Denaturation of circular DNA: Supercoils and overtwist

摘要

The denaturation transition of circular DNA is studied within a Poland-Scheraga-type approach, generalizednto account for the fact that the total linking number (LK), which measures the number of windings of one strandnaround the other, is conserved. In the model the LK conservation is maintained by invoking both overtwistingnand writhing (supercoiling) mechanisms. This generalizes previous studies, which considered each mechanismnseparately. The phase diagram of the model is analyzed as a function of the temperature and the elastic constantnκ associated with the overtwisting energy for any given loop entropy exponent c. As in the case where the twonmechanisms apply separately, the model exhibits no denaturation transition for c u0002 2. For c > 2 and κ = 0 wenfind that the model exhibits a first-order transition. The transition becomes of higher order for any κ > 0. Wenalso calculate the contribution of the two mechanisms separately in maintaining the conservation of the linkingnnumber and find that it is weakly dependent on the loop exponent c.