Base Pairs

A base pair is two nucleotide bases held together by hydrogen bonds across the two strands of a nucleic acid. In DNA adenine always pairs with thymine and guanine with cytosine, and the number of base pairs is the standard measure of the length of a DNA molecule.

The bases are of two types, the larger purines (adenine and guanine) and the smaller pyrimidines (cytosine and thymine). Pairing a purine with a pyrimidine keeps the double helix a uniform width of about 2 nanometers. A–T pairs share two hydrogen bonds and G–C pairs share three, so G–C-rich DNA is more stable and melts at a higher temperature. In RNA, uracil takes the place of thymine.

Neighboring base pairs are stacked about 0.34 nanometers apart, and the helix makes a full turn every 10 to 10.5 base pairs. The human genome has about 3.1 billion base pairs in a haploid set, and the shorter units used are kilobases (thousands) and megabases (millions). Chargaff's rule, that the amount of A equals T and G equals C, was an early clue to the pairing found by James Watson and Francis Crick.

Complementary pairing lets one strand serve as a template for the other, which makes accurate replication and transcription possible. Mispaired bases are detected by mismatch repair. Other pairings, such as the wobble base pair and Hoogsteen pairs, occur in RNA and in special DNA structures.