base pair, a pair of nucleotides in the complementary strands of a DNA molecule that interact through hydrogen bonding across the axis of the helix. One of the nucleotides in each pair is a purine (either adenine or guanine), and the other is a pyrimidine (either thymine or cytosine). Because of distinct hydrogen bonding capacity, adenine always pairs with thymine, and guanine always pairs with cytosine.
