Khan Academy
Khan Academy: Test Prep: Mcat: Biomolecules: Dna: Dna Structure and Function
Provides detailed explanation of DNA's structure and function. Includes illustrations.
Other
Molecule of the Month: Dna (Java)
A good site that begins with the background and branches into the structure, function and applications of technology. This version of the page requires Java, and should open some applets that allow you to see the structures in 3-D.