Ii

Adenine (A)

Figure 12.9. Sketch of the structures of the two pyrimidine nucleotide bases, cytosine <C) and thymine (t), and the two purine bases, guanine (G) and adenine (A). The points of attachment on the desoxyribose sugar of Fig. 12.8, entailing the loss of a hydrogen atom H, are indicated by vertical arrows. The horizontal arrows designate the complementary amino acid pairs.

The histone beads are joined together by lengths of double-stranded DNA in the "linker region" between the beads, shown in Fig. 12.1 lb. The DNA associated with the histories is called chromatin, and the histone bead with encircling DNA strands is called a nucleosome. The linker regions provide the nucleosome sequence with the great flexibility that is required for subsequent stages of folding.

In the next stage of compaction the nucleosomes stack one above the other, alternating between two coiled columns and connected by the linker strands, as shown laid out lengthwise in Fig. 12.1 lc. They now form a structure of 1-mm-long chromatin fibers 30 nm in diameter, a configuration called the "packing of the nucleosomes". The chromatin fibers then undergo the next higher order of folding shown in Fig. 12.lid, and this becomes condensed into 700-nm-wide hyperfoldings of the 300-nm-wide foldings, in the manner shown in Fig. 12.1 le. These various stages of compaction are held in place largely by relatively weak hydrogen bonds, which makes it feasible for the overall structure to unfold, partially or completely, for replication during cell division, or for transcription during the formation of ribonucleic acid (RNA) molecules, which bring about or direct the synthesis

basa pairing.

Figure 12.10. Model of the DNA double helix. The width (2nm), the spacing between nucleotides (0.34 nm), and the length of the repeat unit or pitch (3.4 nm) of the helix are indicated in the upper left. The sugar-phosphate backbones, the arrangements of the nucleotides (C, T, G, A), and the four cases of nucleotide base pairing are sketched in the lower right. [From Mader

of proteins. The final condensed structure of the metaphase chromosome, sketched in Fig. 12.1 If, is small enough to fit inside the nucleus of a cell.

The genome is the complete set of hereditary units called genes, each of which is responsible for a particular structure or function in the body, such as determining the color of one's eyes. In human beings the genome consists of 46 chromosomes (two haploid sets of 23 chromosomes), with an average of about 1600 genes arranged lengthwise along each chromosome. The human haploid genome contains about 3.4 x 109 base pairs, an average of about 150 x 106 base pairs per chromosome.

(a) DNA double helix

(b) chromatin

("beads on string")

(c) Packed nucleosomes

(d) extended section of chromosome

(e) condensed section of chromosome

chromosome

(b) chromatin

("beads on string")

(c) Packed nucleosomes

(e) condensed section of chromosome

30 nm

300 nm

700 nm

1400 nm

Figure 12.11.- Successive twistings and foldings during the packing of DMA into mammalian chromosomes, with the sizes at successive stages given in nanometers. [From R. J. Nossai and H. Lecar, Molecular and Cell Biophysics, Addison-Wesley, Boston, 1991, Fig. 4.9 (p. 118).]

30 nm

300 nm

700 nm

1400 nm

Figure 12.11.- Successive twistings and foldings during the packing of DMA into mammalian chromosomes, with the sizes at successive stages given in nanometers. [From R. J. Nossai and H. Lecar, Molecular and Cell Biophysics, Addison-Wesley, Boston, 1991, Fig. 4.9 (p. 118).]

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