Experimental Analysis of Supercoiling in Twisted Polymer Line

Marlen Mahendraratnam, Allison Stiller, Stephen J Burns


It is well known that DNA has a double helix structure due to chemical interactions between its base paired molecules. These interactions, along with mechanical forces, cause DNA to bend and twist, which controls gene expression and other processes such as transcription and translation. Additionally, due to its extreme length, DNA must supercoil in order to achieve the extreme compact state necessary to fit on histones within a nucleus [1]. Current nano-torsional tests and mathematical models on DNA have hypothesized that at certain critical forces during twisting, DNA forms supercoils due to a torsional buckling instability of the structure; however, there is little experimental research to verify the proposed models of this process [1]. A typical test would have to measure the twist on the DNA molecule while applying a measured force and observing the onset of buckling.

A supercoil is defined in this paper as a loop structure created as a mechanism to release torsional energy. For the experiments described in this paper, an isotropic polymer line was twisted and stretched until it exhibited supercoiling when observed in a mechanical load frame.

Supercoiling was achieved by looping the polymer line through a weight and then knotting the ends around a stable hook. This doubled feature allowed the line to twist in a manner which mimics that of DNA: two strands crossing over each other. The weight was twisted until the first supercoil occurred. The radius of the formed loop was also measured. The data measured supports the hypothesis that the first supercoil occurs when a critical torque, or twist, is reached. Additionally, the results indicate that supercoiling is primarily a strain driven process, as the formation of the supercoils is dependent on shortening of the line caused by applied torsion. Furthermore, the line undergoes plastic deformation even after the supercoils are allowed to relax and the line returns to a state of zero applied torsion.


Supercoiling; DNA Mechanics

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