15.3 DNA

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Fig. 15.2 Polymerized DNA. The so-called3 prime3^, end is at the upper left end, and the5 prime5^, end is at the

lower right (after Ageno, 1967; reproduced with permission of the Accademia dei Lincei)

Each monomer unit has three parts: base, sugar, and phosphate (Fig. 15.2). The sugar

(deoxyribose) and phosphate are always the same; the possibility of storing infor-

mation arises by varying the base, for which there are four possibilities: the purines

adenine (A) and thymine (T), and the pyrimidines cytosine (C) and guanine (G). The

strand running from 5 prime5^, to 3 prime3^, is called the “sense” strand (i.e., it is used to specify

protein sequences via RNA), and the other one the “antisense” (antiparallel) strand.

Mainly, only one strand encodes this information and the complementary one serves

to correct damage (Sect. 14.7.2).

Each base has the very important property of being able to H-bond with one of the

other three, the complementary base, significantly better than any of the others. This

is perhaps the purest, most elementary example of molecular recognition. Hence,

a polymerized chain of monomers can serve as a template for the assembly of a

complementary strand. The purine pairs are linked by only two H-bonds, whereas

the pyrimidines are linked by three (Fig. 15.3). This means that the C–G base-pairing

melts (i.e., the H-bonds are broken) at a higher temperature than the A–T pairing.