Figure 1. (Top) Structure of double-stranded DNA. The backbones of the two intertwined strands, consisting of alternating deoxyribose and phosphate groups, are shown schematically as helices. The bases form pairs between the backbones, linked by hydrogen bonds (represented as white ellipses), with approximately 10 pairs per turn of the double helix. (Bottom) The four basepairs. Adenine and guanine are both purines, each consisting of a linked five- and six-atom ring of carbon and nitrogen. Thymine and cytosine are pyrimidines, consisting of a six-atom ring alone. The points at which each base attaches to the backbone are indicated by arrows: the G:C and A:T basepairs have nearly identical attachment points, and this symmetry is preserved when they are flipped to give C:G and A:T pairs. It is this feature that allows any sequence of bases to be accommodated within a regular double helix.