The sugars and phosphate groups make up the backbone of DNA and therefore provide DNA’s structure; the backbone of DNA does not vary. The sequence of the bases does vary, however, and this is how genetic information is carried. The structure of part of a DNA chain is shown below. The three-dimensional structure of DNA was deduced in 1953 by James Watson and Francis Crick. Their model of DNA had five important features: 1. Two helical chains of polynucleotides are coiled around a common axis; the chains run in opposite directions, as shown in Figure 1. Figure 1. Model of a double-helical DNA. One chain is shown in green and the other in red. The purine and pyrimidine bases are shown in lighter colors than the sugar-phosphate backbone. (a) Axial view. The structure repeats along the helix axis (vertical) at intervals of 34 Å, which corresponds to 10 residues on each chain. (b) A schematic "ribbon" representation of an axial view of DNA. (Courtesy of M. Meselson and F. W. Stahl) (c) Radial view, looking down the helix axis. Reprinted with permission.1 2. The bases are on the inside of the helix, whereas the backbone (the sugars and phosphates) are on the outside.