《生物化学原理》（英文版）INFORMATION PATHWAYS

PART INFORMATION PATHWAYS 24 Genes an 25 DNA Metabolism 948 26 RNA Metabolism 995 27 Protein Metabolism 1034 28 Regulation of Gene Ex 1 e of DNA in1953 The third and final part of this book explores the bio-of coding by chemical mechanisms underlying the apparently con-tic diffraction analysis. tradictory requirements for both genetic continuity and the evolution of living organisms. What is the molecular found impa k hypothesis arose from transmitted from one ge to the next with high wide range of opservations disciplines. fidelity? Hov ding of the struc- that are the raw How is ge-ture of DNA inevitably stimulated questions about its g- sequences of the astonishing variety of prot cules in a living cell? The fund t re rise to the tein, so much of the ma comprising the that a er DNA molecules with mational units form the focal points The second is tran- Part. Modern bi oded in DNA are copied precisely into RNA. function has ble to that stimulate Darwin's the- encoded in messenger RNA is translated on the ribo- ory on the origin of species nearly 150 years ago. un- somes into a polypeptide with a particular sequence of derstanding of ed in . 9 中

T he third and final part of this book explores the bio￾chemical mechanisms underlying the apparently con￾tradictory requirements for both genetic continuity and the evolution of living organisms. What is the molecular nature of genetic material? How is genetic information transmitted from one generation to the next with high fidelity? How do the rare changes in genetic material that are the raw material of evolution arise? How is ge￾netic information ultimately expressed in the amino acid sequences of the astonishing variety of protein mole￾cules in a living cell? The fundamental unit of information in living sys￾tems is the gene. A gene can be defined biochemically as a segment of DNA (or, in a few cases, RNA) that en￾codes the information required to produce a functional biological product. The final product is usually a pro￾tein, so much of the material in Part III concerns genes that encode proteins. A functional gene product might also be one of several classes of RNA molecules. The storage, maintenance, and metabolism of these infor￾mational units form the focal points of our discussion in Part III. Modern biochemical research on gene structure and function has brought to biology a revolution compara￾ble to that stimulated by the publication of Darwin’s the￾ory on the origin of species nearly 150 years ago. An un￾derstanding of how information is stored and used in cells has brought penetrating new insights to some of the most fundamental questions about cellular structure and function. A comprehensive conceptual framework for biochemistry is now unfolding. Today’s understanding of information pathways has arisen from the convergence of genetics, physics, and chemistry in modern biochemistry. This was epitomized by the discovery of the double-helical structure of DNA, postulated by James Watson and Francis Crick in 1953 (see Fig. 8–15). Genetic theory contributed the concept of coding by genes. Physics permitted the determina￾tion of molecular structure by x-ray diffraction analysis. Chemistry revealed the composition of DNA. The pro￾found impact of the Watson-Crick hypothesis arose from its ability to account for a wide range of observations derived from studies in these diverse disciplines. This revolution in our understanding of the struc￾ture of DNA inevitably stimulated questions about its function. The double-helical structure itself clearly sug￾gested how DNA might be copied so that the informa￾tion it contains can be transmitted from one generation to the next. Clarification of how the information in DNA is converted into functional proteins came with the dis￾covery of both messenger RNA and transfer RNA and with the deciphering of the genetic code. These and other major advances gave rise to the central dogma of molecular biology, comprising the three major processes in the cellular utilization of ge￾netic information. The first is replication, the copying of parental DNA to form daughter DNA molecules with identical nucleotide sequences. The second is tran￾scription, the process by which parts of the genetic message encoded in DNA are copied precisely into RNA. The third is translation, whereby the genetic message encoded in messenger RNA is translated on the ribo￾somes into a polypeptide with a particular sequence of amino acids. PART INFORMATION PATHWAYS III 24 Genes and Chromosomes 923 25 DNA Metabolism 948 26 RNA Metabolism 995 27 Protein Metabolism 1034 28 Regulation of Gene Expression 1081 921 8885d_c24_920-947 2/11/04 1:36 PM Page 921 mac76 mac76:385_reb:

885dc24_9222/11/043:11 PM Page922mac76mac76:385reb: Part Il Information Pathways upon which life itself is based. We might expect the for- DNA mation of phosphodiester bonds in DNA or peptide bonds in proteins to be a trivial feat for cells, given the arsenal of enzymatic and chemical tools described in Part Il. However, the framework of patterns and rules established in our examination of metabolic pathways RNA thus far must be enlarged considerably to take into account molecular information. Bonds must be formed Translation between particular subunits in informational biopoly- mers, avoiding either the occurrence or the persistence of sequence errors. This has an enormous impact on the thermodynamics, chemistry, and enzymology of the biosynthetic processes. Formation of a peptide bond re- The central dogma of molecular biology, showing the general path- quires an energy input of only about 21 kJ/mol of bonds tion. The term"dogma"is a misnomer. Introduced by Francis Crick at and can be catalyzed by relatively simple enzymes.But a time when little evidence supported these ideas, the dogma has be. to synthesize a bond between two specific amino acids come a well-established principle. at a particular point in a polypeptide, the cell invests about 125 kJ/mol while making use of more than 200 enzymes, RNA molecules, and specialized proteins. The chemistry involved in peptide bond formation does not change because of this requirement, but additional Part Ill explores these and related pi In processes are layered over the basic reaction to ensure pter 24 we examine the structure, topology, and that the peptide bond is formed between particular ackaging of chromosomes and genes. The processes amino acids. Information is expensive underlying the central dogma are elaborated in Chap- The dynamic interaction between nucleic acids and ters 25 through 27. Finally, we turn to regulation, ex- proteins is another central theme of Part Ill. with the amining how the expression of genetic information is important exception of a few catalytic RNA molecules controlled(Chapter 28) (discussed in Chapters 26 and 27), the processes that A major theme running through these chapters is make up the pathways of cellular information flow are the added complexity inherent in the biosynthesis of catalyzed and regulated by proteins. An understanding macromolecules that contain information. Assembling of these enzymes and other proteins can have practical nucleic acids and proteins with particular sequences of as well as intellectual rewards, because they form the nucleotides and amino acids represents nothing less basis of recombinant dna technology (introduced in than preserving the faithful expression of the template Chapter 9)

Part III explores these and related processes. In Chapter 24 we examine the structure, topology, and packaging of chromosomes and genes. The processes underlying the central dogma are elaborated in Chap￾ters 25 through 27. Finally, we turn to regulation, ex￾amining how the expression of genetic information is controlled (Chapter 28). A major theme running through these chapters is the added complexity inherent in the biosynthesis of macromolecules that contain information. Assembling nucleic acids and proteins with particular sequences of nucleotides and amino acids represents nothing less than preserving the faithful expression of the template upon which life itself is based. We might expect the for￾mation of phosphodiester bonds in DNA or peptide bonds in proteins to be a trivial feat for cells, given the arsenal of enzymatic and chemical tools described in Part II. However, the framework of patterns and rules established in our examination of metabolic pathways thus far must be enlarged considerably to take into account molecular information. Bonds must be formed between particular subunits in informational biopoly￾mers, avoiding either the occurrence or the persistence of sequence errors. This has an enormous impact on the thermodynamics, chemistry, and enzymology of the biosynthetic processes. Formation of a peptide bond re￾quires an energy input of only about 21 kJ/mol of bonds and can be catalyzed by relatively simple enzymes. But to synthesize a bond between two specific amino acids at a particular point in a polypeptide, the cell invests about 125 kJ/mol while making use of more than 200 enzymes, RNA molecules, and specialized proteins. The chemistry involved in peptide bond formation does not change because of this requirement, but additional processes are layered over the basic reaction to ensure that the peptide bond is formed between particular amino acids. Information is expensive. The dynamic interaction between nucleic acids and proteins is another central theme of Part III. With the important exception of a few catalytic RNA molecules (discussed in Chapters 26 and 27), the processes that make up the pathways of cellular information flow are catalyzed and regulated by proteins. An understanding of these enzymes and other proteins can have practical as well as intellectual rewards, because they form the basis of recombinant DNA technology (introduced in Chapter 9). 922 Part III Information Pathways The central dogma of molecular biology, showing the general path￾ways of information flow via replication, transcription, and transla￾tion. The term “dogma” is a misnomer. Introduced by Francis Crick at a time when little evidence supported these ideas, the dogma has be￾come a well-established principle. RNA Protein Transcription Translation Replication DNA 8885d_c24_922 2/11/04 3:11 PM Page 922 mac76 mac76:385_reb:

8885dc24920-9472/11/041:36 PM Page923mac76mac76:385 chapter GENES AND CHROMOSOMES 24.1 Chromosomal Elements 924 tain them (Fig. 24-1). In this chapter we shift our focus 24.2 DNA Supercoiling 930 from the secondary structure of DNA, considered in 24.3 The Structure of Chromosomes 938 Chapter 8, to the extraordinary degree of organization required for the tertiary packaging of DNA into chromo- somes. We first examine the elements within viral and cellular chromosomes, then assess their size and organi- DNA topoisomerases are the magicians of the DNA world. zation. We next consider DNA topology, providing a By allowing DNa strands or double helices to pass through each other, they can solve all of the topological blems of DNA in replication, transcription and other cellular transactions mes Wang, article in Nature Reviews in Molecular Cell Biology, 2002 Supercoiling, in fact, does more for DNa than act as an executive enhancer; it keeps the unruly, spreading DNA inside the cramped confines that the cell has provided Nicholas Cozzarelli, Harvey Lectures, 1993 Most every cell of a multicellular organism contains the same complement of genetic material--its genome. Just look at any human individual for a hint of the wealth of information contained in each human cell Chromosomes the nucleic acid molecules that are the repository of an organisms genetic information, the largest molecules in a cell and may contain thou- sands of genes as well as considerable tracts of inter- genic DNA. The 16 chromosomes in the relatively small genome of the yeast Saccharomyces cerevisiae have molecular masses ranging from1.5×10t1×10°dal- FIGURE 24-1 Bacteriophage T2 protein coat surrounded by its sin- gle, linear molecule of DNA. The DNA was released by lysing the tons, corresponding to DNA molecules with 230,000 to bacteriophage particle in distilled water and allowing the DNA to 1,532,000 contiguous base pairs(bp). Human chromo- spread on the water surface. An undamaged T2 bacteriophage parti somes range up to 279 million bp cle consists of a head structure that tapers to a tail by which the bac. The very size of DNA molecules presents an inter- teriophage attaches itself to the outer surface of a bacterial cell. All esting biological puzzle, given that they are generally the DNA shown in this electron micrograph is normally packaged in. much longer than the cells or viral packages that con- side the phage head

chapter Almost every cell of a multicellular organism contains the same complement of genetic material—its genome. Just look at any human individual for a hint of the wealth of information contained in each human cell. Chromosomes, the nucleic acid molecules that are the repository of an organism’s genetic information, are the largest molecules in a cell and may contain thou￾sands of genes as well as considerable tracts of inter￾genic DNA. The 16 chromosomes in the relatively small genome of the yeast Saccharomyces cerevisiae have molecular masses ranging from 1.5
108 to 1
109 dal￾tons, corresponding to DNA molecules with 230,000 to 1,532,000 contiguous base pairs (bp). Human chromo￾somes range up to 279 million bp. The very size of DNA molecules presents an inter￾esting biological puzzle, given that they are generally much longer than the cells or viral packages that con￾tain them (Fig. 24–1). In this chapter we shift our focus from the secondary structure of DNA, considered in Chapter 8, to the extraordinary degree of organization required for the tertiary packaging of DNA into chromo￾somes. We first examine the elements within viral and cellular chromosomes, then assess their size and organi￾zation. We next consider DNA topology, providing a GENES AND CHROMOSOMES 24.1 Chromosomal Elements 924 24.2 DNA Supercoiling 930 24.3 The Structure of Chromosomes 938 DNA topoisomerases are the magicians of the DNA world. By allowing DNA strands or double helices to pass through each other, they can solve all of the topological problems of DNA in replication, transcription and other cellular transactions. —James Wang, article in Nature Reviews in Molecular Cell Biology, 2002 Supercoiling, in fact, does more for DNA than act as an executive enhancer; it keeps the unruly, spreading DNA inside the cramped confines that the cell has provided for it. —Nicholas Cozzarelli, Harvey Lectures, 1993 24 923 0.5 m FIGURE 24–1 Bacteriophage T2 protein coat surrounded by its sin￾gle, linear molecule of DNA. The DNA was released by lysing the bacteriophage particle in distilled water and allowing the DNA to spread on the water surface. An undamaged T2 bacteriophage parti￾cle consists of a head structure that tapers to a tail by which the bac￾teriophage attaches itself to the outer surface of a bacterial cell. All the DNA shown in this electron micrograph is normally packaged in￾side the phage head. 8885d_c24_920-947 2/11/04 1:36 PM Page 923 mac76 mac76:385_reb:

8885d_c24_920-9472/11/041:36 PM Page924mac76mac76:385 924 Chapter 24 Genes and Chromosomes description of the coiling of DNA molecules. Finally, we catalytic function. DNA also contains other segments or discuss the protein-DNA interactions that organize sequences that have a purely regulatory function. Reg hromosomes into compact structures ulatory sequences provide signals that may denote the beginning or the end of genes, or influence the tran scription of genes, or function as initiation points for 24.1 Chromosomal Elements replication or recombination(Chapter 28). Some genes can be expressed in different ways to generate multiple Cellular DNA contains genes and intergenic both of which may serve functions vital to the IS, gene products from one segment of DNA. The special le transcriptional and translational mechanisms that allow more complex genomes, such as those of C this are described in Chapters 26 through 28 cells, demand increased levels of chromosomal organi- We can make direct estimations of the minimum zation, and this is reflected in the chromosomes struc verall size of genes that encode proteins. As described tural features. We begin by considering the ditterent in detail in Chapter 27, each amino acid of a polypep- types of DNA sequences and structural elements within tide chain is coded for by a sequence of three consec- utive nucleotides in a single strand of DNA(Fig. 24-2) with these"codons"arranged in a sequence that corre- Genes Are Segments of DNA That Code sponds to the sequence of amino acids in the polypep- for Polypeptide Chains and RNAs tide that the gene encodes. a polypeptide chain of 350 amino acid residues(an average-size chain)corre- Our understanding of genes has evolved tremendot over the last century. Classically, a gene was defined as a portion of a chromosome that determines or affects a single character or phenotype(visible property), such DNA mRNA Polypeptide as eye color. George Beadle and Edward Tatum proposed a molecular definition of a gene in 1940. After exposing spores of the fungus Neurospora crassa to x rays and TIlIA other agents known to damage dNa and cause alterations in DNA sequence(mutations ), they detected mutant fungal strains that lacked one or another specific en- zyme, sometimes resulting in the failure of an entire TIIA metabolic pathway. Beadle and Tatum concluded that a gene is a segment of genetic material that determines or codes for one enzyme: the one gene-one enzym hypothesis. Later this concept was broadened to one TIIA gene-one polypeptide, because many genes code for proteins that are not enzymes or for one polypeptide of UACACUUUUG TIIA U a multisubunit protein. The modern biochemical definition of a gene is even more precise. a gene is all the dna that encodes the primary sequence of some final gene product, which can be either a polypeptide or an RNa with a structural or TIIA TIIA TIIA GCcGUUUCU CIlI TIIA termi Template strand FIGURE 24-2 Colinearity of the coding nucleotide sequences of DNA and mRNA and the amino acid sequence of a polypeptide chain. a protein through the intermediary mRNA. One of the DNA strands serves as a template for synthesis of mRNA, which has nucleotide triplets (codons)complementary to those of the DNA. In some bacte. orge W. Bead award L. Tatum al and many eukaryotic genes, coding sequences are interrupted at 1903-198 1909-1975 tervals by regions of noncoding sequences(called introns)

description of the coiling of DNA molecules. Finally, we discuss the protein-DNA interactions that organize chromosomes into compact structures. 24.1 Chromosomal Elements Cellular DNA contains genes and intergenic regions, both of which may serve functions vital to the cell. The more complex genomes, such as those of eukaryotic cells, demand increased levels of chromosomal organi￾zation, and this is reflected in the chromosome’s struc￾tural features. We begin by considering the different types of DNA sequences and structural elements within chromosomes. Genes Are Segments of DNA That Code for Polypeptide Chains and RNAs Our understanding of genes has evolved tremendously over the last century. Classically, a gene was defined as a portion of a chromosome that determines or affects a single character or phenotype (visible property), such as eye color. George Beadle and Edward Tatum proposed a molecular definition of a gene in 1940. After exposing spores of the fungus Neurospora crassa to x rays and other agents known to damage DNA and cause alterations in DNA sequence (mutations), they detected mutant fungal strains that lacked one or another specific en￾zyme, sometimes resulting in the failure of an entire metabolic pathway. Beadle and Tatum concluded that a gene is a segment of genetic material that determines or codes for one enzyme: the one gene–one enzyme hypothesis. Later this concept was broadened to one gene–one polypeptide, because many genes code for proteins that are not enzymes or for one polypeptide of a multisubunit protein. The modern biochemical definition of a gene is even more precise. A gene is all the DNA that encodes the primary sequence of some final gene product, which can be either a polypeptide or an RNA with a structural or catalytic function. DNA also contains other segments or sequences that have a purely regulatory function. Reg￾ulatory sequences provide signals that may denote the beginning or the end of genes, or influence the tran￾scription of genes, or function as initiation points for replication or recombination (Chapter 28). Some genes can be expressed in different ways to generate multiple gene products from one segment of DNA. The special transcriptional and translational mechanisms that allow this are described in Chapters 26 through 28. We can make direct estimations of the minimum overall size of genes that encode proteins. As described in detail in Chapter 27, each amino acid of a polypep￾tide chain is coded for by a sequence of three consec￾utive nucleotides in a single strand of DNA (Fig. 24–2), with these “codons” arranged in a sequence that corre￾sponds to the sequence of amino acids in the polypep￾tide that the gene encodes. A polypeptide chain of 350 amino acid residues (an average-size chain) corre- 924 Chapter 24 Genes and Chromosomes George W. Beadle, 1903–1989 Edward L. Tatum, 1909–1975 U C U A G A C G U G C A G G A C C T U A C A T G A C U T G A U U U A A A G C C C G G G U U C A A 5 3 3 5 DNA mRNA T C T C G T G G A T A C A C T T T T G C C G T T 3 5 Arg Gly Tyr Thr Phe Ala Val Ser Carboxyl terminus Amino terminus Polypeptide Template strand FIGURE 24–2 Colinearity of the coding nucleotide sequences of DNA and mRNA and the amino acid sequence of a polypeptide chain. The triplets of nucleotide units in DNA determine the amino acids in a protein through the intermediary mRNA. One of the DNA strands serves as a template for synthesis of mRNA, which has nucleotide triplets (codons) complementary to those of the DNA. In some bacte￾rial and many eukaryotic genes, coding sequences are interrupted at intervals by regions of noncoding sequences (called introns). 8885d_c24_920-947 2/11/04 1:36 PM Page 924 mac76 mac76:385_reb:

885c249252/12/0411:21 AM Page925mac76mac76:385ebd 24.1 Chromosomal elements sponds to 1, 050 bp. Many genes in eukaryotes and a few double-stranded. a typical medium-sized DNA virus is in prokaryotes are interrupted by noncoding DNA seg- bacteriophage A (lambda), which infects E. coli. In its ments and are therefore considerably longer than this replicative form inside cells, A DNA is a circular double simple calculation would suggest helix. This double-stranded DNA contains 48, 502 bp and How many genes are in a single chromosome? The has a contour length of 17.5 um Bacteriophage x174 Escherichia coli chromosome, one of the prokaryotic is a much smaller DNa virus; the DNA in the viral par genomes that has been completely sequenced, is a cir- ticle is a single-stranded circle, and the double-stranded cular DNA molecule (in the sense of an endless loop replicative form contains 5, 386 bp. Although viral rather than a perfect circle) with 4, 639, 221 bp. These genomes are small, the contour lengths of their dNAs base pairs encode about 4, 300 genes for proteins and are much greater than the long dimensions of the viral another 115 genes for stable RNA molecules. Among eu- particles that contain them. The DNA of bacteriophage karyotes, the approximately 3.2 billion base pairs of the T4, for example, is about 290 times longer than the vi human genome include 30,000 to 35,000 genes on 24 ral particle itself (Table 24-1) different chromosomes Bacteria A single E coli cell contains almost 100 times DNA Molecules Are Much Longer Than the Cellular as much DNA as a bacteriophage A particle. The chro- Packages That contain Them mosome of an E. coli cell is a single double-stranded circular DNA molecule. Its 4, 639, 221 bp have a contour Chromosomal DNAs are often many orders of magni- length of about 1.7 mm, some 850 times the length of tude longer than the cells or viruses in which they are the E coli cell (Fig. 24-3). In addition to the very large found(Fig. 24-1; Table 24-1). This is true of every class circular DNA chromosome in their nucleoid, many bac of organism or parasite teria contain one or more small circular DNa molecules that are free in the cytosol. These extrachromosomal Viruses Viruses are not free-living organisms; rather, elements are called plasmids(Fig. 244; see also they are infectious parasites that use the resources of a p. 311). Most plasmids are only a few thousand base host cell to carry out many of the processes they re- pairs long, but some contain more than 10,000 bp. They quire to propagate. Many viral particles consist of no carry genetic information and undergo replication to more than a genome(usually a single RNA or DNA mol- yield daughter plasmids, which pass into the daughter ecule) surrounded by a protein coat cells at cell division. Plasmids have been found in yeast Almost all plant viruses and some bacterial and an- and other fungi as well as in bacteria. imal viruses have rNa genomes. These genomes tend In many cases plasmids confer no obvious advan to be particularly small. For example, the genomes of tage on their host, and their sole function appears to b mammalian retroviruses such as HIv are about 9, 000 nu- self-propagation. However, some plasmids carry genes cleotides long, and that of the bacteriophage QB has that are useful to the host bacterium. For example 4, 220 nucleotides. Both types of viruses have single- some plasmid genes make a host bacterium resistant stranded RNA genomes to antibacterial agents. Plasmids carrying the gene for The genomes of DNA viruses vary greatly in size the enzyme B-lactamase confer resistance to B-lactam (Table 24-1). Many viral DNAs are circular for at least antibiotics such as penicillin and amoxicillin(see Box part of their life cycle. During viral replication within a 20-1). These and similar plasmids may pass from an host cell, specific types of viral DNA called replicative antibiotic-resistant cell to an antibiotic-sensitive cell of the forms may appear; for example, many linear DNAs be- same or another bacterial species, making the recipient lar and all single-stranded DNAs become cell antibiotic resistant. The extensive use of antibiotics TABLE 24-1 The Sizes of DNA and Viral Particles for Some Bacterial Viruses(Bacteriophages) Size of viral Length of DNA (bp) viral DNA (nm) viral particle(nm) dX174 5,386 1939 39936 14,377 A(lambda) 17.460 14 168889 60800 210 Note: Data on size of DNA are for the replicate form(double- stranded ). The contour length is calculated assuming that

sponds to 1,050 bp. Many genes in eukaryotes and a few in prokaryotes are interrupted by noncoding DNA seg￾ments and are therefore considerably longer than this simple calculation would suggest. How many genes are in a single chromosome? The Escherichia coli chromosome, one of the prokaryotic genomes that has been completely sequenced, is a cir￾cular DNA molecule (in the sense of an endless loop rather than a perfect circle) with 4,639,221 bp. These base pairs encode about 4,300 genes for proteins and another 115 genes for stable RNA molecules. Among eu￾karyotes, the approximately 3.2 billion base pairs of the human genome include 30,000 to 35,000 genes on 24 different chromosomes. DNA Molecules Are Much Longer Than the Cellular Packages That Contain Them Chromosomal DNAs are often many orders of magni￾tude longer than the cells or viruses in which they are found (Fig. 24–1; Table 24–1). This is true of every class of organism or parasite. Viruses Viruses are not free-living organisms; rather, they are infectious parasites that use the resources of a host cell to carry out many of the processes they re￾quire to propagate. Many viral particles consist of no more than a genome (usually a single RNA or DNA mol￾ecule) surrounded by a protein coat. Almost all plant viruses and some bacterial and an￾imal viruses have RNA genomes. These genomes tend to be particularly small. For example, the genomes of mammalian retroviruses such as HIV are about 9,000 nu￾cleotides long, and that of the bacteriophage Q
has 4,220 nucleotides. Both types of viruses have single￾stranded RNA genomes. The genomes of DNA viruses vary greatly in size (Table 24–1). Many viral DNAs are circular for at least part of their life cycle. During viral replication within a host cell, specific types of viral DNA called replicative forms may appear; for example, many linear DNAs be￾come circular and all single-stranded DNAs become double-stranded. A typical medium-sized DNA virus is bacteriophage (lambda), which infects E. coli. In its replicative form inside cells, DNA is a circular double helix. This double-stranded DNA contains 48,502 bp and has a contour length of 17.5 m. Bacteriophage X174 is a much smaller DNA virus; the DNA in the viral par￾ticle is a single-stranded circle, and the double-stranded replicative form contains 5,386 bp. Although viral genomes are small, the contour lengths of their DNAs are much greater than the long dimensions of the viral particles that contain them. The DNA of bacteriophage T4, for example, is about 290 times longer than the vi￾ral particle itself (Table 24–1). Bacteria A single E. coli cell contains almost 100 times as much DNA as a bacteriophage particle. The chro￾mosome of an E. coli cell is a single double-stranded circular DNA molecule. Its 4,639,221 bp have a contour length of about 1.7 mm, some 850 times the length of the E. coli cell (Fig. 24–3). In addition to the very large, circular DNA chromosome in their nucleoid, many bac￾teria contain one or more small circular DNA molecules that are free in the cytosol. These extrachromosomal elements are called plasmids (Fig. 24–4; see also p. 311). Most plasmids are only a few thousand base pairs long, but some contain more than 10,000 bp. They carry genetic information and undergo replication to yield daughter plasmids, which pass into the daughter cells at cell division. Plasmids have been found in yeast and other fungi as well as in bacteria. In many cases plasmids confer no obvious advan￾tage on their host, and their sole function appears to be self-propagation. However, some plasmids carry genes that are useful to the host bacterium. For example, some plasmid genes make a host bacterium resistant to antibacterial agents. Plasmids carrying the gene for the enzyme
-lactamase confer resistance to
-lactam antibiotics such as penicillin and amoxicillin (see Box 20–1). These and similar plasmids may pass from an antibiotic-resistant cell to an antibiotic-sensitive cell of the same or another bacterial species, making the recipient cell antibiotic resistant. The extensive use of antibiotics 24.1 Chromosomal Elements 925 TABLE 24–1 The Sizes of DNA and Viral Particles for Some Bacterial Viruses (Bacteriophages) Size of viral Length of Long dimension of Virus DNA (bp) viral DNA (nm) viral particle (nm) X174 5,386 1,939 25 T7 39,936 14,377 78 (lambda) 48,502 17,460 190 T4 168,889 60,800 210 Note: Data on size of DNA are for the replicative form (double-stranded). The contour length is calculated assuming that each base pair occupies a length of 3.4 Å (see Fig. 8–15). 8885d_c24_925 2/12/04 11:21 AM Page 925 mac76 mac76:385_reb: