They are called plectonemes. They are the analog of the twisted tangles of telephone cords, and can form in DNA that is pulled through a nanoscale channel. This is the finding of a study led by Ulrich Keyser from the University of Cambridge and Cristian Micheletti from SISSA-Scuola Internazionale Superiore di Studi Avanzati in Trieste, conducted in collaboration with Antonio Suma from the Interuniversity Department of Physics of the University of Bari and Politecnico of Bari and the team of Aleksei Aksimentiev from the University of Illinois.
By combining experiments and simulations, the international team revealed what happens when a DNA strand is pulled through a nanopore. The fluid flow accompanying DNA translocation generates a torsion that propagates upstream along the double helix, causing the formation of plectonemes, or supercoils. These tangled structures produce a characteristic signature in the current measured during translocation, a property that had previously gone unnoticed, and possibly confused with the much shorter signal produced by DNA knots.
Studying DNA supercoils is relevant to understanding how genomic DNA is organised in the cell nucleus, where plectonemes may help to maintain order and compactness in chromosomes. The use of nanopores may therefore open new perspectives for studying the action of specific enzymes responsible for generating and eliminating supercoils. The research is published in Phys Rev. X.