《现代遗传学》课程教学资源（参考书籍）【美】D. Peter Snustad、Michael J. Simmons《Principles of Genetics（SIXTH EDITION）》

Three Great Milestones in Genetics 3 century.Mendel carried out his path-breaking research in relative obscurity.He studied the inheritance of different traits in peas, which he grew in the monastery garden.His method involved in- terbreeding plants that showed different traits-for example,short plants were bred with tall plants-to see how the traits were in- herited by the offspring.Mendel's careful analysis enabled him to discern patterns,which led him to postulate the existence of heredi- tary factors responsible for the traits he studied.We now call these factors genes. Mendel studied several genes in the garden pea.Each of the genes was associated with a different trait-for example,plant height,or flower color,or seed texture.He discovered that these genes exist in different forms,which we now call alleles.One form of the gene for height,for example,allows pea plants to grow more than 2 meters tall;another form of this gene limits their growth to about half a meter. Mendel proposed that pea plants carry two copies of each gene. These copies may be the same or different.During reproduction, one of the copies is randomly incorporated into each sex cell or gamete.The female gametes (eggs)unite with the male gametes (sperm)at fertilization to produce single cells,called zygotes,which then develop into new plants.The reduction in gene copies from two to one during gamete formation and the subsequent restoration of two copies during fertilization underlie the rules of inheritance that Mendel discovered. Mendel emphasized that the hereditary factors-that is,the genes-are discrete entities.Different alleles of a gene can be brought together in the same plant through hybridization and can then be separated from each other during the production of gametes.The FIGURE 1.1 Gregor Mendel coexistence of alleles in a plant therefore does not compromise their integrity.Mendel also found that alleles of different genes are inherited independently of each other. These discoveries were published in 1866 in the proceedings of the Natural His- tory Society of Bruinn,the journal of the scientific society in the city where Mendel lived and worked.The article was not much noticed,and Mendel went on to do other things.In 1900,sixteen years after he died,the paper finally came to light,and the sci- ence of genetics was born.In short order,the type of analysis that Mendel pioneered was applied to many kinds of organisms,and with notable success.Of course,not every result fit exactly with Mendel's principles.Exceptions were encountered,and when they were investigated more fully,new insights into the behavior and properties of genes emerged.We will delve into Mendel's research and its applications to the study Nitrogen-containing base of inheritance,including heredity in humans,in Chapter 3,and we will explore some H H ramifications of Mendel's ideas in Chapter 4.In Chapters 5,6,and 7 we will see how Mendel's principles of inheritance are related to the behavior of chromosomes-the Phosphate cellular structures where genes reside. WATSON AND CRICK:THE STRUCTURE OF DNA The rediscovery of Mendel's paper launched a plethora of studies on inheritance in plants,animals,and microorganisms.The big question on everyone's mind was"What is a gene?"In the middle of the twentieth century,this question was finally answered. Genes were shown to consist of complex molecules called nucleic acids. Nucleic acids are made of elementary building blocks called nucleotides(Figure 1.2). Each nucleotide has three components:(1)a sugar molecule;(2)a phosphate molecule Sugar which has acidic chemical properties;and (3)a nitrogen-containing molecule,which FIGURE 1.2 Structure of a nucleotide.The has slightly basic chemical properties.In ribonucleic acid,or RNA,the constituent sugar molecule has three components:a phosphate is ribose;in deoxyribonucleic acid,or DNA,it is deoxyribose.Within RNA or DNA, group,a sugar [in this case deoxyribosel,and a one nucleotide is distinguished from another by its nitrogen-containing base.In RNA, nitrogen-containing base [in this case adenine]

Three Great Milestones in Genetics 3 century. Mendel carried out his path-breaking research in relative obscurity. He studied the inheritance of different traits in peas, which he grew in the monastery garden. His method involved in￾terbreeding plants that showed different traits—for example, short plants were bred with tall plants—to see how the traits were in￾herited by the offspring. Mendel’s careful analysis enabled him to discern patterns, which led him to postulate the existence of heredi￾tary factors responsible for the traits he studied. We now call these factors genes. Mendel studied several genes in the garden pea. Each of the genes was associated with a different trait—for example, plant height, or fl ower color, or seed texture. He discovered that these genes exist in different forms, which we now call alleles. One form of the gene for height, for example, allows pea plants to grow more than 2 meters tall; another form of this gene limits their growth to about half a meter. Mendel proposed that pea plants carry two copies of each gene. These copies may be the same or different. During reproduction, one of the copies is randomly incorporated into each sex cell or gamete. The female gametes (eggs) unite with the male gametes (sperm) at fertilization to produce single cells, called zygotes, which then develop into new plants. The reduction in gene copies from two to one during gamete formation and the subsequent restoration of two copies during fertilization underlie the rules of inheritance that Mendel discovered. Mendel emphasized that the hereditary factors—that is, the genes—are discrete entities. Different alleles of a gene can be brought together in the same plant through hybridization and can then be separated from each other during the production of gametes. The coexistence of alleles in a plant therefore does not compromise their integrity. Mendel also found that alleles of different genes are inherited independently of each other. These discoveries were published in 1866 in the proceedings of the Natural His￾tory Society of Brünn, the journal of the scientifi c society in the city where Mendel lived and worked. The article was not much noticed, and Mendel went on to do other things. In 1900, sixteen years after he died, the paper fi nally came to light, and the sci￾ence of genetics was born. In short order, the type of analysis that Mendel pioneered was applied to many kinds of organisms, and with notable success. Of course, not every result fi t exactly with Mendel’s principles. Exceptions were encountered, and when they were investigated more fully, new insights into the behavior and properties of genes emerged. We will delve into Mendel’s research and its applications to the study of inheritance, including heredity in humans, in Chapter 3, and we will explore some ramifi cations of Mendel’s ideas in Chapter 4. In Chapters 5, 6, and 7 we will see how Mendel’s principles of inheritance are related to the behavior of chromosomes—the cellular structures where genes reside. WATSON AND CRICK: THE STRUCTURE OF DNA The rediscovery of Mendel’s paper launched a plethora of studies on inheritance in plants, animals, and microorganisms. The big question on everyone’s mind was “What is a gene?” In the middle of the twentieth century, this question was fi nally answered. Genes were shown to consist of complex molecules called nucleic acids. Nucleic acids are made of elementary building blocks called nucleotides ( Figure 1.2). Each nucleotide has three components: (1) a sugar molecule; (2) a phosphate molecule, which has acidic chemical properties; and (3) a nitrogen-containing molecule, which has slightly basic chemical properties. In ribonucleic acid, or RNA, the constituent sugar is ribose; in deoxyribonucleic acid, or DNA, it is deoxyribose. Within RNA or DNA, one nucleotide is distinguished from another by its nitrogen-containing base. In RNA, FIGURE 1.1 Gregor Mendel. Phosphate Nitrogen-containing base Sugar O P O O C C C O H H H – O – O H H N H H C C C C C H N N N N C H C H H H FIGURE 1.2 Structure of a nucleotide. The molecule has three components: a phosphate group, a sugar (in this case deoxyribose), and a nitrogen-containing base (in this case adenine).

4 Chapter 1 The Science of Genetics the four kinds of bases are adenine (A),guanine(G),cytosine(C), and uracil(U);in DNA,they are A,G,C,and thymine(T).Thus,in both DNA and RNA there are four kinds of nucleotides,and three of them are shared by both types of nucleic acid molecules. The big breakthrough in the study of nucleic acids came in 1953 when James Watson and Francis Crick Figure 1.3)deduced how nucleotides are organized within DNA.Watson and Crick knew that the nucleotides are linked,one to another,in a chain.The link- ages are formed by chemical interactions between the phosphate of one nucleotide and the sugar of another nucleotide.The nitrogen- containing bases are not involved in these interactions.Thus,a chain of nucleotides consists of a phosphate-sugar backbone to which bases are attached,one base to each sugar in the backbone.From one end of the chain to the other,the bases form a linear sequence characteristic of that particular chain.This sequence of bases is what distinguishes one gene from another.Watson and Crick proposed that DNA molecules consist of two chains of nucleotides(Figure 1.4a). These chains are held together by weak chemical attractions-called hydrogen bonds-between particular pairs of bases;A pairs with T,and G pairs with C.Because of these base-pairing rules,the se- FIGURE 1.3 Francis Crick and James Watson. quence of one nucleotide chain in a double-stranded DNA molecule can be predicted from that of the other.In this sense,then,the two chains of a DNA molecule are complementary. A double-stranded DNA molecule is often called a duplex.Watson and Crick dis- covered that the two strands of a DNA duplex are wound around each other in a helical configuration(Figure 1.4b).These helical molecules can be extraordinarily large. Some contain hundreds of millions of nucleotide pairs,and their end-to-end length exceeds 10 centimeters.Were it not for their extraordinary thinness(about a hundred- millionth of a centimeter),we would be able to see them with the unaided eye. RNA,like DNA,consists of nucleotides linked one to another in a chain.However, unlike DNA,RNA molecules are usually single-stranded.The genes of most organ- isms are composed of DNA,although in some viruses they are made of RNA.We will examine the structures of DNA and RNA in detail in Chapter 9,and we will investigate the genetic significance of these macromolecules in Chapters 10,11,and 12. THE HUMAN GENOME PROJECT:SEQUENCING DNA AND CATALOGUING GENES If geneticists in the first half of the twentieth century dreamed about identifying the stuff that genes are made of,geneticists in the second half of that century dreamed (a) about ways of determining the sequence of bases in DNA mol- e8He888e月8日 ecules.Near the end of the century,their dreams became reality as Base- Hydrogen projects to determine DNA base sequences in several organisms, pairs A99A99199A91sA bonds including humans,took shape.Obtaining the sequence of bases in an organism's DNA-that is,sequencing the DNA-should, Phosphate-Sugar in principle,provide the information needed to analyze all that backbones organism's genes.We refer to the collection of DNA molecules that is characteristic of an organism as its genome.Sequencing the genome is therefore tantamount to sequencing all the organism's genes-and more,for we now know that some of the DNA does (b) not comprise genes.The function of this nongenic DNA is not FIGURE 1.4 DNA,a double-stranded molecule always clear;however,it is present in many genomes,and some- held together by hydrogen bonding between times it is abundant.A Milestone in Genetics:X174,the First paired bases.[a]Two-dimensional representa- DNA Genome Sequenced describes how genome sequencing got started.You can find tion of the structure of a DNA molecule com- this account in the Student Companion site. posed of complementary nucleotide chains. The paragon of all the sequencing programs is the Human Genome Project,a world- (b)A DNA molecule shown as a double helix. wide effort to determine the sequence of approximately 3 billion nucleotide pairs in

4 Chapter 1 The Science of Genetics the four kinds of bases are adenine (A), guanine (G), cytosine (C), and uracil (U); in DNA, they are A, G, C, and thymine (T). Thus, in both DNA and RNA there are four kinds of nucleotides, and three of them are shared by both types of nucleic acid molecules. The big breakthrough in the study of nucleic acids came in 1953 when James Watson and Francis Crick ( Figure 1.3) deduced how nucleotides are organized within DNA. Watson and Crick knew that the nucleotides are linked, one to another, in a chain. The link￾ages are formed by chemical interactions between the phosphate of one nucleotide and the sugar of another nucleotide. The nitrogen￾containing bases are not involved in these interactions. Thus, a chain of nucleotides consists of a phosphate-sugar backbone to which bases are attached, one base to each sugar in the backbone. From one end of the chain to the other, the bases form a linear sequence characteristic of that particular chain. This sequence of bases is what distinguishes one gene from another. Watson and Crick proposed that DNA molecules consist of two chains of nucleotides ( Figure 1.4a). These chains are held together by weak chemical attractions—called hydrogen bonds—between particular pairs of bases; A pairs with T, and G pairs with C. Because of these base-pairing rules, the se￾quence of one nucleotide chain in a double-stranded DNA molecule can be predicted from that of the other. In this sense, then, the two chains of a DNA molecule are complementary. A double-stranded DNA molecule is often called a duplex. Watson and Crick dis￾covered that the two strands of a DNA duplex are wound around each other in a helical confi guration ( Figure 1.4b). These helical molecules can be extraordinarily large. Some contain hundreds of millions of nucleotide pairs, and their end-to-end length exceeds 10 centimeters. Were it not for their extraordinary thinness (about a hundred￾millionth of a centimeter), we would be able to see them with the unaided eye. RNA, like DNA, consists of nucleotides linked one to another in a chain. However, unlike DNA, RNA molecules are usually single-stranded. The genes of most organ￾isms are composed of DNA, although in some viruses they are made of RNA. We will examine the structures of DNA and RNA in detail in Chapter 9, and we will investigate the genetic signifi cance of these macromolecules in Chapters 10, 11, and 12. THE HUMAN GENOME PROJECT: SEQUENCING DNA AND CATALOGUING GENES If geneticists in the fi rst half of the twentieth century dreamed about identifying the stuff that genes are made of, geneticists in the second half of that century dreamed about ways of determining the sequence of bases in DNA mol￾ecules. Near the end of the century, their dreams became reality as projects to determine DNA base sequences in several organisms, including humans, took shape. Obtaining the sequence of bases in an organism’s DNA—that is, sequencing the DNA—should, in principle, provide the information needed to analyze all that organism’s genes. We refer to the collection of DNA molecules that is characteristic of an organism as its genome. Sequencing the genome is therefore tantamount to sequencing all the organism’s genes—and more, for we now know that some of the DNA does not comprise genes. The function of this nongenic DNA is not always clear; however, it is present in many genomes, and some￾times it is abundant. A Milestone in Genetics: X174, the First DNA Genome Sequenced describes how genome sequencing got started. You can fi nd this account in the Student Companion site. The paragon of all the sequencing programs is the Human Genome Project, a world￾wide effort to determine the sequence of approximately 3 billion nucleotide pairs in FIGURE 1.3 Francis Crick and James Watson. Phosphate-Sugar backbones C T A G C A T G T A T A A T A C T G C G G C G C G C T G A C (a) (b) Hydrogen bonds Base pairs FIGURE 1.4 DNA, a double-stranded molecule held together by hydrogen bonding between paired bases. (a) Two-dimensional representa￾tion of the structure of a DNA molecule com￾posed of complementary nucleotide chains. (b) A DNA molecule shown as a double helix.

Three Great Milestones in Genetics 5 human DNA.As initially conceived,the Human Genome Project was to involve collaborations among researchers in many different countries,and much of the work was to be funded by their gov- ernments.However,a privately funded project initiated by Craig Venter,a scientist and entrepreneur,soon developed alongside the publicly funded project.In 2001 all these efforts culminated in the publication of two lengthy articles about the human genome.The articles reported that 2.7 billion nucleotide pairs of human DNA had been sequenced.Computer analysis of this DNA suggested that the human genome contained between 30,000 and 40,000 genes. More recent analyses have revised the human gene number down- ward,to around 20,500.These genes have been catalogued by loca- tion,structure,and potential function.Efforts are now focused on studying how they influence the myriad characteristics of humans. The genomes of many other organisms-bacteria,fungi,plants protists,and animals-have also been sequenced.Much of this work has been done under the auspices of the Human Genome Project, FIGURE 1.5 A researcher loading samples into an automated or under projects closely allied to it.Initially the sequencing efforts DNA sequencer. were focused on organisms that are especially favorable for genetic research.In many places in this book,we explore ways in which researchers have used these model organisms to advance genetic knowledge.Current sequencing projects have moved beyond the model organisms to diverse plants,animals,and microbes.For ex- ample,the genomes of the mosquito and the malaria parasite that it carries have both been sequenced,as have the genomes of the honeybee,the poplar tree,and the sea squirt. Some of the targets of these sequencing projects have a medical,agricultural,or com- mercial significance;others simply help us to understand how genomes are organized and how they have diversified during the history of life on Earth. All the DNA sequencing projects have transformed genetics in a fundamental way. Genes can now be studied at the molecular level with relative ease,and vast numbers of genes can be studied simultaneously.This approach to genetics,rooted in the analysis of the DNA sequences that make up a genome,is called genomics.It has been made possible by advances in DNA sequencing technology,robotics,and computer science Figure 1.5).Researchers are now able to construct and scan enormous databases con- taining DNA sequences to address questions about genetics.Although there are a large number of useful databases currently available,we will focus on the databases assem- bled by the National Center for Biotecbnology Information (NCBI),maintained by the U.S. National Institutes of Health.The NCBI databases-available free on the web at http:// www.ncbi.nih.gov-are invaluable repositories of information about genes,proteins, genomes,publications,and other important data in the fields of genetics,biochemistry, and molecular biology.They contain the complete nucleotide sequences of all genomes that have been sequenced to date,and they are continually updated.In addition,the NCBI web site contains tools that can be used to search for specific items of inter- est-gene and protein sequences,research articles,and so on.In Chapter 15,we will introduce you to some of these tools,and throughout this book,we will encourage you to visit the NCBI web site at the end of each chapter to answer specific questions. Gregor Mendel postulated the existence of particulate factors-now called genes-to explain bow traits are KEY POINTS inberited. .Alleles,the alternate forms of genes,account for beritable differences among individuals. .James Watson and Francis Crick elucidated the structure of DNA,a macromolecule composed of two complementary cbains of nucleotides. DNA is the bereditary material in all life forms except some types of viruses,in wbich RNA is the bereditary material. .Tbe Human Genome Project determined the sequence of nucleotides in the DNA of the buman genome. Sequencing the DNA of a genome provides the data to identify and catalogue all tbe genes of an organism

Three Great Milestones in Genetics 5 human DNA. As initially conceived, the Human Genome Project was to involve collaborations among researchers in many different countries, and much of the work was to be funded by their gov￾ernments. However, a privately funded project initiated by Craig Venter, a scientist and entrepreneur, soon developed alongside the publicly funded project. In 2001 all these efforts culminated in the publication of two lengthy articles about the human genome. The articles reported that 2.7 billion nucleotide pairs of human DNA had been sequenced. Computer analysis of this DNA suggested that the human genome contained between 30,000 and 40,000 genes. More recent analyses have revised the human gene number down￾ward, to around 20,500. These genes have been catalogued by loca￾tion, structure, and potential function. Efforts are now focused on studying how they infl uence the myriad characteristics of humans. The genomes of many other organisms—bacteria, fungi, plants, protists, and animals—have also been sequenced. Much of this work has been done under the auspices of the Human Genome Project, or under projects closely allied to it. Initially the sequencing efforts were focused on organisms that are especially favorable for genetic research. In many places in this book, we explore ways in which researchers have used these model organisms to advance genetic knowledge. Current sequencing projects have moved beyond the model organisms to diverse plants, animals, and microbes. For ex￾ample, the genomes of the mosquito and the malaria parasite that it carries have both been sequenced, as have the genomes of the honeybee, the poplar tree, and the sea squirt. Some of the targets of these sequencing projects have a medical, agricultural, or com￾mercial signifi cance; others simply help us to understand how genomes are organized and how they have diversifi ed during the history of life on Earth. All the DNA sequencing projects have transformed genetics in a fundamental way. Genes can now be studied at the molecular level with relative ease, and vast numbers of genes can be studied simultaneously. This approach to genetics, rooted in the analysis of the DNA sequences that make up a genome, is called genomics. It has been made possible by advances in DNA sequencing technology, robotics, and computer science ( Figure 1.5). Researchers are now able to construct and scan enormous databases con￾taining DNA sequences to address questions about genetics. Although there are a large number of useful databases currently available, we will focus on the databases assem￾bled by the National Center for Biotechnology Information (NCBI), maintained by the U.S. National Institutes of Health. The NCBI databases—available free on the web at http:// www.ncbi.nih.gov—are invaluable repositories of information about genes, proteins, genomes, publications, and other important data in the fi elds of genetics, biochemistry, and molecular biology. They contain the complete nucleotide sequences of all genomes that have been sequenced to date, and they are continually updated. In addition, the NCBI web site contains tools that can be used to search for specifi c items of inter￾est—gene and protein sequences, research articles, and so on. In Chapter 15, we will introduce you to some of these tools, and throughout this book, we will encourage you to visit the NCBI web site at the end of each chapter to answer specifi c questions. FIGURE 1.5 A researcher loading samples into an automated DNA sequencer.  Gregor Mendel postulated the existence of particulate factors—now called genes—to explain how traits are inherited.  Alleles, the alternate forms of genes, account for heritable differences among individuals.  James Watson and Francis Crick elucidated the structure of DNA, a macromolecule composed of two complementary chains of nucleotides.  DNA is the hereditary material in all life forms except some types of viruses, in which RNA is the hereditary material.  The Human Genome Project determined the sequence of nucleotides in the DNA of the human genome.  Sequencing the DNA of a genome provides the data to identify and catalogue all the genes of an organism. KEY POINTS

6 Chapter 1 The Science of Genetics DNA as the Genetic Material In biology information flows from DNA to RNA In all cellular organisms,the genetic material is DNA.This material to protein. must be able to replicate so that copies can be transmitted from cell to cell and from parents to offspring;it must contain information to direct cellular activities and to guide the development,functioning, and behavior of organisms;and it must be able to cbange so that over time,groups of organisms can adapt to different circumstances. DNA REPLICATION:PROPAGATING GENETIC INFORMATION The genetic material of an organism is transmitted from a mother cell to its daugh- ters during cell division.It is also transmitted from parents to their offspring during reproduction.The faithful transmission of genetic material from one cell or organism to another is based on the ability of double-stranded DNA molecules to be replicated. DNA replication is extraordinarily exact.Molecules consisting of hundreds of millions of nucleotide pairs are duplicated with few,if any,mistakes. The process of DNA replication is based on the complementary nature of the strands that make up duplex DNA molecules(Figure 1.6).These strands are held together by relatively weak hydrogen bonds between specific base pairs-A paired with T,and G paired with C.When these bonds are broken,the separated strands can serve as templates for the synthesis of new partner strands.The new strands are assembled by the stepwise incorporation of nucleotides opposite to nucleotides in the template strands.This incorporation conforms to the base-pairing rules.Thus,the sequence of nucleotides in a strand being synthesized is dictated by the sequence of nucleotides in the template strand.At the end of the replication process,each template strand is paired with a newly synthesized partner strand.Thus,two identical DNA duplexes are created from one original duplex. The process of DNA replication does not occur spontaneously.Like most bio- chemical processes,it is catalyzed by enzymes.We will explore the details of DNA replication,including the roles played by different enzymes,in Chapter 10. A:T A: A:TE A:T ☒CGC ☒CGC G ☒CG G:C> GC GC ZC:G■ ☒C:G A:TE DA:TE A:T A:TE T:A T:A T:A IT:A A:TE A:T A:T A:T G C C T:A 7 G:C2 IT:A T:A G A A C i Parental DNA Separation of Synthesis of new Two identical molecule parental strands complementary strands daughter DNA molecules FIGURE 1.6 DNA replication.The two strands in the parental molecule are oriented in opposite directions (see arrows).These strands separate and new strands are synthesized using the parental strands as templates. When replication is completed,two identical double-stranded DNA molecules have been produced

6 Chapter 1 The Science of Genetics In all cellular organisms, the genetic material is DNA. This material must be able to replicate so that copies can be transmitted from cell to cell and from parents to offspring; it must contain information to direct cellular activities and to guide the development, functioning, and behavior of organisms; and it must be able to change so that over time, groups of organisms can adapt to different circumstances. DNA REPLICATION: PROPAGATING GENETIC INFORMATION The genetic material of an organism is transmitted from a mother cell to its daugh￾ters during cell division. It is also transmitted from parents to their offspring during reproduction. The faithful transmission of genetic material from one cell or organism to another is based on the ability of double-stranded DNA molecules to be replicated. DNA replication is extraordinarily exact. Molecules consisting of hundreds of millions of nucleotide pairs are duplicated with few, if any, mistakes. The process of DNA replication is based on the complementary nature of the strands that make up duplex DNA molecules ( Figure 1.6). These strands are held together by relatively weak hydrogen bonds between specifi c base pairs—A paired with T, and G paired with C. When these bonds are broken, the separated strands can serve as templates for the synthesis of new partner strands. The new strands are assembled by the stepwise incorporation of nucleotides opposite to nucleotides in the template strands. This incorporation conforms to the base-pairing rules. Thus, the sequence of nucleotides in a strand being synthesized is dictated by the sequence of nucleotides in the template strand. At the end of the replication process, each template strand is paired with a newly synthesized partner strand. Thus, two identical DNA duplexes are created from one original duplex. The process of DNA replication does not occur spontaneously. Like most bio￾chemical processes, it is catalyzed by enzymes. We will explore the details of DNA replication, including the roles played by different enzymes, in Chapter 10. In biology information flows from DNA to RNA to protein. DNA as the Genetic Material TA CG CG CG C G TA C G A T T A T A T A TA GC TA CG CG CG C G TA C G A T T A T A T A TA GC TA CG CG CG C G TA C G A T T A T A T A TA GC TA CG C G T A TA GC A T G C G C A T C G T A T A Parental DNA molecule Separation of parental strands Synthesis of new complementary strands Two identical daughter DNA molecules TA CG C G T A TA GC A T CG CG TA C G T A T A CG CG C G TA T A T A + FIGURE 1.6 DNA replication. The two strands in the parental molecule are oriented in opposite directions (see arrows). These strands separate and new strands are synthesized using the parental strands as templates. When replication is completed, two identical double-stranded DNA molecules have been produced.

DNA as the Genetic Material 7 GENE EXPRESSION:USING GENETIC INFORMATION DNA molecules contain information to direct the activities of cells and to guide the development,functioning,and behavior of the organisms that comprise these cells. This information is encoded in sequences of nucleotides within the DNA molecules of the genome.Among cellular organisms,the smallest known genome is that of Mycoplasma genitaliun:580,070 nucleotide pairs.By contrast,the human genome consists of 3.2 billion nucleotide pairs.In these and all other genomes,the information contained within the DNA is organized into the units we call genes.An M.genitalium has 482 genes,whereas a human sperm cell has around 20,500.Each gene is a stretch of nucleotide pairs along the length of a DNA molecule.A particular DNA molecule may contain thousands of different genes.In an M.genitalium cell,all the genes are situated on one DNA molecule-the single chromosome of this organism.In a human sperm cell,the genes are situated on 23 different DNA molecules corresponding to the 23 chromosomes in the cell.Most of the DNA in M.genitaliunt comprises genes, whereas most of the DNA in humans does not-that is,most of the human DNA is nongenic.We will investigate the genic and nongenic composition of genomes in many places in this book,especially in Chapter 15. How is the information within individual genes organized and expressed?This ques- tion is central in genetics,and we will turn our attention to it in Chapters 11 and 12. Here,suffice it to say that most genes contain the instructions for the synthesis of proteins.Each protein consists of one or more chains of amino acids.These chains are called polypeptides.The 20 different kinds of amino acids that occur naturally can be combined in myriad ways to form polypeptides.Each polypeptide has a characteristic sequence of amino acids.Some polypeptides are short-just a few amino acids long- whereas others are enormous-thousands of amino acids long. The sequence of amino acids in a polypeptide is specified by a sequence of elemen- tary coding units within a gene.These elementary coding units,called codons,are trip- lets of adjacent nucleotides.A typical gene may contain hundreds or even thousands of codons.Each codon specifies the incorporation of an amino acid into a polypeptide. Thus,the information encoded within a gene is used to direct the synthesis of a polypep- tide,which is often referred to as the gene's product.Sometimes,depending on how the coding information is utilized,a gene may encode several polypeptides;however,these polypeptides are usually all related by sharing some common sequence of amino acids. The expression of genetic information to form a polypeptide is a two-stage pro- cess(Figure 1.7).First,the information contained in a gene's DNA is copied into a molecule of RNA.The RNA is assembled in stepwise fashion along one of the strands of the DNA duplex.During this assembly process,A in the RNA pairs with T in the DNA,G in the RNA pairs with C in the DNA,C in the RNA pairs with G in the DNA,and U in the RNA pairs with A in the DNA.Thus,the nucleotide sequence of the RNA is determined by the nucleotide sequence of a strand of DNA in the gene. The process that produces this RNA molecule is called transcription,and the RNA itself is called a transcript.The RNA transcript eventually separates from its DNA template and,in some organisms,is altered by the addition,deletion,or modification of nucleotides.The finished molecule,called the messenger RNA or simply mRNA,con- tains all the information needed for the synthesis of a polypeptide. The second stage in the expression of a gene's information is called translation At this stage,the gene's mRNA acts as a template for the synthesis of a polypeptide Each of the gene's codons,now present within the sequence of the mRNA,specifies the incorporation of a particular amino acid into the polypeptide chain.One amino acid is added at a time.Thus,the polypeptide is synthesized stepwise by reading the codons in order.When the polypeptide is finished,it dissociates from the mRNA, folds into a precise three-dimensional shape,and then carries out its role in the cell. Some polypeptides are altered by the removal of the first amino acid,which is usually methionine,in the sequence. We refer to the collection of all the different proteins in an organism as its proteome. Humans,with around 20,500 genes,may have hundreds of thousands of different proteins

DNA as the Genetic Material 7 GENE EXPRESSION: USING GENETIC INFORMATION DNA molecules contain information to direct the activities of cells and to guide the development, functioning, and behavior of the organisms that comprise these cells. This information is encoded in sequences of nucleotides within the DNA molecules of the genome. Among cellular organisms, the smallest known genome is that of Mycoplasma genitalium: 580,070 nucleotide pairs. By contrast, the human genome consists of 3.2 billion nucleotide pairs. In these and all other genomes, the information contained within the DNA is organized into the units we call genes. An M. genitalium has 482 genes, whereas a human sperm cell has around 20,500. Each gene is a stretch of nucleotide pairs along the length of a DNA molecule. A particular DNA molecule may contain thousands of different genes. In an M. genitalium cell, all the genes are situated on one DNA molecule—the single chromosome of this organism. In a human sperm cell, the genes are situated on 23 different DNA molecules corresponding to the 23 chromosomes in the cell. Most of the DNA in M. genitalium comprises genes, whereas most of the DNA in humans does not—that is, most of the human DNA is nongenic. We will investigate the genic and nongenic composition of genomes in many places in this book, especially in Chapter 15. How is the information within individual genes organized and expressed? This ques￾tion is central in genetics, and we will turn our attention to it in Chapters 11 and 12. Here, suffi ce it to say that most genes contain the instructions for the synthesis of proteins. Each protein consists of one or more chains of amino acids. These chains are called polypeptides. The 20 different kinds of amino acids that occur naturally can be combined in myriad ways to form polypeptides. Each polypeptide has a characteristic sequence of amino acids. Some polypeptides are short—just a few amino acids long— whereas others are enormous—thousands of amino acids long. The sequence of amino acids in a polypeptide is specifi ed by a sequence of elemen￾tary coding units within a gene. These elementary coding units, called codons, are trip￾lets of adjacent nucleotides. A typical gene may contain hundreds or even thousands of codons. Each codon specifi es the incorporation of an amino acid into a polypeptide. Thus, the information encoded within a gene is used to direct the synthesis of a polypep￾tide, which is often referred to as the gene’s product. Sometimes, depending on how the coding information is utilized, a gene may encode several polypeptides; however, these polypeptides are usually all related by sharing some common sequence of amino acids. The expression of genetic information to form a polypeptide is a two-stage pro￾cess ( Figure 1.7). First, the information contained in a gene’s DNA is copied into a molecule of RNA. The RNA is assembled in stepwise fashion along one of the strands of the DNA duplex. During this assembly process, A in the RNA pairs with T in the DNA, G in the RNA pairs with C in the DNA, C in the RNA pairs with G in the DNA, and U in the RNA pairs with A in the DNA. Thus, the nucleotide sequence of the RNA is determined by the nucleotide sequence of a strand of DNA in the gene. The process that produces this RNA molecule is called transcription, and the RNA itself is called a transcript. The RNA transcript eventually separates from its DNA template and, in some organisms, is altered by the addition, deletion, or modifi cation of nucleotides. The fi nished molecule, called the messenger RNA or simply mRNA, con￾tains all the information needed for the synthesis of a polypeptide. The second stage in the expression of a gene’s information is called translation. At this stage, the gene’s mRNA acts as a template for the synthesis of a polypeptide. Each of the gene’s codons, now present within the sequence of the mRNA, specifi es the incorporation of a particular amino acid into the polypeptide chain. One amino acid is added at a time. Thus, the polypeptide is synthesized stepwise by reading the codons in order. When the polypeptide is fi nished, it dissociates from the mRNA, folds into a precise three-dimensional shape, and then carries out its role in the cell. Some polypeptides are altered by the removal of the fi rst amino acid, which is usually methionine, in the sequence. We refer to the collection of all the different proteins in an organism as its proteome. Humans, with around 20,500 genes, may have hundreds of thousands of different proteins