© Wikimedia Commons, Public Domain. Author: brian0918, 22 November 2009.
This animation, created with a sophisticated molecular modeling program, shows detail of a DNA molecule's structure. The nitrogenous bases are on the outside of the double helix, and the hydrogen bonds connecting the bases in pairs form the ladder-like rungs. Note that the molecule twists clockwise rather than counterclockwise. Most DNA is a right-handed helix like this one, though other forms of the molecule exit in nature.