DNA double helix

View of a short piece of a DNA double helix. This is a molecular model generated from X-ray diffraction data. Carbon atoms are shown in black, nitrogen in blue, oxygen in red and phosphorus in purple. Hydrogen atoms are not shown. This is the standard double helical "B" form of DNA solved by Watson and Crick. The two sugar phosphate backbones are linked by the interaction of the base pairs - C with G and A with T. This piece is of DNA is approx. 20 bp long. A single gene is often tens of thousands of base pairs long.