Evidence for a Solenoid Phase of Supercoiled DNA

TL;DR

This study reveals a solenoid phase in supercoiled DNA, characterized by stepwise plectoneme lengthening and improved modeling of DNA mechanics, enhancing understanding of DNA supercoiling behavior.

Contribution

It introduces evidence for a solenoid phase in supercoiled DNA and develops a model incorporating writhe fluctuations to better describe DNA mechanics.

Findings

Discontinuous extension changes during DNA overwinding.

Stepwise plectoneme lengthening observed.

Enhanced model for DNA torsional modulus.

Abstract

In mechanical manipulation experiments, a single DNA molecule overwound at constant force undergoes a discontinuous drop in extension as it buckles and forms a superhelical loop (a plectoneme). Further overwinding the DNA, we observe an unanticipated cascade of highly regular discontinuous extension changes associated with stepwise plectoneme lengthening. This phenomenon is consistent with a model in which the force-extended DNA forms barriers to plectoneme lengthening caused by topological writhe. Furthermore, accounting for writhe in a fluctuating solenoid gives an improved description of the measured force-dependent effective torsional modulus of DNA, providing a reliable formula to estimate DNA torque. Our data and model thus provide context for further measurements and theories that capture the structures and mechanics of supercoiled biopolymers.