Deoxyribonucleic acid (DNA) encodes the genetic information of all known living organisms. The structure of DNA is a double helix, consisting of two polymeric chains of ribose units connected by phosphate linkages. Each ribose unit is bonded to one of four bases: the purines adenine (A) and guanine (G) and the pyrimidines thymine (T) and cytosine (C). The two strands of the helix are connected by hydrogen bonds, with unique pairing between the bases A-T and G-C. The genetic code is specified by a particular sequence of A, G, T and C bases. For example, a fragment ⋯GATTACA⋯ in one strand, is paired with the complementary sequence ⋯CTAATGT⋯ in the other. Human DNA contains slightly more than 3 billion base pairs.
In this Demonstration, you can select either the "adenine–thymine" (A-T) or the "guanine–cytosine" (G-C) base pair. Then, by using the , sliders (or the mouse) and adjusting the rotation angle, you can move the T or C until hydrogen bonds to the complementary base A or G can form. Hydrogen bonds consist of linear sequences of the atoms N-HN or N-HO. Successful hydrogen bonds are shown by thick dashed lines. The A-T connection consists of two hydrogen bonds, while the G-C connection has three hydrogen bonds. Each base is bonded to a ribose molecule, at the position marked by a yellow sphere.