- any of the pairs of the hydrogen-bonded purine and pyrimidine bases that form the links between the sugar-phosphate backbones of nucleic acid molecules: the pairs are adenine and thymine in DNA, adenine and uracil in RNA, and guanine and cytosine in both DNA and RNA.
- The pair of nitrogenous bases that connects the complementary strands of DNA or of double-stranded RNA and consists of a purine linked by hydrogen bonds to a pyrimidine: adenine-thymine and guanine-cytosine in DNA, and adenine-uracil and guanine-cytosine in RNA.
- Any of the pairs of nucleotides connecting the complementary strands of a molecule of DNA or RNA and consisting of a purine linked to a pyrimidine by hydrogen bonds. The base pairs are adenine-thymine and guanine-cytosine in DNA, and adenine-uracil and guanine-cytosine in RNA or in hybrid DNA-RNA pairing. Base pairs may be thought of as the rungs of the DNA ladder.