Denaturation of Circular DNA: Supercoil Mechanism

TL;DR

This paper extends the Poland-Scheraga model to analyze the denaturation transition in circular DNA, revealing how supercoiling influences the order of the transition and identifying a condensation transition at fixed linking number.

Contribution

It introduces a model incorporating winding degrees of freedom in circular DNA, showing how supercoiling affects the denaturation transition's nature and order.

Findings

First order transition for c>2 becomes continuous as c approaches 2

Melting under fixed linking number is a condensation transition

Transition order depends on the loop exponent c

Abstract

The denaturation transition which takes place in circular DNA is analyzed by extending the Poland-Scheraga model to include the winding degrees of freedom. We consider the case of a homopolymer whereby the winding number of the double stranded helix, released by a loop denaturation, is absorbed by \emph{supercoils}. We find that as in the case of linear DNA, the order of the transition is determined by the loop exponent $c$. However the first order transition displayed by the PS model for $c>2$ in linear DNA is replaced by a continuous transition with arbitrarily high order as $c$ approaches 2, while the second-order transition found in the linear case in the regime $1<c\le2$ disappears. In addition, our analysis reveals that melting under fixed linking number is a \emph{condensation transition}, where the condensate is a macroscopic loop which appears above the critical temperature.