Genes and DNA

A BEGINNER'S GUIDE TO GENETICS

AND ITS APPLICATIONS

CHARLOTTE K. OMOTO AND PAUL F. LURQUIN

COLUMBIA UNIVERSITY PRESS

NEW YORK

Columbia University Press

Publishers Since 1893

New York Chichester, West Sussex

Copyright © 2004 Columbia University Press

All rights reserved

Library of Congress Cataloging-in-Publication Data Omoto, Charlotte K.

Genes and DNA : a beginner's guide to genetics and its applications / Charlotte K. Omoto and Paul F. Lurquin. p. cm.

Includes bibliographical references and index. ISBN 0-231-13012-0 (cloth : alk. paper) ISBN 0-231-13013-9 (pbk. : alk. paper)

1. Genetics. 2. Molecular genetics. I. Lurquin, Paul F. II. Title.

QH430.O47 2004 576.5—dc22

2003062584

Columbia University Press books are printed on permanent and durable acid-free paper.

Printed in the United States of America c 10 9 8 7 6 5 4 3 2 1

IN MEMORY OF OUR MOTHERS

Contents

Acknowledgments xiii

List of Contributors xv

Preface: Why Is Genetics Important? xvii

Chapter 1. What Are Genes? 1

DNA 1

DNA Can Be Specifically Stained and Observed in Cells 2

DNA Determines Genetic Properties in Bacteria 3

DNA Is a Double Helix 6

Transfer of Genetic Information to Progeny 9

DNA Can Be Replicated in the Test Tube 10

Summary 17

Try This at Home: Extract DNA from Vegetables in Your Kitchen 1 7

Box 1.1. PCR and Identification 15

Chapter 2. Inheritance of Single-Gene Traits 19

Plants Are Good Organisms for the Study of Inheritance 20

Genes Do Not Blend 21

Rules of Inheritance 22

Behavior of Chromosomes 23

The Punnett Square 24

Incomplete Dominance 28

Sex Is Also Determined by Inheritance Rules 29

Summary 33

Chapter 3. Mendelian Traits in Humans 34

Blood Types 34

Sex-Linked Traits: Hemophilia 35

Sex-Linked Traits: Color Blindness 3 7

Prostate and Breast Cancer 3 7

Genetic Metabolic Diseases 39

Sickle-cell Anemia 41

Hemochromatosis 42

Another Sex-Influenced Trait: Male Pattern Baldness 44

Dominant Genetic Diseases 44

Pedigree Analysis 45

Summary 50

Try This at Home: Pedigree Game 50

Box 3.1. Warning on Diet Products 40

Chapter 4. From Genes to Phenotype 53

Transcription 54

Translation 56

Changes in DNA Modify the Amino Acid Sequences of Proteins 61

Gene Regulation 63

Summary 67 Try This at Home: DNA Replication, Transcription, and Translation Game 68

Box 4.1 Why People Are Saving Their Babies' Cord Blood 65

Chapter 5. Using Bacteria as Protein Factories 70

Tools for Manufacturing Proteins 70

Using Restriction Enzymes and Plasmids to Clone a Gene 73

Producing Human Proteins in E. Coli 74

Medically Important Human Proteins Made in E. Coli 76

Summary 78

Chapter 6. Genetically Modified Plants

What Are Genetically Modified Organisms (GMOs)?

Agrobacterium-Mediated Gene Transfer 80

Biolistics 81

Genetic Modifications 86

Genetically Modified Nonfood Plants 88

Ecological Issues 90

Labeling Issues and Food Safety 91

Summary 92

Box 6.1: Detecting Foreign Genes in Genetically Modified Plants 85

Chapter 7. When Things Go Wrong 93

Errors in Chromosome Number 94

Multiple Sets of Chromosomes 94

Looking at Our Chromosomes 96

Changes in the DNA Base Sequence 97

Triplet Repeat Errors 102

Summary 103

Chapter 8. Mutagens, Teratogens, and Human Reproduction 104

Spontaneous Mutations 104

Mutagens 105

How Do We Detect Mutagens? 107

Teratogens 109

Human Reproduction 112

Summary 113 Box 8.1: Why There Were Few Thalidomide-Caused Birth Defects in the United States 110

Chapter 9. Linkage and Mapping: Gene Discovery 114

There Are Many Genes on Each Chromosome 114

Independent Assortment of Genes 115

Linkage 118

Recombination 119

Linkage to a DNA Marker 123

The Human Genome Project and Others 131

Discovering Disease Genes in Humans 132

Summary 133 Try This at Home: Independent Assortment of Chromosomes and the Making of a Unique Individual 1 33

Try This at Home: Explore Genetics Databases 1 35

Box 9.1: Identifying Disease Genes Using Restriction

Fragment Length Polymorphism 126 Box 9.2: Identifying a Disease-Resistance Gene in Barley

Through Map-Based Cloning 128

Chapter 10. Genetics of Populations and Genetic Testing 138

Why Don't We Observe 3 to 1 Ratios of Dominant Versus

Recessive Traits in Populations? 138 Predicting the Genotype of the Next Generation Using the Punnett Square 139 Conditions for Observing Constant Gene and

Genotype Frequencies 142

Another Application of the Hardy-Weinberg Law 143

Predicting Gene Frequency for a Recessive Trait 145

Gene Frequencies Vary in Different Populations 147

Newborn Testing and Conditional Probability 148

Predicting Genotype Frequency for Sex-Linked Traits 151

Summary 152

Chapter 11. Survival of the Fittest? 153

What Is Meant by Fitness? 153

Selection Requires Variation. 154

Selection Can Result in Reduced Genetic Diversity 155

Natural Selection Determined Skin Color in Humans 156

Fitness Depends Upon the Environment 157

Selection and Antibiotic-Resistant Bacteria 158

Heterozygous Advantage 161

Why Do Dominant Genetic Diseases Exist? 1 63

Small Populations 164

Summary 170 Box 11.1: DNA Sequences Provide Clues to Human Evolution:

The Founder Effect in Prehistoric Africa 165 Try This at Home: Demonstrations of the Effects of Small Population Size 171

Chapter 12. Nature Versus Nurture 175

Polygenic Traits Are Additive 176

Polygenic Traits Exhibit Continuous Variation in Phenotype 178

Polygenic Traits Are Influenced by the Environment 180

Measuring Variance in Traits and Estimating Heritability 181 Twin Studies Are Helpful in Studying Polygenic Traits in Humans 183

Quantitative Traits in Medicine and Agriculture 1 84

Summary 186

Chapter 13. Genetically Modified Animals and the Applications of Gene Technology for Humans 187

Cloning Animals by the Nuclear-Transfer Technique 187

Genetically Modifying Animals Using Embryonic Stem Cells 1 88

Uses of Genetically Modified Animals 1 90

Human Gene Therapy 192

Human Reproductive Cloning 195

Human Therapeutic Cloning 197

Summary 197

Appendix A. Internet Resources 199

Appendix B. Glossary of Scientific Names of Organisms 203

Appendix C. Glossary of Human Genetic Diseases 205

Appendix D. Glossary of Terms 207

Index 213

Acknowledgments

WE APPRECIATE THE HELP of our colleagues Kirstin Malm, Valerie Lynch-Holm, and Norah McCabe in making the images for some of the figures. We appreciate the suggestion from Noreen Warren for testing corn products for genetic modification and for providing more information about Thalidomide. We thank Andris Kleinhofs for help with and images used in box 9.2. Mark Michelsen improved many of the figures to make them suitable for publication. We gratefully thank our colleagues Paul Verrell and Martin Morgan for reading and critiquing a draft of our manuscript. Thanks also to our students who did the same thing in addition to, of course, taking exams on the subject material of this book. Thanks also to three anonymous reviewers whose contributions enhanced our manuscript. Robin Smith, senior editor for the sciences at Columbia University Press, encouraged us at every step of the way. Last but not least, we appreciate the partial financial support from the Honors College and the College of Sciences of Washington State University.

Contributors

DAVID HANSEN

Minnesota Science, University of Minnesota ANDRIS KLEINHOFS

Department of Crop and Soil Sciences, Washington State University

VALERIE LYNCH-HOLM

EM Center, Washington State University

KIRSTIN MALM

School of Molecular Biosciences, Washington State University NORAH MCCABE

School of Molecular Biosciences, Washington State University

N OREEN WARREN Madison Area Technical College

Preface: Why Is Genetics Important?

PERHAPS YOU'VE HEARD THE MEDIA use the terms "cloning," "genetically modified organisms," "DNA fingerprinting," and "genetic testing." But have you ever discovered what these terms really mean? Our goal is to help you to become more familiar with words like these. We will provide you with clear and straightforward genetic principles that are relevant to your everyday life and help you understand the many applications of modern genetic techniques.

Genetics is one of the greatest adventures in science. This book will help you explore everything from the foundations of genetics, a little over a century ago, to modern genetic applications, including the genetic engineering of plant products that you probably eat on a regular basis. You will learn about medical, legal, and ethical aspects of genetics, as well as the impact of genetics on our society; this impact is mind-boggling. For example, as little as fifteen years ago, it was unthinkable that deoxyribonucleic acid (DNA) would play any role in the prosecution and conviction of criminals. The use of DNA testing is now routine, and many people suggest that DNA testing should be made available for cases that came to trial before this technology was available. Likewise, paternity suits often ended up in mistrials because incontrovertible evidence could not be obtained when only simple blood tests were available. But now no one would consider paternity suits without DNA evidence.

We are discovering many genetic diseases and finding out more about them. The first catalog of human genetic diseases listed fewer than 1,500. Now this catalog is updated almost daily as an Internet resource, part of Online Mendelian Inheritance in Man, with almost 8,000 genetic diseases as of this writing. Every state in the United States, and most of the developed world, now tests newborn babies for genetic diseases, and the number of diseases that are tested for is increasing.

A few years ago, genetically modified corn did not exist. Now, you might eat it for breakfast. As you may know, the existence of genetically modified crops on the shelves of our supermarkets has contributed to street riots in Seattle and New York City. The few examples listed above already demonstrate that the science of genetics is important for society. It is easy to form an opinion about modern genetics without really knowing all that much about it. But we need to be well informed to make good decisions about these important issues. Our ultimate purpose in writing this book is to help you make informed decisions about genetics.

To achieve this goal, you will learn how genes determine the characteristics of all life forms and how these genes are passed from parents to progeny. Next you will find out how genetic analysis was able to proceed before we even knew that DNA exists. You will also examine different types of changes in our genetic material and how these changes affect how we look, act, and feel. You will look at the impact of our environment on our genetic material. Then you will learn how our understanding properties of DNA has led to genetic engineering and its medical applications. You will be introduced to the genetics of populations, in which it is no longer single individuals, but rather large collections of individuals, that are studied. This branch of genetics is the key to the study of species conservation. Population genetics also leads directly to the concepts upon which evolution by natural selection is based. Finally, you will discover the role of genetics and the environment in determining traits. This area of genetics is important in agriculture as well as medicine.

We hope you enjoy learning about genetics with our book. But first and foremost we hope that after reading it, you will understand genetics and will be able to confidently make decisions regarding genetics and genetic technology that affect your life.

Genes and DNA

CHAPTER 1

What Are Genes?

YOU MAY ALREADY KNOW THAT GENES are made of DNA (short for deoxyribonucleic acid). More interesting than knowing this is understanding how we know that DNA is the basis for heredity and understanding the importance of the structure of DNA for inheritance. You will see in this chapter that DNA and its structure are the keys to understanding inheritance.

DNA has a fascinating history. The Swiss scientist Friedrich Miescher discovered DNA near the end of the nineteenth century. Miescher never knew that the substance he had isolated from sperm and pus (yes, pus!) would turn out to be so critical to the understanding of life. He died several decades before the function of DNA and its famous double-helical structure were uncovered. After Miescher, other scientists tried to identify the chemical composition of sperm, reasoning that sperm must carry the genetic material to the next generation. These scientists also reasoned that sperm cells have very little excess cellular material other than the hereditary material found in the sperm head. In fact, DNA constitutes over 60 percent of the sperm head; the remainder is mostly protein.

For a long time after Miescher's discovery, DNA was thought to be a simple molecule, consisting of nucleotides strung together like beads on a string. Each nucleotide is composed of a sugar (deoxyri-bose) chemically linked to phosphorus atoms and one of four different nitrogenous bases (so called because they contain a significant number of nitrogen atoms). The nitrogenous bases are adenine, guanine, cytosine, and thymine. These four bases are abbreviated as A, G, C, and T. Nothing known about the DNA molecule suggested that it could play any role in heredity. The structure of DNA seemed much too simple to account for the many already known hereditary traits. But then scientists found that the building blocks of DNA—the nucleotides—were repeated hundreds of times in the DNA molecule. As techniques to isolate DNA from living cells improved, the number of nucleotides in a DNA molecule was found to be in the thousands, and then in the hundreds of thousands. Scientists had discovered that DNA is a polymer, much like many plastics such as polyethylene and polypropylene, except that DNA is a very long polymer with millions of nucleotides, As, Gs, Cs, and Ts.

Yet nobody knew what living cells did with this polymer, nor did anybody know the structure of the DNA molecule. In fact, some scientists believed that only animals and bacterial cells possessed DNA and that plants were devoid of it. Since plants, as well as animals and bacteria, all had well-defined genetic characteristics (for example, flower color for plants, shape for animals, and pathogenicity for bacteria), DNA could not be the genetic material, so the logic went. We now know that plants do contain DNA, and that the failure to isolate it from them was due to the use of crude techniques. In fact, for geneticists, plants, animals, and bacteria are largely similar in spite of their great diversity. This is because their hereditary properties are all based on the existence of one substance: DNA.

DNA Can Be Specifically Stained and Observed in Cells

An important step in the development of ideas about the chemical nature of the genetic material was the ability to stain DNA. In the 1920s, German biochemist Robert Feulgen developed a way to specifically stain DNA. He then used this method to stain DNA in living tissue. The Feulgen reaction, as it is now called, specifically colors DNA purple. The stained cells can then be viewed under the microscope. Feulgen used this technique on all kinds of tissues from animals, plants, and protozoa. Under the microscope, the purple DNA stain was found in a central compartment of all these cells. The compartment is given the name nucleus, plural nuclei. Feulgen found that the nuclei of all of these cells, including the nuclei of plant cells, became stained. This definitively proved that plants had DNA and that the DNA of cells is located in the nucleus.

With the Feulgen stain, scientists had a tool to measure the amount of DNA present in cells. In 1950, in a paper entitled "Constancy of DNA in Plant Nuclei," Hewson Swift at the University of Chicago showed that all cells from different parts of a corn plant had a constant amount of DNA. Furthermore, the amount of DNA in pollen was half that found in, for example, the leaf and root cells. He found that rapidly dividing cells in the root tip and other cells prior to cell division had twice as much as DNA. These are what one would expect of the genetic material (see chapter 2). If DNA was the genetic material, its amount should be constant in all the cells of the organism regardless of the size of the cell.

An even more interesting observation was made using the Feulgen stain: DNA changes shape as cells divide. Most of the time, the Feul-gen stain showed an amorphous purple sphere in the nucleus, without any substructure. But just before cells divide, the DNA becomes condensed into sausage-looking structures called chromosomes. It was found that the number and shape of these condensed chromosomes was the same in different body cells of the same organism. Furthermore, one could see that in sperm cells the number of chromosomes was halved. We would expect that the amount of hereditary material in the gametes, sperm or pollen and egg or ova, would be half that found in the nonreproductive cells of the organism.

DNA Determines Genetic Properties in Bacteria

That DNA is indeed the genetic material was demonstrated in bacteria in 1944. A team led by the Canadian Oswald Avery at Rockefeller University in New York made this landmark discovery. Their biological material was the bacterium Streptococcus pneumoniae, which, as its name indicates, causes pneumonia. Avery's laboratory possessed two strains of these bacteria. One strain infects mice with pneumonia (the "virulent" strain), and the other strain does not (the "avirulent" strain). The two strains look different when growing in a petri dish: The virulent strain grows as a smooth, slimy, large collection of cells known as a colony. The avirulent strain produced small, well-defined, rough-looking colonies (figure 1.1). Thus the bacterial strains are distinguishable by two characteristics: physical appearance and the strain's ability or inability to cause pneumonia. These characteristics are hereditary. Avery knew this because he grew the two bacterial strains for a long time, during which the cells divided many times, but this produced no change in the bacteria's ability to cause pneumonia or the bacteria's physical appearance.

Avery did an experiment that suggested that hereditary properties like virulence and appearance could be exchanged between cells. He knew, as we do, that heating bacterial cells kills them. Indeed, when the smooth, virulent bacteria are heated, they are killed and no longer infect mice with pneumonia. But when one mixes these heat-killed virulent bacteria with live, avirulent rough bacteria, one finds that this mixture can kill mice. Avery reasoned that the genetic material from the heat-killed virulent bacteria changed the properties of the rough strain. In this case, he reasoned, DNA purified from the smooth strain would change the characteristics of the rough strain, but only if the DNA could get into the rough, avirulent bacteria. Avery's research team purified DNA from the smooth, slimy strain and added the purified DNA to the rough cells. They observed that a few smooth and slimy cells appeared in the culture of rough cells exposed to the DNA! This "transformation," as they called it, was stable over many cell divisions. Avery and his team interpreted these observations to mean that DNA was the genetic material.

However, to make sure that the DNA did not contain contaminants that could have been the true genetic material, they did further experiments with a variety of enzymes. Suffice it to say that enzymes are proteins that catalyze all sorts of reactions. Biologists commonly give names to these enzymes by putting the suffix "-ase" after the name of the substances they act upon. For example, deoxyribonuclease is an enzyme that destroys DNA by cutting it into its nucleotide building blocks. Similarly, a protease is an enzyme that destroys proteins. When Avery and his coworkers added deoxyribonuclease to their purified DNA and then added this mixture to rough cells, no smooth, slimy cells were recovered. This meant that destroying DNA destroyed the transforming activity. On the other hand, adding proteases to the DNA had no effect on its transforming activity. If contaminating proteins in their DNA samples had been responsible for the transforming activity,

Figure 1.1 Virulent and Avirulent Streptococcus Pneumoniae Colonies on an Agar Plate. The large, slimy, smooth colonies on the left are virulent, and the small, distinct colonies on the right are avirulent. Plate and photograph courtesy of Kirstin Malm.

the addition of proteases should have destroyed this activity. It did not. This means that proteins were not responsible for the observed transformation. There it was: the genetic material—the genes—of Streptococcus pneumoniae was not made of proteins, but of DNA.

Avery's work had been done under carefully controlled conditions, and his conclusions were straightforward. Yet nobody at the time believed him! Why was that so? It turns out that as recently as the late 1940s, scientists were convinced that protein, not DNA, was the genetic material. Why? Proteins seemed like a much more likely candidate to be the genetic material. Proteins existed in innumerable varieties that differed enormously among living species. For this reason, proteins were thought to constitute the true genetic material. Another factor is that in 1944 World War II was still raging, and Avery's discovery must have been seen as of little consequence when people were dying on the battlefields and in bombed-out cities. Finally, some people thought that DNA was possibly the genetic material of some rare bacterial species, but certainly it could not be responsible for the hereditary properties of higher life forms, such as animals and plants. Of course, the skeptics were dead wrong, as we know. Such an important discovery should have earned its authors a Nobel Prize. However, Avery was sixty-seven at the time he made this important discovery, and he died eleven years later. Recognition of important discoveries often takes decades. Since the Nobel Prize is not granted posthumously, Avery was unable to be recognized for his important work.

We know today that DNA is an almost universal genetic material, and that genes present in simple viruses, bacteria, plants, and animals are all made of DNA. Amazingly, some viruses are made of a chemical very similar to DNA, ribonucleic acid (RNA), where the base thymine (T) is replaced by uracil (U) and where the sugar is ribose, not deoxyribose.

DNA Is a Double Helix

By the late 1940s biochemists knew that DNA was a very long polymer made up of millions of nucleotides. Each nucleotide contains one of the four nitrogenous bases (A, T, G, or C) linked to a deoxyribose unit, in turn linked to a chemical group containing a phosphorus atom. DNA is held together by bonds between the phosphate and the deoxyribose units. Therefore, one speaks of the DNA's "sugar-phosphate backbone" (figure 1.2.A). In those years, it was also known that in all DNA samples isolated from widely different species (human, yeast, and bacteria, for example), the amount of adenine (A) was always equal to the amount of thymine (T). Similarly, guanine (G) was always equal to cytidine (C). Nobody knew how to explain this, but the observation suggested some regularity in the DNA molecule.

A breakthrough occurred when Rosalind Franklin, a researcher at King's College in London, England, succeeded in crystallizing DNA in the early 1950s. Crystals are formed when identical molecules are packed in a very organized fashion. This is rather simple to do for a small molecule. Perhaps you have made sugar crystals by putting a string into a solution saturated with sugar. This happens because the rough structure of the string initiates crystal formation. Once some sugar molecules attach to the string, other sugar molecules can fit in like bricks in a wall. Because DNA is such a large molecule, it does not form crystals readily. Why was it so important to obtain DNA crystals? One can take advantage of the very regular arrangement of the same molecules in a crystal to determine their structure. There existed at the time a well-established technique used to determine the arrangement of atoms inside a crystal. This technique is called X-ray crystallography,

Figure 1.2 Diagrams of DNA. A. A flat diagram shows two strands of DNA each with four bases. Note that each strand is held together by a "sugar-phosphate backbone." The two strands run in opposite directions, thus the left-hand strand is shown upside down. The strands are held together by weak bonds, called hydrogen bonds, between the bases. B. A diagram of the double-helix structure ofDNA. Note the sugar-phosphate backbone in opposite directions shown by the arrows. The rungs of the ladder represent the bases held together by the weak hydrogen bonds. C. A detailed diagram of two bases from two opposing strands of DNA. The phosphates are shown as shaded circles and sugars as shaded pentagons. The dotted lines connecting the bases are weak hydrogen bonds.

Figure 1.2 Diagrams of DNA. A. A flat diagram shows two strands of DNA each with four bases. Note that each strand is held together by a "sugar-phosphate backbone." The two strands run in opposite directions, thus the left-hand strand is shown upside down. The strands are held together by weak bonds, called hydrogen bonds, between the bases. B. A diagram of the double-helix structure ofDNA. Note the sugar-phosphate backbone in opposite directions shown by the arrows. The rungs of the ladder represent the bases held together by the weak hydrogen bonds. C. A detailed diagram of two bases from two opposing strands of DNA. The phosphates are shown as shaded circles and sugars as shaded pentagons. The dotted lines connecting the bases are weak hydrogen bonds.

and, as its name indicates, it consists of illuminating a target crystal with X rays. The regular arrangement of atoms in a crystal deflects X rays and forms spots in concentric rings on a photographic film. The more organized the structure, the more spots are formed farther out in the ring. By noting the location and intensity of these spots, one can then determine the relative positions of the atoms in the crystal and determine the three-dimensional structure of the crystallized molecules. Thus, Rosalind Franklin obtained the first high-quality X-ray data for a DNA crystal (figure 1.3).

At this time, James Watson, a young American postdoctoral scientist, and Francis Crick, an English physicist working on his Ph.D. dissertation, were both at Cambridge University. These two struck up a collaboration to solve the problem of the structure of DNA. Neither of them had done any previous work with DNA. They were thus novices, although Crick knew the theory of X-ray crystallography very well. Indeed, Watson and Crick never did a single experiment to solve the structure of DNA. All the experimental results had been obtained by Rosalind Franklin and later repeated by her boss, Maurice Wilkins. One day, while he was visiting the King's College researchers, Watson saw a photograph of DNA made by Franklin. The arrangement of the spots radiating out in an X shape immediately suggested to him that DNA must be a helical molecule. Back in Cambridge, Watson convinced Crick of that interpretation, and model building started. After a few days of trial and error, they had a helical molecule that was also consistent with Franklin's X-ray crystallography data. DNA was a double helix in which the sugar-phosphate backbones are on the outside, while the bases are on the inside of the molecule (figure 1.2.B). This structure was held together by weak bonds between an A and a T, indicated by the dotted lines in the figure, and similarly G was held to a C. This pairing of A with T and G with C is called "complementary base pairing." This explained why the number of As was always equal to the number of Ts, and Gs always equal to Cs. The discovery of the double-helical structure of DNA was published in a short report in 1953, and Watson, Crick, and Wilkins received the Nobel Prize in 1962.

A very sad aspect to this story is that Rosalind Franklin did not receive recognition at the time for her major contribution. She died in 1958 at the young age of thirty-seven of ovarian cancer, unable to share the honors unquestionably due to her too.

Figure 1.3 Photograph of the X-ray diffraction pattern ofDNA. Produced by Rosalind Franklin. Transfer of Genetic Information to Progeny

In their short article, Watson and Crick announced cryptically that "it has not escaped our notice that the specific [base] pairing we have postulated immediately suggests a possible copying mechanism for the genetic material." What did they mean by that? First, since DNA is the genetic material, it contains all the instructions necessary to "tell" a living cell what it is supposed to do. Next, as cells divide, as for example in the case of bacteria or human cells, the progeny of a cell must contain the same genetic instructions as the original cell. Thus, there must exist a mechanism that copies DNA faithfully, to ensure that progeny cells contain the same genetic material as the maternal cell. When Watson and Crick envisioned a copying mechanism for DNA, none of the details of this mechanism was known. However, simply by looking at the double-helical structure of DNA, they understood the basics of the replication process. To understand how they arrived at this interpretation, we must go back to the complementary base pairing that holds the DNA double helix together: an A always faces a T and a G always faces a C.

If DNA is the genetic material, cells must contain the necessary machinery (like enzymes) that can "read" the base sequence in DNA. With the structure of DNA depicted in figure 1.2, we realize that if the two DNA strands become separated, each strand has the information to specify the order of A, T, G, and C in the other strand. What is the result of this "reading" process? Figure 1.2.C shows that because of the A-T and G-C complementary base-pair arrangement, whenever an A is read in one of the strands of the original DNA molecule, the cellular machinery must incorporate a T in the newly growing opposite strand. But, since the A in the original molecule faced a T, the T in that strand must be read in such a way that the cellular machinery incorporates an A in the other growing strand. The result is that an original A-T pair is now copied into two A-T pairs, each new one present in the two replicated DNA molecules. This copying mechanism occurs at the level of each individual base pair, ensuring that the two resultant double-helical molecules are identical to the original DNA double helix. We say that the two DNA strands are used as templates for the synthesis of two complementary, new DNA strands.

We now know that this is how DNA replicates. The machinery that performs DNA replication is very complex and involves dozens of proteins. One key enzyme in the process of DNA replication is called DNA polymerase. This enzyme "reads" the bases present in the template strands and incorporates the complementary bases into the growing new strands. DNA replication is extremely accurate, but it is not absolutely perfect. Mistakes made by DNA polymerase result in the incorporation of a "wrong" base (like putting in a G opposite an A instead of a T), and these errors are one of the causes of spontaneous mutations, the ultimate source of genetic variation.

DNA Can Be Replicated in the Test Tube

Geneticists now have a good understanding of the many ingredients that are necessary for DNA replication. We need to have the enzyme DNA polymerase to do the job, as well as the building blocks of DNA, nucleotides. In the natural process of DNA replication, the two strands of DNA are separated and the enzyme DNA polymerase binds to short double-stranded stretches positioned next to the single-stranded DNA to be copied.

Today, it is possible to make significant amounts of DNA in the test tube, using a method that partially imitates the mechanism used by living cells. This method is called the polymerase chain reaction (PCR) and was invented in 1986 by Kary Mullis. Mullis won the Nobel Prize in 1993 and candidly confesses that he came up with the idea on a surfing trip, while high on drugs.

We have seen that each strand of a double-stranded DNA is used as template for DNA replication. DNA normally is held together as a double-stranded molecule by weak bonds between the complementary bases (figure 1.2). It turns out that we can separate the two strands of double-stranded DNA simply by heating up a DNA solution close to the boiling point. At high temperature, the weak bonds that link the two strands together are broken, and the two strands of DNA separate. If the solution is cooled, the double-stranded DNA can reform. As in the cell, DNA polymerase in the test tube needs to have short stretches of double-stranded structure to copy the single stranded regions. By 1986, chemists could make short pieces of single-stranded DNA of a predetermined nucleotide sequence. If we know the base sequence of a piece of DNA, it is possible to synthesize a short piece of DNA whose base sequence is complementary. When the solution is cooled, short strands of DNA can more easily find their complementary strand than long strands. To reform the original long double-stranded DNA, the solution must be cooled slowly. Thus if the solution is fast-cooled, the process of short pieces making a partial double-stranded structure wins out over the long, complementary DNA sequences coming together (figure 1.4.C).

The short pieces of DNA that form the short double-stranded regions are known as "primers." In order to copy both strands of the double-stranded DNA, each strand must have a primer. The relative position of the complementary regions to these primers determines the size of the DNA piece that will be made. As we just saw, these primers can be made in the lab, and they will form weak bonds with single-stranded DNA with a complementary sequence. Therefore, by adding DNA polymerase together with the building blocks of DNA (nucleotides, A, T, G, and C), one can copy DNA in the test tube (figure 1.4). The problem, though, is that one can copy DNA only once

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