A simple model of DNA superhelices in solution.

Closed circular DNA molecules in aqueous solution take the form of interwound superhelices over a wide range of superhelix densities. We describe a very simple model of such a superhelix in which twisting and bending forces are in balance, subject both to topological constraints and to a limitation on the distance of closet approach of the interwound duplexes of the superhelix. The model is consistent with some of the observed physical properties of closed circular DNA, and suggests that there may be severe limits to the range of allowable geometries for the superhelix structure of minimum energy.