DNA, B form, 20 Base Pairs. A computer-drawn simulation of the colored plastic space-filling molecular models used by chemists, with spheres at the Van-der-Waals radii of the respective atoms. The colors are the ones used with the standard models: hydrogen is white, nitrogen is blue, oxygen is red, and phosphorus is yellow. An exception is carbon, which is not colored black but rather dark blue to distinguish it from the back background. This model shows 20 bast pairs of DNA in the crystalline B form, first studied by Watson and Crick. It is thought that DNA in solution forms a similar double helix, except that there are 10.5 base pairs per turn of the helix, rather than exactly 10, as shown here for the crystal.