Discontinuities at the DNA supercoiling transition

TL;DR

This paper investigates the abrupt transition in DNA supercoiling, providing experimental evidence and a theoretical framework to understand the discontinuities in extension and torque during plectoneme formation.

Contribution

It introduces a theory for the coexisting plectonemic state with four key parameters and tests plectoneme models against experimental data, highlighting discrepancies.

Findings

Discontinuities in extension and torque are observed at the supercoiling transition.

A four-parameter theory predicts how these discontinuities vary with DNA length.

Discrepancies between models and experiments are explained by the coexisting state parameters.

Abstract

While slowly turning the ends of a single molecule of DNA at constant applied force, a discontinuity was recently observed at the supercoiling transition, when a small plectoneme is suddenly formed. This can be understood as an abrupt transition into a state in which stretched and plectonemic DNA coexist. We argue that there should be discontinuities in both the extension and the torque at the transition, and provide experimental evidence for both. To predict the sizes of these discontinuities and how they change with the overall length of DNA, we organize a theory for the coexisting plectonemic state in terms of four length-independent parameters. We also test plectoneme theories, including our own elastic rod simulation, finding discrepancies with experiment that can be understood in terms of the four coexisting state parameters.