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Dynamics of DNA Supercoils

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Science  05 Oct 2012:
Vol. 338, Issue 6103, pp. 94-97
DOI: 10.1126/science.1225810

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Watching Supercoiled DNA

The DNA double helix can undergo additional twisting, or supercoiling, that plays a role in transciption and protein binding, in part by bringing distant DNA locations together. The process forms intertwined loops, called plectonemes, and van Loenhout et al. (p. 94, published online 13 September; see the Perspective by Sheinin and Wang) visualized plectoneme dynamics of fluorescently labeled, 21-kilobase tethered DNA molecules using magnetic tweezers to apply twisting forces. Plectonemes could diffuse along the DNA, but move more rapidly if they “hopped”—nucleating a plectoneme at a new position.

Abstract

DNA in cells exhibits a supercoiled state in which the double helix is additionally twisted to form extended intertwined loops called plectonemes. Although supercoiling is vital to many cellular processes, its dynamics remain elusive. In this work, we directly visualize the dynamics of individual plectonemes. We observe that multiple plectonemes can be present and that their number depends on applied stretching force and ionic strength. Plectonemes moved along DNA by diffusion or, unexpectedly, by a fast hopping process that facilitated very rapid (<20 milliseconds) long-range plectoneme displacement by nucleating a new plectoneme at a distant position. These observations directly reveal the dynamics of plectonemes and identify a mode of movement that allows long-distance reorganization of the conformation of the genome on a millisecond time scale.

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