A simple demonstration of How Intermolecular Forces Make DNA helical

The DNA double helix provides a beautiful and easy to understand example of how intermolecular forces combine to determine macromolecular structure. A simple consideration of hydrogen bonds, dispersion forces, and ionic interactions explains why DNA is most stable as a helix. A model easily made from boxes and string illustrates the principles clearly. I present this demonstration to my general chemistry students after intermolecular forces have been introduced. The model may also be used in more advanced classes to explain how intercalators, such as ethidium bromide, unwind DNA when they wedge between base pairs.