Topoisomerases are enzymes that act on the topology of DNA. The double-helical configuration that DNA strands naturally reside in makes them difficult to separate, and yet they must be separated if enzymes are to transcribe the sequences that encode proteins, or if chromosomes are to be replicated. In so-called circular DNA, in which double helical segment is bent around and joined in a circle, the two strands are topologically linked, or knotted. They cannot be separated by any process that does not involve the breaking of strands. Topoisomerases catalyze and guide the unknotting of DNA.
Many respond to "supercoiling.
The insertion of viral DNA into chromosomes and other forms of recombination can also require the action of topoisomerases.
See also: DNA topology