Conformational fluctuations of DNA helix.

Treatment of a homogeneous sample of circular DNA molecules that contain at least one single-strand scission per molecule with ligase yields a population of covalently closed circles heterogeneous in the topological winding number alpha. Under proper conditions, the product molecules with different values of alpha can all be resolved by gel electrophoresis. The distributions in alpha for several DNAs, covalently closed at different temperatures, have been determined and the results have been interpreted in terms of thermal fluctuations of the DNA helix.