Type I topoisomerase
（重定向自Topoisomerase I）
![](/Images/godic/202502/16/PDB_1vcc_EBI5407.jpg")
![Structure of topo III bound to single stranded DNA (pdb ID 1I7D). Note that the DNA resembles B-form DNA](/Images/godic/202502/16/TopoI5407.png")
![Important active site residues of Topoisomerase IB. DNA is colored raspberry.](/Images/godic/202502/16/Topoisomerase_IB_Active_Site5407.jpg")
![Active site of Topoisomerase IB. DNA is colored raspberry with a light-blue backbone. (PDB ID = 1A36)](/Images/godic/202502/16/1a36_topoisomerase-IB-active-site5407.png")
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.