Figure 1.

Topological states of DNA. DNA containing no torsional stress is considered “relaxed” (top middle). Underwinding or overwinding the DNA results in negatively supercoiled [(−)SC, top left] or positively supercoiled [(+)SC, top right] DNA. DNA supercoiling is depicted as writhe for visual clarity, but twist and writhe are interconvertible within these molecules. Intermolecular catenanes (middle) and intramolecular knots (bottom) can also form in DNA. In these cases, twist and writhe are not interconvertible. Type II enzymes (human topoisomerase IIα, hTIIα; human topoisomerase IIβ, hTIIβ; gyrase; topoisomerase IV, topo IV) that can perform each of the reactions to alter topological states are also listed. Created with BioRender.com.