FIGURE 5-4 Comparison of A-Form and B-Form DNA. The sugar-phosphate backbones of the two polynucleotide strands, which are on the outside in both structures, are shown in red and blue; the bases (lighter shades) are oriented inward. (a) The B form of DNA has about 10.5 base pairs per helical turn. Adjacent stacked base pairs are 0.34 nm apart. (b) The more compact A form of DNA has 11 base pairs per turn, with a much deeper major groove and a much shallower minor groove than B-form DNA.

[Part (a) data from R. Wing et al., 1980, Nature 287:755, PDB ID 1bna. Part (b) data from B. N. Conner et al., 1984, J. Mol. Biol. 174:663, PDB ID 1ana.]