[Why is the denaturation process of super-helical DNA so uncooperative?]

A theoretical model is proposed for the helix-coil transition in circular covalently closed DNA. In the melting interval the topological constrains for such a molecule are considered to affect both the inner state of the melted regions and the spatial configuration of the whole molecule ("writhing"). A good agreement is achieved between the theoretically calculated and experimentally obtained parameters of the denaturation process. A conclusion is drawn, that at the early stages of thermodenaturation (approximately till the transition midpoint) the effect of topological constrains is realised mainly through the writhing of the molecule. The denatured regions exist in the form of relaxed loops.