形成DNA、RNA單體以及編碼遺傳信息的化學結構，也就是組成堿基對的堿基包括A、G、T、C、U。嚴格地說，堿基對是一對相互匹配的堿基（即A：T，G：C，A：U相互作用）被氫鍵連接起來。然而，它常被用來衡量DNA和RNA的長度（盡管RNA是單鏈）。它還與核苷酸互換使用，盡管后者是由一個五碳糖、磷酸和一個堿基組成。

The chemical structures that form the units of DNA and RNA and that encode genetic information i.e. the bases that make up the base-pairs are A, G, T, C and U (see DNA). Strictly, a base pair is a pair of complementary bases (i.e. the A:T, G:C and A:U interactions) held together by hydrogen bonds. However, this term is used loosely as a measure of DNA and RNA length (although RNA is single-stranded). The term is also used interchangeably with “nucleotide” although the latter is composed of a five-carbon sugar joined to a phosphate group and one of the bases.