Opening image: The Watson-Crick double helix. The 20-bp DNA is shown here as a stick model. Hydrogen bonds between the base pairs are shown as dashed green lines.
The DNA double helix consists of a stack of planar base pairs between two spiraling sugar-phosphate backbones. Base pairs prefer to stack on one another rather than being exposed to water. JSmol's ability to rotate molecules allows you to appreciate the helical shape of the DNA molecule.