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 headchapter 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: