Figure 1.

Biologically relevant topological structures of DNA. Depicted are schematics of the three topological forms of DNA that topoisomerases maintain and modulate. For simplicity, each line represents a double-stranded DNA helix, as shown by the upper left inset. As indicated by the arrow sizes, most type-2 topoisomerases shift the DNA topology equilibrium toward relaxing, unknotting and decatenating. Bacterial DNA gyrase and archaeal reverse gyrase are unique enzymes that introduce supercoils into DNA. This figure is reproduced from (3).