An enzyme that participates in the overwinding or underwinding of DNA, controling the topological state of DNA replication during mitosis.
The winding problem of DNA arises due to the intertwined nature of its double-helical structure. During DNA replication and transcription, DNA becomes overwound ahead of a replication fork. If left unabated, this torsion would eventually stop the ability of DNA or RNA polymerases involved in these processes to continue down the DNA strand.
It binds to DNA and cut the phosphate backbone of either one or both the DNA strands, unwinding the DNA. And the DNA backbone is resealed at the end of the process.

(Types)
• Type I: Cuts one strand of a DNA double helix.
- - Type IA: Forms a covalent intermediate with the 5' end of DNA.
- - Type IB: Form a covalent intermediate with the 3' end of DNA.
- - Type IC (topoisomerase V): Similar mechanism with type IB.
• Type II: Cuts both strands of one DNA double helix.
- - Type IIA
- - Type IIB

By blocking the reaction of resealing the breaks in the DNA ultimately can result in cellular death (apoptosis). Topoisomerase inhibitors are cytotoxic agents that represent highly relevant for therapeutics against infectious and cancerous cells.

Cf. camptothecin, quinolone

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