A base pair is 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; in each case, the pairing involves a purine and a pyrimidine.
Source: National Human Genome Research Institute