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15

The Molecules of Life

Fig. 15.3 The hydrogen-bonding patterns of complementary bases (thymine [T], adenine [A], gua-

nine [G], cytosine [C], moving round clockwise from the upper left) (after Ageno, 1967; reproduced

with permission of the Accademia dei Lincei). In RNA, uracil (U) replaces thymine (i.e., the methyl

group on the base is replaced by hydrogen) and the ribose has a hydroxyl group. The lower pair is

denoted by CpG (Sect. 14.8.4)

As expected from their aromatic structure, the bases are planar. Figure 15.4 shows

the formation of the double helix. The genes of most organisms are formed by such

a double helix. The melting of the H-bonds as the temperature is raised is highly

coöperative (due to the repulsive electrostatic force between the charged phosphate

groups). On average, the separation into single stranded DNA occurs at about 80Superscript degrees^◦C

(at about 90Superscript degrees^◦C for sequences rich in C–G pairs, and at about 65Superscript degrees^◦C for sequences

rich in A–T pairs). These melting temperatures are lower at extremes of pH. Melting

leads to the complete separation of the two chains, which is made use of in artificial