From ISOGG Wiki
A base pair is two chemical bases bonded to one another forming a "rung of the DNA ladder". The DNA molecule consists of two strands that wind around each other like a twisted ladder. Each strand has a backbone made of alternating sugar (deoxyribose) and phosphate groups. Attached to each sugar is one of four bases - adenine (A), cytosine (C), guanine (G), or thymine (T). The two strands are held together by hydrogen bonds between the bases, with adenine forming a base pair with thymine, and cytosine forming a base pair with guanine.
This article uses material in the public domain from the Talking Glossary of Genetic Terms and is reproduced courtesy of the National Human Genome Research Institute.