Enzymes capable of altering the degree of supercoiling of double-stranded DNA molecules. Various topoisomerases can increase or relax supercoiling, convert single-stranded rings to intertwined double-stranded rings, tie and untie knots in single stranded and duplex rings or catenate and decatenate duplex rings. Any enzyme that cleaves only one strand of a DNA duplex and then reseals it is classified as a type I topoisomerase (Topo I). Type II topoisomerases (Topo II) change DNA topology by breaking and rejoining double-stranded DNA.
