Supercoiled DNA: Structure

Supercoiling is introduced into DNA molecules when the double helix is twisted around its own axis in three‐dimensional space. Experimental techniques that are sensitive to molecular shape can be used to analyse the topological states of DNA, but the approaches used most successfully are high-speed centrifugation, high-resolution microscopy and gel electrophoresis. Generally, DNA molecules are negatively supercoiled inside cells, although the level of supercoiling is not equal throughout the genome and many supercoils may be constrained by bound proteins. Supercoiling increases the free energy of DNA and influences DNA metabolism by promoting or hindering specific enzymatic processes. DNA topoisomerases are the main enzymes that regulate DNA topology and several different types of enzymes are present in all cells.