A base pair is an association of two nucleobases through hydrogen bonds in a compatible arrangement. It commonly joins opposite strands of a nucleic acid double helix, but can also join two regions of one folded RNA strand. Canonical Watson–Crick DNA base pairs match adenine with thymine and guanine with cytosine; canonical RNA base pairs use adenine with uracil instead. The nucleobases belong to complete nucleotides, including their attached sugars and backbone phosphate. See NHGRI: base pair.
The abbreviation bp also measures the length of a paired region. Our ACGTAC/TGCATG duplex has six base pairs, containing twelve nucleotides altogether. Its upper strand contains two adenine, two cytosine, one guanine, and one thymine nucleobase; the lower contains one, one, two, and two respectively. Combined, the counts are three of each nucleobase. Equality of total adenine and thymine, or of total guanine and cytosine, need not hold within either individual strand.
