page_title: Type I topoisomerase
section_title: Type IA topoisomerases
hierarchical_section_title: Type I topoisomerase / Type IA topoisomerases
caption_attribution_description: English: topo III bound to DNA generated by Pymol using pdb from PDB.
context_page_description: In molecular biology Type I topoisomerases are enzymes that cut one of the two strands of double-stranded DNA, relax the strand, and reanneal the strand. They are further subdivided into two structurally and mechanistically distinct topoisomerases: type IA and type IB.
  Type IA topoisomerases change the linking number of a circular DNA strand by units of strictly 1.
  Type IB topoisomerases change the linking number by multiples of 1.
  Historically, type IA topoisomerases are referred to as prokaryotic topo I, while type IB topoisomerases are referred to as eukaryotic topoisomerase. This distinction, however, no longer applies as type IA and type IB topoisomerases exist in all domains of life.
  Functionally, these subclasses perform very specialized functions. Prokaryotic topoisomerase I can only relax negative supercoiled DNA, whereas eukaryotic topoisomerase I can introduce positive supercoils, separating the DNA of daughter chromosomes after DNA replication, and relax DNA.
caption: Structure of topo III bound to single stranded DNA (pdb ID 1I7D). Note that the DNA resembles B-form DNA
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