DNA topoisomerase is a class of enzymes existing in the nucleus, which can catalyze the breaking and binding of DNA strands and thus control the topological state of DNA. Topoisomerase participates in the regulation of the superhelix structure template. There are two main topoisomerases in mammals. DNA topoisomerase I catalyzes the change of topoisomerase state of DNA replication by forming a transient single chain cleavage - binding cycle. Topoisomerase II, in contrast, changes the topological state of THE DNA by causing a momentary break of the double-stranded enzyme bridge, then opening it up and re-closing it.