Watson-Crick base-pairing relationships

Figure 20.4 The bases present in RNA and DNA and the Watson-Crick base pairing relationships. Uracil is present in RNA but is replaced by thymine in DNA that is, the pairs C-G and T-A are found in DNA the pairs C-G but U-A are found in RNA. The pairing is brought about by hydrogen bonding, indicated by a broken line.

This is consistent with there not being enough space (20 °) for two purines to fit within the helix and too much space for two pyrimidines to get close enough to each other to form hydrogen bonds between them. These relationships are often called the rules of Watson-Crick base pairing. 

Watson and Crick (see Volume 1) immediately saw the relationship of the double helix to genetic replication. They proposed that each strand of the chromosome serves as a template to specify a new, complementary DNA strand. A template is a pattern for making something DNA acts as a template because each strand specifies the new daughter strand by base-pairing. This template feature makes DNA replication semiconservative after replication, each daughter chromosome has one strand of newly synthesized DNA and one strand of DNA from the parental chromosome. See Figure 8-6. 

When replication occurs, the two strands of the Watson-Crick double helix must separate so that each can serve as a template for the synthesis of its complement. Since the two strands are complementary to one another, each bears a definite sequence relationship to the other. When one strand acts as a template, it directs the synthesis of its complement. The product of the synthesis directed by each template strand is therefore a duplex molecule that is identical to the starting duplex. The process is accurate because of the specificity of base pairing and because the protein apparatus that catalyzes the replication can remove mismatched bases. 

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