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 (n).