the complex of two heterocyclic nucleic acid bases, one a pyrimidine and the other a purine, brought about by hydrogen bonding between the purine and the pyrimidine; base pairing is the essential element in the structure of DNA proposed by J. Watson and F. Crick in 1953; usually guanine is paired with cytosine (G·C), and adenine with thymine (A·T) or uracil (A·U). SYN: nucleoside pair, nucleotide pair.
