Topoisomerases are enzymes that regulate the overwinding or underwinding of DNA. The winding problem of DNA arises due to the intertwined nature of its double-helical structure. Topoisomerases are isomerase enzymes that act on the topology of DNA. Type I topoisomerase cuts one strand of a DNA double helix, relaxation occurs, and then the cut strand is reannealed. Type I topoisomerases are subdivided into two subclasses: type IA topoisomerases, which share many structural and mechanistic features with the type II topoisomerases, and type IB topoisomerases, which utilize a controlled rotary mechanism. Type II topoisomerase cuts both strands of one DNA double helix, pass another unbroken DNA helix through it, and then reanneal the cut strands. This class is also split into two subclasses: type IIA and type IIB topoisomerases, which possess similar structure and mechanisms.
