Distributions of topological states in circular DNA

A distinctive feature of closed circular DNA molecules is their particular topological state, which cannot be altered by any conformational rearrangem ent short of breaking at least one strand. This topological constraint open s unique possibilities for experimental studies of the distributions of top ological states created in different ways. Primarily, the equilibrium distr ibutions of topological properties are considered in the review. It is desc ribed how such distributions can be obtained and measured experimentally, a nd how they can be computed. Comparison of the calculated and measured equi librium distributions over the linking number of complementary strands, equ ilibrium fractions of knots and links formed by circular molecules has prov ided much valuable information about the properties of the double helix. St udy of the steady-state fraction of knots and links created by type II DNA topoisomerases has revealed a surprising property of the enzymes: their abi lity to reduce these fractions considerably below the equilibrium level.