The DNA double helix. A, Diagrammatic model of the helical structure, showing its dimensions, the major and minor grooves, the periodicity of the bases, and the antiparallel orientation of the backbone chains (represented by ribbons). The base pairs (represented by rods) are perpendicular to the axis and lie stacked one on another. B, The chemical structure of the backbone and bases of DNA, showing the sugar phosphate linkages of the backbone and the hydrogen bonding between the base pairs. There are two hydrogen bonds between adenine and thymine, and three between cytosine and guanine.