《大学英语读写》课程教学资源（学习资料）生物工程专业英语（合肥学院）Specialized English in Biotechnology

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1 Contents Lesson 1 What is Biotechnology? ................................................................................................ 2 Lesson 2 Where Did Biotechnology Begin? .................................................................................. 3 Lesson 3 Brief History of Biotechnology...................................................................................... 5 Lesson 4 Dogma, DNA, and Enzymes.......................................................................................... 7 Lesson 5 Polymerase Chain Reaction - Xeroxing DNA .................................................................. 9 Lesson 6 Monoclonal Antibody Technology.................................................................................11 Lesson 7 The Human Genome Project.........................................................................................13 Lesson 8 Whose Genome is It, Anyway? .....................................................................................16 Lesson 9 Agriculture - An Overview...........................................................................................18 Lesson 10 Gene Gun Speeds Search for New Orchid Colors ..........................................................21 Lesson 11 Transforming Plants...................................................................................................23 Lesson 12 Animals and Animal Health ........................................................................................26 Lesson 13 Biomining ................................................................................................................28 Lesson 14 Biofuel.....................................................................................................................29 Lesson 15 New Foods and Food Producers..................................................................................31 Lesson 16 Blazing a Genetic Trail in Medicine.............................................................................34 Reading materials.....................................................................................................................36

Lesson 1 What is Biotechnology? Biotechnology in one formor another has flourished since prehistoric times When the first human beings realized that they could plant their own crops and breed their own animals.they learned to use biotechnology.The discovery that fruit juices fermented into wine.or that milk could be converted into cheeseor yogurt,or that beer could be made by fermenting solutions of malt and hops began the study of biotechnology.When the first bakers found that they coud make a of spongy bread rather than a firm. thin cracker,they were acting as fledgling biotechnologists.The first animal breeders.realizing that different physical traits could be either magnified or lost by mating appropriate pairs of animals,engaged in the manipulations of biotechnology What then is biotechnology?The term brings to mind many different things.Some think of developing new typesof animls.Others dream of almost unlimited sources of human therapeutic drugs. Still othersisn the possibility of growing rops thatemore utrtiousand naturally pest-resistant tofeed a rapidly growing world population.many first-thoughtespon as there are people to whom the question can be posed In its purest form,the term"biotechnology"refers to the use of living organisms or their products to modify human health and the human environment.Prehistoric biotechnologists did this as they used yeast cells toraise bread dough and to beverages,and bacterial cel tomake and yogurts and as they bred their strong.productive animals to make even stronger and more productive offspring Throughout human history,we have learned a great deal about the different organisms that our ancestors used so effectively.The marked increase in our understanding of these organisms and theircel products gains us the ability to control the many functions of various cells and organisms.Using the techniques of gene splicing and recombinant DNA technology,we can now actually combine the genetic elements of two or more living cells.Functioning lengths of DNA can be taken from one organism and placed into the cells of another organism.Asaresu for example,we can cause bacterial cellsto produce human molecules.Cows can produce more milk for the same amount of feed.And we can synthesize therapeutic molecules that have never before existed. 2

2 Lesson 1 What is Biotechnology? Biotechnology in one form or another has flourished since prehistoric times. When the first human beings realized that they could plant their own crops and breed their own animals, they learned to use biotechnology. The discovery that fruit juices fermented into wine, or that milk could be converted into cheese or yogurt, or that beer could be made by fermenting solutions of malt and hops began the study of biotechnology. When the first bakers found that they could make a soft, spongy bread rather than a firm, thin cracker, they were acting as fledgling biotechnologists. The first animal breeders, realizing that different physical traits could be either magnified or lost by mating appropriate pairs of animals, engaged in the manipulations of biotechnology. What then is biotechnology? The term brings to mind many different things. Some think of developing new types of animals. Others dream of almost unlimited sources of human therapeutic drugs. Still others envision the possibility of growing crops that are more nutritious and naturally pest-resistant to feed a rapidly growing world population. This question elicits almost as many first-thought responses as there are people to whom the question can be posed. In its purest form, the term "biotechnology" refers to the use of living organisms or their products to modify human health and the human environment. Prehistoric biotechnologists did this as they used yeast cells to raise bread dough and to ferment alcoholic beverages, and bacterial cells to make cheeses and yogurts and as they bred their strong, productive animals to make even stronger and more productive offspring. Throughout human history, we have learned a great deal about the different organisms that our ancestors used so effectively. The marked increase in our understanding of these organisms and their cell products gains us the ability to control the many functions of various cells and organisms. Using the techniques of gene splicing and recombinant DNA technology, we can now actually combine the genetic elements of two or more living cells. Functioning lengths of DNA can be taken from one organism and placed into the cells of another organism. As a result, for example, we can cause bacterial cells to produce human molecules. Cows can produce more milk for the same amount of feed. And we can synthesize therapeutic molecules that have never before existed

Lesson 2 Where Did Biotechnology Begin? With the Basics Certain practices that we would now elassify as applications of biotechnology have been in use since man's earliest days Nearly 10.000 years ago,our ancestors were producing wine,beer,and bread by using fermentation,a natural process in which the biological activity of one-celed organisms plays a critical role. In fermentation,microorganisms such as bacteria,yeasts,and moldse mixed with ingredients that provide them with food.As they digest this food,the organisms produce two eritical by-produets,carbon dioxide In beer making yeast cells break down starch and sugar (present in cereal grains)to form alcohol, the frothor head,of the beer results from the carbon dioxide gas that the cells produce.In simple terms. the living cells rearrange chemical elements toform new products that they need to live and reproduce. By happy coincidence,in the process of doing so.they help make a popular beverage. Bread baking is also dependent on the action of yeast cells.The bread dough contains nutrients that these cells digest for their own sustenance.The digestion process generates alcohol (which contributes to that wonderful aroma of baking bread)and carbon dioxide gas(which makes the dough rise and forms the honeycomb texture of the baked loaf). Discovery of the fementation allowed early peoples to produce foods by allowing live organisms toact on other ingredients.But our found that,by manipulating the conditions under which the femmentation took pace,they could improve both the quality and the yield of the ingredients themselves Crop Improvement Although plant is a relatively moder discipline,its fundamental techniques have been applied throughout human history.When early man went through the crucial ransition from nomadic hunter to settled farmer,cultivated crops became vital for survival.These primitive farmers although ignorant of the natural principles at work,found that they could increase the yield and improve the taste of crops by selecting seeds from particularly desirable plants 3

3 Lesson 2 Where Did Biotechnology Begin? With the Basics Certain practices that we would now classify as applications of biotechnology have been in use since man's earliest days. Nearly 10,000 years ago, our ancestors were producing wine, beer, and bread by using fermentation, a natural process in which the biological activity of one-celled organisms plays a critical role. In fermentation, microorganisms such as bacteria, yeasts, and molds are mixed with ingredients that provide them with food. As they digest this food, the organisms produce two critical by-products, carbon dioxide gas and alcohol. In beer making, yeast cells break down starch and sugar (present in cereal grains) to form alcohol; the froth, or head, of the beer results from the carbon dioxide gas that the cells produce. In simple terms, the living cells rearrange chemical elements to form new products that they need to live and reproduce. By happy coincidence, in the process of doing so, they help make a popular beverage. Bread baking is also dependent on the action of yeast cells. The bread dough contains nutrients that these cells digest for their own sustenance. The digestion process generates alcohol (which contributes to that wonderful aroma of baking bread) and carbon dioxide gas (which makes the dough rise and forms the honeycomb texture of the baked loaf). Discovery of the fermentation process allowed early peoples to produce foods by allowing live organisms to act on other ingredients. But our ancestors also found that, by manipulating the conditions under which the fermentation took place, they could improve both the quality and the yield of the ingredients themselves. Crop Improvement Although plant science is a relatively modern discipline, its fundamental techniques have been applied throughout human history. When early man went through the crucial transition from nomadic hunter to settled farmer, cultivated crops became vital for survival. These primitive farmers, although ignorant of the natural principles at work, found that they could increase the yield and improve the taste of crops by selecting seeds from particularly desirable plants

Farmers long ago noted that they coud improve each year's harvest by using seed from only the best plants of the current crop Plants that,for example,gave the highest yield,stayed the healthiest during periods of drought or disease.or were easiest to harvest tended to produce future generations with these same characteristics.Through several years of careful seed selection.farmers could maintain and strengthen such desirable traits. The possibilities for improving plants expanded as a result of Gregor Mendel's investigations in the mid-1860s of hereditary traits in peas.Once the genetic basis of heredity was understood,the benefitsof cross-breeding.or hybridization,became apparent:plants with different desirable traits could be used to cultivate a later generation that combined these characteristics. An understanding of the scientific principles behind fermentation and crop improvement practices has come only in the last hundred years.But the early,crude techniques,even without the benefit of sophisticated laboratories and automated equipment,were a true practice of bitechnology guiding natural processes to improve man's physical and economic well-being Harnessing Microbes for Health Every student of chemistry knows the shape of a Buchner funnel,but they may be unaware that the distinguished German ientist it was named after made the vital discovery (in 1897)that enzymes extracted from yeast are effective in converting sugar into alcohol.Major outbreaks of disease in overerowded industrial cities led eventually to the introduction,in the early years of the present century, of large-scale sewage purification systems based on microbial activity.By this time it had proved possible to generate certain key industrial chemicals(glycerol,acetone,and butanol)using bacteria. Another major beneficial legacy of early 20th century biotechnology was the discovery by Alexander Fleming (in 1928)of penicillin,an antibiotic derived from the mold Penicillium.Large-scale production of penicillin was achieved in the 1940s.However,the revolution in understanding the chemical basis of cell function that stemmed from the post-war emergence of molecular biology was still to come.It was this exciting phase of bioscience that led to the recent explosive development of biotechnology. 4

4 Farmers long ago noted that they could improve each succeeding year's harvest by using seed from only the best plants of the current crop. Plants that, for example, gave the highest yield, stayed the healthiest during periods of drought or disease, or were easiest to harvest tended to produce future generations with these same characteristics. Through several years of careful seed selection, farmers could maintain and strengthen such desirable traits. The possibilities for improving plants expanded as a result of Gregor Mendel's investigations in the mid-1860s of hereditary traits in peas. Once the genetic basis of heredity was understood, the benefits of cross-breeding, or hybridization, became apparent: plants with different desirable traits could be used to cultivate a later generation that combined these characteristics. An understanding of the scientific principles behind fermentation and crop improvement practices has come only in the last hundred years. But the early, crude techniques, even without the benefit of sophisticated laboratories and automated equipment, were a true practice of biotechnology guiding natural processes to improve man's physical and economic well-being. Harnessing Microbes for Health Every student of chemistry knows the shape of a Buchner funnel, but they may be unaware that the distinguished German scientist it was named after made the vital discovery (in 1897) that enzymes extracted from yeast are effective in converting sugar into alcohol. Major outbreaks of disease in overcrowded industrial cities led eventually to the introduction, in the early years of the present century, of large-scale sewage purification systems based on microbial activity. By this time it had proved possible to generate certain key industrial chemicals (glycerol, acetone, and butanol) using bacteria. Another major beneficial legacy of early 20th century biotechnology was the discovery by Alexander Fleming (in 1928) of penicillin, an antibiotic derived from the mold Penicillium. Large-scale production of penicillin was achieved in the 1940s. However, the revolution in understanding the chemical basis of cell function that stemmed from the post-war emergence of molecular biology was still to come. It was this exciting phase of bioscience that led to the recent explosive development of biotechnology

Lesson 3 Brief History of Biotechnology world of"engineered"products that are based in the natural world rather than on chemical and industrial processes. Biotechnology has been described as"Janus-faced"This implies that there are two sides.On one. techniques allow DNA to be manipulated to move genes from one organism to another.On the other,it involves relatively new technologies whose consequences are untested and should be met with caution. The termbiotchnologywas coind inby Karl Ereky,an Hungarian engineer.At that time,the term meant all the lines of work by which products are produced from raw materials with the aid of living organisms.Ereky envisioned a biochemical age similar to the stone and iron ages. A common misconception among teachers is the thought that biotechnology includes only DNA and genetic engineering To keep students abreast of current knowledge,teachers sometimes have emphasized the techniques of DNA science as the "end-and-all"of biotechnology.This trend has also led to a misunderstanding in the general population.Biotechnology is NOT new.Man has been manipulating living things to solve problems and improve his way of life for millennia.Early agriculture concentrated on producing food.Plants and animals were selectively bred,and microorganisms were used to make food items such as beverages,cheese,and bread The late eighteenth century and the beginning of the nineteenth century saw the advent of vaccinations crop rotation involving leguminous crops and animal drawn machinery.The end of the nineteenth century was a milestone of biology.Microorganisms were discovered.Mendel's workon genetics was accomplished,and institutes for investigating fermentation and other microbial processes were established by Koch,Pasteur,and Lister. Biotechnology at the beginning of the twentieth century began to bring industry and agriculture together.During World War I,fermentation processes were developed that produced acetone from starch and paint solvents for the rapidly growing automobile industry.Work in the 1930s was geared toward using surplus agricultural products to supply industry instead of importsor petrochemicals.The advent of World War II brought the manufacture of penicillin.The biotechnical focus moved to 5

5 Lesson 3 Brief History of Biotechnology Biotechnology seems to be leading a sudden new biological revolution. It has brought us to the brink of a world of "engineered" products that are based in the natural world rather than on chemical and industrial processes. Biotechnology has been described as "Janus-faced." This implies that there are two sides. On one, techniques allow DNA to be manipulated to move genes from one organism to another. On the other, it involves relatively new technologies whose consequences are untested and should be met with caution. The term "biotechnology" was coined in 1919 by Karl Ereky, an Hungarian engineer. At that time, the term meant all the lines of work by which products are produced from raw materials with the aid of living organisms. Ereky envisioned a biochemical age similar to the stone and iron ages. A common misconception among teachers is the thought that biotechnology includes only DNA and genetic engineering. To keep students abreast of current knowledge, teachers sometimes have emphasized the techniques of DNA science as the "end-and-all" of biotechnology. This trend has also led to a misunderstanding in the general population. Biotechnology is NOT new. Man has been manipulating living things to solve problems and improve his way of life for millennia. Early agriculture concentrated on producing food. Plants and animals were selectively bred, and microorganisms were used to make food items such as beverages, cheese, and bread. The late eighteenth century and the beginning of the nineteenth century saw the advent of vaccinations, crop rotation involving leguminous crops, and animal drawn machinery. The end of the nineteenth century was a milestone of biology. Microorganisms were discovered, Mendel's work on genetics was accomplished, and institutes for investigating fermentation and other microbial processes were established by Koch, Pasteur, and Lister. Biotechnology at the beginning of the twentieth century began to bring industry and agriculture together. During World War I, fermentation processes were developed that produced acetone from starch and paint solvents for the rapidly growing automobile industry. Work in the 1930s was geared toward using surplus agricultural products to supply industry instead of imports or petrochemicals. The advent of World War II brought the manufacture of penicillin. The biotechnical focus moved to pharmaceuticals

Thecold war"years were dominated by work with microorganisms in preparation for biological warfare, as well as antibiotics and fermentation proces Biotechnology is currently being used in many areas including agriculture,bioremediation,food processing.and energy production.DNA fingerprinting is becoming a common practice in forensics. Similar techniques were used recently to identify the bones of the last Car of Russia and several members of his family.Production of insulin and other medicines is accomplished through nof vectors that now carry the chosen gene.Immunoassays are used not only in medicine for drug level and pregnancy testing.but also by farmers to aid in detection of unsafe levels of pesticides,herbicides.and toxinson crops and in animal products provide rapid field tests for industrial chemicals in ground water,sediment,and soil.In agriculture genetic engineering is being used to produce plants that are resistant to insects,weeds,and plant diseases. A current agricutral very involves the tomat.Arecentarticle in the New Yorker magazine compared the discovery of the edible tomato that came about by early biotechnology with the new "Flavr-Sav tomato brought about through modern techniques.In the very near future.you will be given the opportunity to bite into the Flavr-Savr tomato,the first food created by the use of recombinant DNA technology ever to go on sae What will you think as you raise the tomato to your mouth?Will you hesitate?This moment may be for you as it was for Robert Gibbon Johnson in 1820 on the steps of the courthouse inSalem,New Jersey. Prior to this moment,the tomato was widely believed to be poisonous.As a large crowd watched, wo tomatoes and changed forever the human-tomato relationship.Since that time. man has sought to produce the supermarket tomato with that"ackyard favor Americans also want that tomato available year-round. New biotechnological techniques have permitted scientists to manipulate desired traits.Prior to the advancement of the methods ofrecombinant DNA,scientists were limited tothe techniques of their time cros-pollination,selective breeding.pesticides,and herbicides Today's biotechnology has its"oin chemistry,physics,and biology.The explosion in techniques has resulted in three major branches of biotechnology:geneticengineering.diagnostic techniquesand techniques 6

6 The "cold war" years were dominated by work with microorganisms in preparation for biological warfare, as well as antibiotics and fermentation processes. Biotechnology is currently being used in many areas including agriculture, bioremediation, food processing, and energy production. DNA fingerprinting is becoming a common practice in forensics. Similar techniques were used recently to identify the bones of the last Czar of Russia and several members of his family. Production of insulin and other medicines is accomplished through cloning of vectors that now carry the chosen gene. Immunoassays are used not only in medicine for drug level and pregnancy testing, but also by farmers to aid in detection of unsafe levels of pesticides, herbicides, and toxins on crops and in animal products. These assays also provide rapid field tests for industrial chemicals in ground water, sediment, and soil. In agriculture, genetic engineering is being used to produce plants that are resistant to insects, weeds, and plant diseases. A current agricultural controversy involves the tomato. A recent article in the New Yorker magazine compared the discovery of the edible tomato that came about by early biotechnology with the new "Flavr-Savr" tomato brought about through modern techniques. In the very near future, you will be given the opportunity to bite into the Flavr-Savr tomato, the first food created by the use of recombinant DNA technology ever to go on sale. What will you think as you raise the tomato to your mouth? Will you hesitate? This moment may be for you as it was for Robert Gibbon Johnson in 1820 on the steps of the courthouse in Salem, New Jersey. Prior to this moment, the tomato was widely believed to be poisonous. As a large crowd watched, Johnson consumed two tomatoes and changed forever the human-tomato relationship. Since that time, man has sought to produce the supermarket tomato with that "backyard flavor." Americans also want that tomato available year-round. New biotechnological techniques have permitted scientists to manipulate desired traits. Prior to the advancement of the methods of recombinant DNA, scientists were limited to the techniques of their time - cross-pollination, selective breeding, pesticides, and herbicides. Today's biotechnology has its "roots" in chemistry, physics, and biology . The explosion in techniques has resulted in three major branches of biotechnology: genetic engineering, diagnostic techniques, and cell/tissue techniques

Lesson 4 Dogma,DNA,and Enzymes The Central Dogma Though it comes as no surprise that the composition of DNA between different organisms is different,it is not immediately obvious why the muscle cells,blood cells,and brain cells of any one particular vertebrate areso different in their structure and composition when the DNA of every one of their cells is identical.This is the key to one of the most exciting areas of modern cell biology.In different cell types different sets of the total number of genes(genome)e expressed.In other words,different regions of the DNA are"active"in the muscle cells,blood cells,and brain cells. To understand how this difference in DNA activity can lead to differences in cell structure and composition,it is necessary to consider what is ofen known as the central dogma of molecular biology: "DNA makes RNA makes protein"In molecular terms,a gene is that portion of DNA that encodes for a single protein.The dictum "one gene makes one protein"has required some modification with the discovery that some proteins are composed of several polypeptide chains,but makes one polypeptide"rule does hold. DNA Contains the Blueprint for all Cell Proteins Messenger RNA isa precise copy (transeript)of the coded seq of nucleicacid bases in DNA,and this message is translated into a unique protein on specialist organelles(ribosomes)present in the cytoplasm of all cells.Proteins,which are largely made up of carbon (C),hydrogen(H),oxygen(0). and nitrogen(N),are constructed from 20 different,common amino acids.The versatility of proteins,the workhorse molecules of the cell,stems from the immense variety of molecular shapes that can be created by linking amino acids together in different.The smaller proteins consist of only a few dozen amino acids,whereas the larger ones may contain in excess of 200 amino acids,all linked together in a linear chain by peptide bonds As the proteins are released from the ribosome.they nique shapes,under the inluence of chemical forces that depend on the particular sequence of amino acids.So the protein primary seqence. encoded in the gene and faithfully transcribed and translated into an amino acid chain,determines the three-dimensional structure of the emerging molecule.The human body possesses some 30,000 different 7

7 Lesson 4 Dogma, DNA, and Enzymes The Central Dogma Though it comes as no surprise that the composition of DNA between different organisms is different, it is not immediately obvious why the muscle cells, blood cells, and brain cells of any one particular vertebrate are so different in their structure and composition when the DNA of every one of their cells is identical. This is the key to one of the most exciting areas of modern cell biology. In different cell types, different sets of the total number of genes (genome) are expressed. In other words, different regions of the DNA are "active" in the muscle cells, blood cells, and brain cells. To understand how this difference in DNA activity can lead to differences in cell structure and composition, it is necessary to consider what is often known as the central dogma of molecular biology: "DNA makes RNA makes protein." In molecular terms, a gene is that portion of DNA that encodes for a single protein. The dictum "one gene makes one protein" has required some modification with the discovery that some proteins are composed of several different polypeptide chains, but the "one gene makes one polypeptide" rule does hold. DNA Contains the Blueprint for all Cell Proteins Messenger RNA is a precise copy (transcript) of the coded sequence of nucleic acid bases in DNA, and this message is translated into a unique protein molecule on specialist organelles (ribosomes) present in the cytoplasm of all cells. Proteins, which are largely made up of carbon (C), hydrogen (H), oxygen (0), and nitrogen (N), are constructed from 20 different, common amino acids. The versatility of proteins, the workhorse molecules of the cell, stems from the immense variety of molecular shapes that can be created by linking amino acids together in different sequences. The smaller proteins consist of only a few dozen amino acids, whereas the larger ones may contain in excess of 200 amino acids, all linked together in a linear chain by peptide bonds. As the proteins are released from the ribosome, they fold into unique shapes, under the influence of chemical forces that depend on the particular sequence of amino acids. So the protein primary sequence, encoded in the gene and faithfully transcribed and translated into an amino acid chain, determines the three-dimensional structure of the emerging molecule. The human body possesses some 30,000 different

Lesson 5 Polymerase Chain Reaction-Xeroxing DNA Who would have thought a bacterium hanging out in a hot spring in Yelowstone National Park would spark a revolutionary new laboratory technique?The polymerase chain reaction,now widely used in research laboratories and doctor's officesrelies on the ability of DNA-copying enzymes to remain stable at high temperatures No problem for the sultry bacterium from Yellowstone tha now helps scientists produce millions of copies of a single DNA segment ina matter of hours. In nature,most organisms copy their DNA in the same way.The PCR mimics this process,only it does it ina test tube.When any cell divides,enzymes called polymerases make a copy of all the DNA in each chromosome.The first step in this process isto"two DNA chains of the double helix.As the two strands separate,DNA polymerase makes a copy using each strand as atemplate The four nucleotide bases,the building blocks of every piece of DNA,are represented by the letters A.C.Gand T,which stand for their chemical names:adenine.ytosine,guanine.and thymine.The Aon one strand always pairs with the Ton the other.whereas Calways pairs with G.The two strands are said to be complementary toeach other. To copy DNA.polymerase requires two other components a supply of the four nucleotide bases and something called a primer.DNA polymerases,whether from humans,bacteria,or viruses,cannot copya chain of DNA without a short sequence of nucleotides to"prime"the process or get it started Sothe cell has another enzyme called a primase that actually makes the first few nucleotides of the copy.This stretch of DNA is called a primer.Once the primer is made,the polymerase can take over making the rest of the new chain A PCR vial contains all the necessary components for DNA duplication:a piece of DNA,large quantities of the four nucleotides,large quantities of the primer sequence,and DNA polymerase.The polymerase is the Taq polymerase.named for which it was isolated. The three parts of the polymerase chain reaction are carried ou in the same vial.but at different temperatures.The first part of the process separates the two DNA chains in the double helix.This is done simply by heating the vial to-95 degrees centigrade (about 165 degrees Fahrenheit)for 30 sconds 9

9 Lesson 5 Polymerase Chain Reaction - Xeroxing DNA Who would have thought a bacterium hanging out in a hot spring in Yellowstone National Park would spark a revolutionary new laboratory technique? The polymerase chain reaction, now widely used in research laboratories and doctor's offices, relies on the ability of DNA-copying enzymes to remain stable at high temperatures. No problem for Thermus aquaticus, the sultry bacterium from Yellowstone that now helps scientists produce millions of copies of a single DNA segment in a matter of hours. In nature, most organisms copy their DNA in the same way. The PCR mimics this process, only it does it in a test tube. When any cell divides, enzymes called polymerases make a copy of all the DNA in each chromosome. The first step in this process is to "unzip" the two DNA chains of the double helix. As the two strands separate, DNA polymerase makes a copy using each strand as a template. The four nucleotide bases, the building blocks of every piece of DNA, are represented by the letters A, C, G, and T, which stand for their chemical names: adenine, cytosine, guanine, and thymine. The A on one strand always pairs with the T on the other, whereas C always pairs with G. The two strands are said to be complementary to each other. To copy DNA, polymerase requires two other components: a supply of the four nucleotide bases and something called a primer. DNA polymerases, whether from humans, bacteria, or viruses, cannot copy a chain of DNA without a short sequence of nucleotides to "prime" the process, or get it started. So the cell has another enzyme called a primase that actually makes the first few nucleotides of the copy. This stretch of DNA is called a primer. Once the primer is made, the polymerase can take over making the rest of the new chain. A PCR vial contains all the necessary components for DNA duplication: a piece of DNA, large quantities of the four nucleotides, large quantities of the primer sequence, and DNA polymerase. The polymerase is the Taq polymerase, named for Thermus aquaticus, from which it was isolated. The three parts of the polymerase chain reaction are carried out in the same vial, but at different temperatures. The first part of the process separates the two DNA chains in the double helix. This is done simply by heating the vial to 90-95 degrees centigrade (about 165 degrees Fahrenheit) for 30 seconds

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