DNA Supercoils

The structure of DNA is ordinarily represented as a double helix.  In fact, functional DNA found within cells has an additional level of complexity – the double helix also twists upon itself resulting in “extended intertwined loops” called plectonemes.  Since it is well established that there is close and necessary relationship between structure and function in the biological realm, it is of immense scientific interest to understand the dynamics of this supercoiling property.

DNA, of course, possesses the blueprint upon which life is based – it contains the information that is used to construct the structural and enzymatic proteins that are essential for life.  In order to fulfill its role successfully, the genomic processes depend upon exquisite and precise mechanisms to control the expression of genes.  Furthermore, since the complex structure of DNA involving supercoils plays a pivotal role in these control mechanisms, it would be of interest to understand the dynamics of the individual plectonemes.

Current understanding of supercoiling suggests that this phenomenon is caused by the movement of proteins along the path of the DNA molecule.  This movement produces perturbations in the DNA structure causing the DNA to twist or writhe- the coiling of the DNA around itself.  The overall impact of these conformational changes induces both local and global effects.

A locally-derived distortion or destabilization of the DNA can alter transcription – the process by which the information contained in genes is transcribed to messenger RNA (m-RNA) – or induce binding to the DNA.  A global change in the overall conformation of DNA can bring distant sections of the DNA together resulting in genetic recombination.

Heretofore, it has proved to be an immense technological problem to study the actual dynamics of supercoiling since analysis has relied, almost exclusively, upon static imaging.  Dr. M.T.J. van Loenhout and his colleagues from the Delft University of Technology, Department of Bionanoscience, Kavti Institute of Nanoscience in Delft Netherlands have overcome this obstacle by designing what they refer to as, “single-molecule magnetic tweezers.”  With this new analytical tool, they have been able to study the real-time dynamics of individual plectonemes.

Van Loenhout and his co-workers have found that plectonemes move along the DNA by simple diffusion or what they refer to as, “fast hopping” that enables long range plectoneme displacement.  These conclusions as to the nature of the supercoiling of DNA are extremely important for they help elucidate the dynamics of a process that is fundamental to the nature of DNA within living cells.