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
University of Cincinnati: Dna Structure and Function
This site explores the history and the science of DNA. Content also examines and considers the possibilities of genetic engineering and mutation.
University of Arizona
Biology Project: Cell Cycle and Mitosis Tutorial: Dna Basics
Tutorial on DNA basics that goes along with an online quiz over the material. Use the naviagation buttons at the bottom of the page to go to the quiz over the material.
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.