Entry courtesy of
The National Human Genome Research Institute (NHGRI)

Base pair

Two bases which form a "rung of the DNA ladder." A DNA nucleotide is made of a molecule of sugar, a molecule of phosphoric acid, and a molecule called a base. The bases are the "letters" that spell out the genetic code. In DNA, the code letters are A, T, G, and C, which stand for the chemicals adenine, thymine, guanine, and cytosine, respectively. In base pairing, adenine always pairs with thymine, and guanine always pairs with cytosine.

Audio (launches external player)

Dr. Robert Nussbaum, of the National Human Genome Research Institute's Laboratory of Genetic Disease Research, defines base pair.