The Replication of DNA

According to the proposal of Watson and Crick (1953a), a DNA molecule consists of two polynucleotide chains wound helically about a common axis. The nitrogen base (adenine, guanine, thymine, or cytosine) at each level on one chain is hydrogen-bonded to the base at the same level on the other chain. Structural requirements allow the occurrence of only the hydrogen-bonded base pairs adenine-thymine and guanine-cytosine, resulting in a detailed complementariness between the two chains. The self-complementariness of this structure suggested to its inventors (Watson and Crick, 1953b) a definite and structurally plausible hypothesis for the replication of the DNA molecule. According to this idea, the elementary replicative act is a molecular duplication. The two chains separate, exposing the hydrogen-bonding sites of the bases. Then, in accord with the base-pairing restrictions, each chain serves as a template for the synthesis of its complement. Accordingly, each daughter molecule contains one...