Identification of Phase Transition and Twist-Stretch Coupling During DNA Supercoiling

Abstract

As a single DNA molecule is supercoiled under tension, the onset of a phase transition is most definitively revealed by a torque plateau. However, in the absence of direct torque detection, a change in the observed extension has often been used to identify the onset of a phase transition. Here by directly measuring torque using an angular optical trap, we show that an extension maximum, which has previously been assumed to be indicative of the onset of a phase transition from a B to sc-P-form DNA, in fact does not coincide with the onset of a torque plateau at a phase transition.