LABORATORY analysis to drug discovery, are really challenges in information technology. This is not surprising Got It? Do you think modern research tools derived from genomics and bioinformatics will change the practice of medicine? How? If a genetic test revealed that you had a 1 in 100 chance of develop- ing a disease like type 2 diabetes, which can be prevented with lifestyle changes like eating a as well as information about DNA sequences of elegans and the soil-dwelling amoeba healthier diet and exercising more, some of Drosophila’s fly relatives. Dictyostelium discoideum. would you change your behavior? Databases like FlyBase are also useful to sci- A key goal is to make sure that all of these What if the risk were 1 in 10? entists working with other organisms, like mice databases can “talk” to each other. That way, or humans. A researcher who discovers a new similar discoveries in different organisms— mammalian gene may consult FlyBase to see if the important, common threads of all How is genetic engineering similar fruit flies have a similar gene and if the database biology—can be identified quickly and to traditional farming? How is it contains hints about what the gene does. Since analyzed further. different? the functions of many genes are retained during For this database communication to evolution, knowing what a gene does in one work, researchers in different fields must organism often provides valuable clues about use the same terms to describe biological A biotechnology company uses what it does in another organism, even if the processes. The development and use of genetic information from a patient two species are only distantly related. such a universal “ontology”—a common volunteer and develops an effec- Several other communities of researchers language—is helping scientists analyze the tive, profitable medicine. Should have created their own databases, including complex network of biology that underlies the patient know that he or she WormBase and DictyBase, dedicated to the our health. was part of this process? Why or investigation of the roundworm Caenorhabditis why not? What if the research did not lead to any medical advance? 88 National Institute of General Medical Sciences Glossary Amino acid | A building block of proteins. Comparative Genomics | The study There are 20 amino acids, each of which is of human genetics by comparisons with the coded for by three adjacent nucleotides in a genetics of other organisms. DNA sequence. Diploid | Having two copies of each Anticipation | The disease process in which chromosome. symptoms show up earlier and are increasingly DNA | Abbreviation for deoxyribonucleic acid, severe in each generation. the molecule that contains the genetic code for all Biofilm | A slime layer that develops naturally life forms except for a few viruses. It consists of when bacteria congregate on surfaces. two long, twisted chains made up of nucleotides. Each nucleotide contains one base, one phosphate Bioinformatics | The field of biology specializ- molecule, and the sugar molecule deoxyribose. ing in developing hardware and software to store The bases in DNA nucleotides are adenine, and analyze the huge amounts of data being thymine, guanine, and cytosine. generated by life scientists. DNA chip | See microarray. Biotechnology | The industrial use of living organisms or biological methods derived through DNA polymerase | An enzyme that copies basic research; examples range from genetic engi- DNA. neering to making cheese or bread. Enzyme | A substance (often a protein) that Chromatin | The organization and dense pack- speeds up, or catalyzes, a chemical reaction with- aging of DNA in the nucleus of cells. out being permanently altered or consumed. Chromosome | A cellular structure containing Epigenetics | The study of heritable changes in genes. Chromosomes are composed of DNA and gene function that occur without a change in the proteins. Humans have 23 pairs of chromosomes DNA sequence. in each body cell, one of each pair from the Eukaryote | An organism whose cells have mother and the other from the father. a membrane-bound nucleus. Circadian | Pertaining to a period of about Exon | A DNA sequence in a gene that codes 24 hours; applied especially to rhythmic biologi- for a gene product. cal repetition like the sleep-wake cycle. Gene | A segment of a DNA molecule that Clone | In genetics, the process of making many contains information for making a protein or, copies of a gene. The term also refers to the isola- sometimes, an RNA molecule. tion and manipulation of a gene. The New Genetics I Glossary 89 Gene chip | See microarray. Meiosis | The type of cell division that creates egg and sperm cells. Gene expression | The process by which genes are first converted to messenger RNA and Microarray | Sometimes called a gene chip or then to proteins. a DNA chip. Microarrays consist of large num- bers of molecules (often, but not always, DNA) Genetics | The scientific study of genes and distributed in rows in a very small space. heredity— of how particular qualities or traits Microarrays permit scientists to study gene are transmitted from parents to offspring. expression by providing a snapshot of all the Genome | All of an organism’s genetic material. genes that are active in a cell at a particular time. Genomics | A “scaled-up” version of genetic MicroRNA | A short piece of single-stranded research in which scientists can look at large RNA that does not encode a protein and controls numbers or all of the genes in an organism at the expression of genes. the same time. Mitochondrion | The cell’s power plant, Haploid | Having one copy of each chromo- supplying the energy to carry out all of the cell’s some, as in a sperm or egg. jobs. Each cell contains up to 1,000 mitochon- Haplotype | A set of closely linked genes or dria. The structures contain their own small DNA polymorphisms inherited as a unit. genomes, called mitochondrial DNA. Histone | A type of protein found in chromo- Mutation | A change in a DNA sequence. somes; histones attached to DNA resemble Nucleotide | A building block of DNA or “beads on a string.” RNA. It includes one base, one phosphate mole- Homeobox | A DNA sequence found in genes cule, and one sugar molecule (deoxyribose in involved in the regulation of the development DNA, ribose in RNA). of animals, fungi, and plants. Nucleus | The structure in the eukaryotic cell Imprinting | The phenomenon in which a gene containing most of its genetic material. may be expressed differently in an offspring Pharmacogenetics | The study of how peo- depending on whether it was inherited from ple’s genetic make-up affects their responses the father or the mother. to medicines. Intron | A DNA sequence, or the RNA sequence transcribed from it, that interrupts the sequences coding for a gene product (exon). 90 National Institute of General Medical Sciences Protein | A molecule or complex of molecules RNA splicing | The process by which introns consisting of subunits called amino acids. are removed and exons are joined together Proteins are the cell’s main building materials from an RNA transcript to produce an mRNA and do most of a cell’s work. molecule. Recombinant DNA | Hybrid DNA produced Sequencing | Sometimes called DNA sequenc- in the laboratory by joining pieces of DNA from ing or gene sequencing. Discovering the exact different sources. order of the building blocks (see nucleotides) of a particular piece of DNA. Replication | The process by which DNA copies itself in order to make a new genome to Systems biology | A field that seeks to study pass on to a daughter cell. the relationships and interactions between vari- ous parts of a biological system (metabolic Ribosome | The cell structure in which pro- pathways, organelles, cells, and organisms) and teins are manufactured. Most cells contain to integrate this information to understand how thousands of ribosomes. biological systems function. RNA | Abbreviation for ribonucleic acid, the Telomere | A repeated DNA sequence that caps molecule that carries out DNA’s instructions for the ends of chromosomes. making proteins. It consists of one long chain made up of nucleotides. Each nucleotide contains Transcription | The first major step in gene one base, one phosphate molecule, and the sugar expression, in which the information coded in molecule ribose. The bases in RNA nucleotides DNA is copied into a molecule of RNA. are adenine, uracil, guanine, and cytosine. Translation | The second major step in gene RNA interference (RNAi) | A gene-silencing expression, in which the instructions encoded in process in which double-stranded RNAs trigger RNA are carried out by making a protein or start- the destruction of specific RNAs. ing or stopping protein synthesis. RNA polymerase | An enzyme that transcribes Variant | A different version of a gene, one that a DNA sequence, creating mRNA. has a slightly different sequence of nucleotides. What Is NIGMS? Discrimination Prohibited Accessibility The National Institute of General Medical Sciences Under provisions of applicable public laws enacted This publication can be made available in (NIGMS) supports basic biomedical research on by Congress since 1964, no person in the United formats that are more accessible to people genes, proteins, and cells. 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