8885d_c24_920-9472/11/041:36 PM Page941mac76mac76:385 24.3 The structure of chromosomes TABLE 24-3 Types and Properties of Histones Content of basic amino Number of acids (% of total) Molecular amino acid Histone residues H1 21,130 223 29.5 113 10.9 125 16.0 15,273 35 19.6 11,236 10.8 13.7 The sizes of these histones vary somewhat from species to species. The numbers given here are for bovine histones. DNA bound DNA. Histone cores do not bind randomly to DNA; rather, they tend to position themselves at certain locations. This positioning is not fully understood but in some cases appears to depend on a local abundance of A=T base pairs in the dna helix where it is in contact with the histones(Fig. 24-29). The tight wrapping of the dna around the nucleosome's histone core requires compression of the minor groove of the helix at thes points, and a cluster of two or three a=f base pairs Histone makes this compression more likely ALk=0 Other proteins are required for the positioning of he nucleosome cores on DNA. In several organisms certain proteins bind to a specific dNa sequence and then facilitate the formation of a nucleosome core nearby. Precise positioning of nucleosome cores can play a role in the expression of some eukaryotic genes (Chapter 28) A-T pairs abundant One(net) negative Histone core FIGURE 24-28 Chromatin assembly. (a)Relaxed, closed-circular DNA.(b)Binding of a histone core to form a nucleosome induces on negative supercoil. In the absence of any strand breaks, a positive supercoil must form elsewhere in the DNA (ALk=0).(c) Relaxation FIGURE 24-29 Positioning of a nucleosome to make optimal use of of this positive supercoil by a topoisomerase leaves one net negative A=T base pairs where the histone core is in contact with the minor percoll(△=-1) groove of the DNA helix
bound DNA. Histone cores do not bind randomly to DNA; rather, they tend to position themselves at certain locations. This positioning is not fully understood but in some cases appears to depend on a local abundance of AUT base pairs in the DNA helix where it is in contact with the histones (Fig. 24–29). The tight wrapping of the DNA around the nucleosome’s histone core requires compression of the minor groove of the helix at these points, and a cluster of two or three AUT base pairs makes this compression more likely. Other proteins are required for the positioning of some nucleosome cores on DNA. In several organisms, certain proteins bind to a specific DNA sequence and then facilitate the formation of a nucleosome core nearby. Precise positioning of nucleosome cores can play a role in the expression of some eukaryotic genes (Chapter 28). 24.3 The Structure of Chromosomes 941 TABLE 24–3 Types and Properties of Histones Number of Content of basic amino Molecular amino acid acids (% of total) Histone weight residues Lys Arg H1* 21,130 223 29.5 11.3 H2A* 13,960 129 10.9 19.3 H2B* 13,774 125 16.0 16.4 H3 15,273 135 19.6 13.3 H4 11,236 102 10.8 13.7 * The sizes of these histones vary somewhat from species to species. The numbers given here are for bovine histones. FIGURE 24–28 Chromatin assembly. (a) Relaxed, closed-circular DNA. (b) Binding of a histone core to form a nucleosome induces one negative supercoil. In the absence of any strand breaks, a positive supercoil must form elsewhere in the DNA (Lk 0). (c) Relaxation of this positive supercoil by a topoisomerase leaves one net negative supercoil (Lk 1). DNA Histone core (a) (b) (c) One (net) negative supercoil Lk 0 Lk 1 topoisomerase Bound negative supercoil (solenoidal) Unbound positive supercoil (plectonemic) DNA Histone core A T pairs abundant FIGURE 24–29 Positioning of a nucleosome to make optimal use of AUT base pairs where the histone core is in contact with the minor groove of the DNA helix. 8885d_c24_920-947 2/11/04 1:36 PM Page 941 mac76 mac76:385_reb:
885024-920-9472/11041:36age942nac76ma76:385律 42 Chapter 24 Genes and Chromosomes FIGURE 24-30 The 30 nm fiber, a higher-order organization of nu leosomes. (a) Schematic illustration of the probable structure of the iber, showing nucleosome packing. (b) Electron micrograph Nucleosomes Are Packed into Successively FIGURE 24-31 A partially unraveled human chromosome, revealing Higher Order Structures numerous loops of DNA attached to a scaffoldlike structure. rapping of dna around a nucleosome core compacts the dnalength about sevenfold. The overall compaction in a chromosome, however, is greater than 10,000-fold- amounts of histone HI (located in the interior of the ample evidence for even higher orders of structural or- fiber) and topoisomerase Il. The presence of topoiso- ganization. In chromosomes isolated by very gentle merase lI further emphasizes the relationship between methods, nucleosome cores appear to be organized into DNA underwinding and chromatin structure. Topoiso- structure called the 30 nm fiber(Fig. 24-30). This merase Il is so important to the maintenance of chro- packing requires one molecule of histone HI per nucle- matin structure that inhibitors of this enzyme can osome core Organization into 30 nm fibers does not ex- tend over the entire chromosome but is punctuated by regions bound by sequence-specific(nonhistone )DNA binding proteins. The 30 nm structure also appears to depend on the transcriptional activity of the particular 30 nm Fiber Histone region of DNA Regions in which genes are being tran- scribed are apparently in a less-ordered state that con- little, if any, histon The 30 nm fber. a second level of chromatin or- ganization, provides an approximately 100-fold com H2A paction of the DNA. The higher levels of folding are not et understood, but it appears that certain regions of DNA associate with a nuclear scaffold(Fig. 24-31). The scaffold-associated regions are separated by loops of FIGURE 24-32 Loops of chromosomal DNA attached to a nuclear DNA with perhaps 20 to 100 kbp. The DNA in a loop scaffold. The DNA in the loops is packaged as 30 nm fibers, so the may contain a set of related genes. For example, in loops are the next level of organization. Loops often contain groups Drosophila complete sets of histone-coding genes seem of genes with related functions. Complete sets of histone-coding genes, to cluster together in loops that are bounded by scaf- as shown in this schematic illustration, appear to be clustered in loops fold attachment sites (Fig. 24-32). The scaffold itself of this kind. Unlike most genes, histone genes occur in multiple copies appears to contain several proteins, notably large in many eukaryotic genomes
Nucleosomes Are Packed into Successively Higher Order Structures Wrapping of DNA around a nucleosome core compacts the DNA length about sevenfold. The overall compaction in a chromosome, however, is greater than 10,000-fold— ample evidence for even higher orders of structural organization. In chromosomes isolated by very gentle methods, nucleosome cores appear to be organized into a structure called the 30 nm fiber (Fig. 24–30). This packing requires one molecule of histone H1 per nucleosome core. Organization into 30 nm fibers does not extend over the entire chromosome but is punctuated by regions bound by sequence-specific (nonhistone) DNAbinding proteins. The 30 nm structure also appears to depend on the transcriptional activity of the particular region of DNA. Regions in which genes are being transcribed are apparently in a less-ordered state that contains little, if any, histone H1. The 30 nm fiber, a second level of chromatin organization, provides an approximately 100-fold compaction of the DNA. The higher levels of folding are not yet understood, but it appears that certain regions of DNA associate with a nuclear scaffold (Fig. 24–31). The scaffold-associated regions are separated by loops of DNA with perhaps 20 to 100 kbp. The DNA in a loop may contain a set of related genes. For example, in Drosophila complete sets of histone-coding genes seem to cluster together in loops that are bounded by scaffold attachment sites (Fig. 24–32). The scaffold itself appears to contain several proteins, notably large amounts of histone H1 (located in the interior of the fiber) and topoisomerase II. The presence of topoisomerase II further emphasizes the relationship between DNA underwinding and chromatin structure. Topoisomerase II is so important to the maintenance of chromatin structure that inhibitors of this enzyme can kill 942 Chapter 24 Genes and Chromosomes FIGURE 24–30 The 30 nm fiber, a higher-order organization of nucleosomes. (a) Schematic illustration of the probable structure of the fiber, showing nucleosome packing. (b) Electron micrograph. FIGURE 24–31 A partially unraveled human chromosome, revealing numerous loops of DNA attached to a scaffoldlike structure. 30 nm 30 nm Fiber Histone genes Nuclear scaffold H1 H3 H4 H2B H2A FIGURE 24–32 Loops of chromosomal DNA attached to a nuclear scaffold. The DNA in the loops is packaged as 30 nm fibers, so the loops are the next level of organization. Loops often contain groups of genes with related functions. Complete sets of histone-coding genes, as shown in this schematic illustration, appear to be clustered in loops of this kind. Unlike most genes, histone genes occur in multiple copies in many eukaryotic genomes. (a) (b) 8885d_c24_920-947 2/11/04 1:36 PM Page 942 mac76 mac76:385_reb:
885024-920-9472/11041:36age943nac76ma76:385 24.3 The structure of chromosomes 943 rapidly dividing cells. Several drugs used in cancer chemotherapy are topoisomerase ll inhibitors that allow the enzyme to promote strand breakage but not the re sealing of the breaks Evidence exists for additional layers of organization in eukaryotic chromosomes, each dramatically enhanc. ing the degree of compaction. One model for achieving this compaction is illustrated in Figure 24-33. Higher order chromatin structure probably varies from chro- mosome to chromosome, from one region to the next in (10 coil a single chromosome, and from moment to moment in the life of a cell. No single model can adequately de scribe these structures. Nevertheless, the principle is clear: DNA compaction in eukaryotic chromosomes is likely to involve coils upon coils upon coils Dimensional Packaging of Nuclear Chromosomes One coil O rosettes) Condensed Chromosome structures Are Maintained by SMC Proteins a third major class of chromatin proteins, in addition to the histones and topoisomerases, is the SMc proteins One rosette (structural maintenance of chromosomes). The primary (6 loops) Nuclear structure of SMc proteins consists of five distinct do- mains(Fig. 24-34a). The amino-and carboxyl-terminal globular domains, N and C, each of which has part of an ATP hydrolytic site, are connected by two regions of a-helical coiled-coil motifs(see Fig 4-11) that are joined One loop by a hinge domain. The proteins are generally dimeric, (-75,000bp) forming a V-shaped complex that is thought to be tied together through their hinge domains(Fig. 24-34b) One N and one c domain come together to form a complete ATP hydrolytic site at each end of the V Proteins in the SMC family are found in all types of organisms, from bacteria to humans. Eukaryotes have 30 nm Fiber two major types, cohesins and condensins(Fig. 24-4 The cohesins play a substantial role in linking together sister chromatids immediately after replication and keeping them together as the chromosomes condense to metaphase. This linkage is essential if chromosomes Beads-on. astr are to segregate properly at cell division. The detailed form of mechanism by which cohesins link sister chromosomes chromatin and the role of ATP hydrolysis, are not yet understood The condensins are essential to the condensation of chromosomes as cells enter mitosis In the laboratory condensins bind to dna in a that creates pos- tive supercoils; that is, condensin binding causes the DNA to become overwound, in contrast to the unde DNA winding induced by the binding of nucleosomes. It is not yet clear how this helps to compact the chromatin though one possibility is presented in Figure 24-35 Bacterial DNA Is Also Highly Organized FIGURE 24-33 Compaction of DNA in a eukaryotic chromosome anization that We now turn briefly to the structure of bacterial chro- in the chromosomes of eukaryotes. The levels take the form of coils mosomes. Bacterial DNA is compacted in a structure upon coils In cells, the higher-order structures (above the 30 nm fibers) called the nucleoid, which can occupy a significant are unlikely to be as uniform as depicted here
rapidly dividing cells. Several drugs used in cancer chemotherapy are topoisomerase II inhibitors that allow the enzyme to promote strand breakage but not the resealing of the breaks. Evidence exists for additional layers of organization in eukaryotic chromosomes, each dramatically enhancing the degree of compaction. One model for achieving this compaction is illustrated in Figure 24–33. Higherorder chromatin structure probably varies from chromosome to chromosome, from one region to the next in a single chromosome, and from moment to moment in the life of a cell. No single model can adequately describe these structures. Nevertheless, the principle is clear: DNA compaction in eukaryotic chromosomes is likely to involve coils upon coils upon coils . . . ThreeDimensional Packaging of Nuclear Chromosomes Condensed Chromosome Structures Are Maintained by SMC Proteins A third major class of chromatin proteins, in addition to the histones and topoisomerases, is the SMC proteins (structural maintenance of chromosomes). The primary structure of SMC proteins consists of five distinct domains (Fig. 24–34a). The amino- and carboxyl-terminal globular domains, N and C, each of which has part of an ATP hydrolytic site, are connected by two regions of -helical coiled-coil motifs (see Fig. 4–11) that are joined by a hinge domain. The proteins are generally dimeric, forming a V-shaped complex that is thought to be tied together through their hinge domains (Fig. 24–34b). One N and one C domain come together to form a complete ATP hydrolytic site at each end of the V. Proteins in the SMC family are found in all types of organisms, from bacteria to humans. Eukaryotes have two major types, cohesins and condensins (Fig. 24–25). The cohesins play a substantial role in linking together sister chromatids immediately after replication and keeping them together as the chromosomes condense to metaphase. This linkage is essential if chromosomes are to segregate properly at cell division. The detailed mechanism by which cohesins link sister chromosomes, and the role of ATP hydrolysis, are not yet understood. The condensins are essential to the condensation of chromosomes as cells enter mitosis. In the laboratory, condensins bind to DNA in a manner that creates positive supercoils; that is, condensin binding causes the DNA to become overwound, in contrast to the underwinding induced by the binding of nucleosomes. It is not yet clear how this helps to compact the chromatin, although one possibility is presented in Figure 24–35. Bacterial DNA Is Also Highly Organized We now turn briefly to the structure of bacterial chromosomes. Bacterial DNA is compacted in a structure called the nucleoid, which can occupy a significant 24.3 The Structure of Chromosomes 943 Nuclear scaffold Two chromatids (10 coils each) One coil (30 rosettes) One rosette (6 loops) One loop (~75,000 bp) 30 nm Fiber “Beads-ona-string” form of chromatin DNA FIGURE 24–33 Compaction of DNA in a eukaryotic chromosome. Model for levels of organization that could provide DNA compaction in the chromosomes of eukaryotes. The levels take the form of coils upon coils. In cells, the higher-order structures (above the 30 nm fibers) are unlikely to be as uniform as depicted here. 8885d_c24_920-947 2/11/04 1:36 PM Page 943 mac76 mac76:385_reb:
885024-920-9472/11041:36age944nac76ma76:385律 944 Chapter 24 Genes and Chromosomes Coiled coil Coiled coil Condensin (+)(+)如opme Relaxed dna FIGURE 24-35 Model for the effect of condensins on DNA super- coiling. Binding of condensins to a closed-circular DNA in the pre ence of topoisomerase I leads to the production of positive supercoils (+) Wrapping of the DNA about the condensin introduces positive supercoils because it wraps in the opposite sense to a solenoidal su- percoil (see Fig. 24-24). The compensating negative supercoils(-)that (c) appear elsewhere in the DNA are then relaxed by topoisomerase I In the chromosome, it is the wrapping of the DNA about condensin that ontribute to dna condensatio namic molecule, possibly reflecting a requirement for more ready access to its genetic information. The bac- terial cell division cycle can be as short as 15 min whereas a typical eukaryotic cell may not divide for hours or even months. In addition, a much greater raction of prokaryotic DNa is used to encode rNA and/or protein products. Higher rates of cellular me tabolism in bacteria mean that a much higher propor- tion of the dna is being transcribed or replicated at FIGURE 24-34 Structure of SMC proteins. (a)The five domains of a given time than in most eukaryotic cells the SMC primary structure. N and C denoted the amino-terminal and rboxyl-terminal domains, respectively. (b) Each polypeptide is oled so that the two coiled-coil domains wrap around each other and the N and C domains come together to form a complete ATP form the dimeric V-shaped molecule. (c) Electron micrograph of SMC eins from Bacillus subtilis raction of the cell volume(Fig. 24-36). The DNA ap- pears to be attached at one or more points to the inner surface of the plasma membrane. Much less is known about the structure of the nucleoid than of eu- karyotic chromatin. In E. coli, a scaffoldlike structure appears to organize the circular chromosome into a series of looped domains, as described above for chro- natin. Bacterial DNA does not seem to have any struc ture comparable to the local organization provided by nucleosomes in eukaryotes. Histonelike proteins are abundant in E. coli-the best-characterized example FIGURE 24-36 E coli cells showing nucleoids. The DNA is staine is a two-subunit protein called HU (Mr 19,000)-but with a dye that fluoresces when exposed to UV light. The light area these proteins bind and dissociate within minutes, and defines the nucleoid. Note that some cells have replicated their DNA no regular, stable DNA-histone structure has been but have not yet undergone cell division and hence have multiple found. The bacterial chromosome is a relatively dy
fraction of the cell volume (Fig. 24–36). The DNA appears to be attached at one or more points to the inner surface of the plasma membrane. Much less is known about the structure of the nucleoid than of eukaryotic chromatin. In E. coli, a scaffoldlike structure appears to organize the circular chromosome into a series of looped domains, as described above for chromatin. Bacterial DNA does not seem to have any structure comparable to the local organization provided by nucleosomes in eukaryotes. Histonelike proteins are abundant in E. coli—the best-characterized example is a two-subunit protein called HU (Mr 19,000)—but these proteins bind and dissociate within minutes, and no regular, stable DNA-histone structure has been found. The bacterial chromosome is a relatively dynamic molecule, possibly reflecting a requirement for more ready access to its genetic information. The bacterial cell division cycle can be as short as 15 min, whereas a typical eukaryotic cell may not divide for hours or even months. In addition, a much greater fraction of prokaryotic DNA is used to encode RNA and/or protein products. Higher rates of cellular metabolism in bacteria mean that a much higher proportion of the DNA is being transcribed or replicated at a given time than in most eukaryotic cells. 944 Chapter 24 Genes and Chromosomes Condensin + Relaxed DNA (–) topoisomerase I (–) (+)(+) (+)(+) FIGURE 24–35 Model for the effect of condensins on DNA supercoiling. Binding of condensins to a closed-circular DNA in the presence of topoisomerase I leads to the production of positive supercoils (). Wrapping of the DNA about the condensin introduces positive supercoils because it wraps in the opposite sense to a solenoidal supercoil (see Fig. 24–24). The compensating negative supercoils () that appear elsewhere in the DNA are then relaxed by topoisomerase I. In the chromosome, it is the wrapping of the DNA about condensin that may contribute to DNA condensation. 2 m FIGURE 24–36 E. coli cells showing nucleoids. The DNA is stained with a dye that fluoresces when exposed to UV light. The light area defines the nucleoid. Note that some cells have replicated their DNA but have not yet undergone cell division and hence have multiple nucleoids. ATP ATP (a) N Hinge Coiled coil Coiled coil 50 nm C (b) (c) FIGURE 24–34 Structure of SMC proteins. (a) The five domains of the SMC primary structure. N and C denoted the amino-terminal and carboxyl-terminal domains, respectively. (b) Each polypeptide is folded so that the two coiled-coil domains wrap around each other and the N and C domains come together to form a complete ATPbinding site. Two of these domains are linked at the hinge region to form the dimeric V-shaped molecule. (c) Electron micrograph of SMC proteins from Bacillus subtilis. 8885d_c24_920-947 2/11/04 1:36 PM Page 944 mac76 mac76:385_reb:
850249452/12/041:22Mage945mac76mac76:385reb Chapter 24 Further Reading with this overview of the complexity of DNA struc I Nucleosomes are organized into 30 nm fibers ture, we are now ready to turn, in the next chapter, to d the fibers are extensively folded to provide a discussion of dna metabolism the 10,000-fold compaction required to fit a typical eukaryotic chromosome into a cell SUMMARY 24. 3 The Structure of chromosomes nucleus. The higher-order folding involves attachment to a nuclear scaffold that contains I The fundamental unit of organization in the histone Hl, topoisomerase Il, and SMC chromatin of eukaryotic cells is the proteins nucleosome, which consists of histones and a Bacterial chromosomes are also extensively 200 bp segment of DNA. A core protein compacted into the nucleoid, but the particle containing eight histones(two copies chromosome appears to be much more each of histones H2A, H2B, H3, and H4)is dynamic and irregular in structure than encircled by a segment of DNA (about 146 bp) eukaryotic chromatin, reflecting the shorter cell in the form of a left-handed solenoidal cycle and very active metabolism of a bacterial supercoil Key Terms Terms in bold are defined in the glossary. exon 928 enwinding genome 923 simple-sequence linking number 933 hromatin 938 chromosome 923 DNA 929 phenotype 924 satellite dna nucleosome 938 mutation 92 centromere superhelical fiber 942 reg density 933 SMC proteins 943 sequence 924 supercoil 930 cohesins 943 relaxed DNA 930 condensins 943 intron 928 plectonemic 937 nucleoid 94 Further Reading Genera Goffeau, A, Barrell, B.G., Bussey, H, Davis, R W, Dujon, B Blattner, F.R., Plunkett, G, Il, Bloch, C.A., Perna, N.T. Feldmann, H, Galibert, F, Hoheisel, J D, Jacq, C. Burland, V, Riley, M, collado- Vides, J, Glasner, J D, Rode, johnston, M, et al. (1996) Life with 6000 genes. Science 274, C.K., Mayhew, G F, et al.(1997 The complete genome 546.563-567 sequence of Escherichia coli K-12 Science 277, 1453-1474. Report of the first complete sequence of a eukaryotic genome. the yeast Saccharomyces cerevisiae. Cozzarelli, N.R.& Wang, J C(eds)(1990) DNA Topology and Greider, C.w.& Blackburn, EH(1996)Telomeres,telomerase Its Biological Effects, Cold Spring Harbor Laboratory Press, Cold and cancer. Sci. Am. 274 (February), 92-97 pring Harbor, NY. Huxley, C.(1997) Mammalian artificial chromosomes and chromo ormberg, A.& Baker, TA.(1991) DNA Replication, 2nd edn, w. H. Freeman Company, New York. Lander, E.S., Linton, L.M., Birren, B, Nusbaum, C, Zody, a good place to start for further information on the structure M.C., Baldwin, J, Devon, K, Dewar, K, Doyle, M. nd function of DNA FitzHugh, W, et al. (2001) Initial sequencing and analysis of the Lodish, H, Berk, A, Matsudaira, P, Kaiser, C.A., Krieger, human genome. Nature 409, 860-921 M, Scott, M.P., Zipursky, S L, Darnell, J(2003)Molecular One of the first reports on the draft sequence of the human Cell Biology, 5th edn, W H. Freeman Company, New York. Another excellent general reference Long, M, de Souza, S.J.,& Gilbert, w.(1995)Evolution of the Genes and Chromosomes tron-exon structure of eukaryotic genes. Curr: Opin. Genet. De.5,774-778. Bromham, L(2002)The human zoo: endogenous retroviruses in the human genome. Trends Ecol Evolut. 17, 91-97 McEachern, M.J., Krauskopf, A,& Blackburn, EH(2000) A thorough description of one of the transposon classes that Telomeres and their control. Annu. Re. Genet. 34. 331-358. makes up a large part of the human genome
With this overview of the complexity of DNA structure, we are now ready to turn, in the next chapter, to a discussion of DNA metabolism. SUMMARY 24.3 The Structure of Chromosomes ■ The fundamental unit of organization in the chromatin of eukaryotic cells is the nucleosome, which consists of histones and a 200 bp segment of DNA. A core protein particle containing eight histones (two copies each of histones H2A, H2B, H3, and H4) is encircled by a segment of DNA (about 146 bp) in the form of a left-handed solenoidal supercoil. ■ Nucleosomes are organized into 30 nm fibers, and the fibers are extensively folded to provide the 10,000-fold compaction required to fit a typical eukaryotic chromosome into a cell nucleus. The higher-order folding involves attachment to a nuclear scaffold that contains histone H1, topoisomerase II, and SMC proteins. ■ Bacterial chromosomes are also extensively compacted into the nucleoid, but the chromosome appears to be much more dynamic and irregular in structure than eukaryotic chromatin, reflecting the shorter cell cycle and very active metabolism of a bacterial cell. Chapter 24 Further Reading 945 Key Terms gene 921 genome 923 chromosome 923 phenotype 924 mutation 924 regulatory sequence 924 plasmid 925 intron 928 exon 928 simple-sequence DNA 929 satellite DNA 929 centromere 930 telomere 930 supercoil 930 relaxed DNA 930 topology 931 underwinding 932 linking number 933 specific linking difference () 933 superhelical density 933 topoisomers 934 topoisomerases 935 plectonemic 937 solenoidal 937 chromatin 938 histones 938 nucleosome 938 30 nm fiber 942 SMC proteins 943 cohesins 943 condensins 943 nucleoid 943 Terms in bold are defined in the glossary. Further Reading General Blattner, F.R., Plunkett, G., III, Bloch, C.A., Perna, N.T., Burland, V., Riley, M., Collado-Vides, J., Glasner, J.D., Rode, C.K., Mayhew, G.F., et al. (1997) The complete genome sequence of Escherichia coli K-12. Science 277, 1453–1474. New secrets of this common laboratory organism are revealed. Cozzarelli, N.R. & Wang, J.C. (eds) (1990) DNA Topology and Its Biological Effects, Cold Spring Harbor Laboratory Press, Cold Spring Harbor, NY. Kornberg, A. & Baker, T.A. (1991) DNA Replication, 2nd edn, W. H. Freeman & Company, New York. A good place to start for further information on the structure and function of DNA. Lodish, H., Berk, A., Matsudaira, P., Kaiser, C.A., Krieger, M., Scott, M.P., Zipursky, S.L., & Darnell, J. (2003) Molecular Cell Biology, 5th edn, W. H. Freeman & Company, New York. Another excellent general reference. Genes and Chromosomes Bromham, L. (2002) The human zoo: endogenous retroviruses in the human genome. Trends Ecol. Evolut. 17, 91–97. A thorough description of one of the transposon classes that makes up a large part of the human genome. Goffeau, A., Barrell, B.G., Bussey, H., Davis, R.W., Dujon, B., Feldmann, H., Galibert, F., Hoheisel, J.D., Jacq, C., Johnston, M., et al. (1996) Life with 6000 genes. Science 274, 546, 563–567. Report of the first complete sequence of a eukaryotic genome, the yeast Saccharomyces cerevisiae. Greider, C.W. & Blackburn, E.H. (1996) Telomeres, telomerase and cancer. Sci. Am. 274 (February), 92–97. Huxley, C. (1997) Mammalian artificial chromosomes and chromosome transgenics. Trends Genet. 13, 345–347. Lander, E.S., Linton, L.M., Birren, B., Nusbaum, C., Zody, M.C., Baldwin, J., Devon, K., Dewar, K., Doyle, M., FitzHugh, W., et al. (2001) Initial sequencing and analysis of the human genome. Nature 409, 860–921. One of the first reports on the draft sequence of the human genome, with lots of analysis and many associated articles. Long, M., de Souza, S.J., & Gilbert, W. (1995) Evolution of the intron-exon structure of eukaryotic genes. Curr. Opin. Genet. Dev. 5, 774–778. McEachern, M.J., Krauskopf, A., & Blackburn, E.H. (2000) Telomeres and their control. Annu. Rev. Genet. 34, 331–358. 8885d_c24_945 2/12/04 11:22 AM Page 945 mac76 mac76:385_reb: