DNA is supercoiled state can be relaxed by an enzymatic process. Which enzyme catalyses relaxation of supercoiled DNA?
Supercoiling is torsional strain generated in a closed DNA duplex, and topoisomerases are the enzymes that relieve this strain by transiently cleaving the phosphodiester backbone and allowing controlled strand passage.
Type-I topoisomerase makes a single-strand nick, lets the intact strand rotate about the break, and reseals it, changing the linking number in steps of one and relaxing negative and, in eukaryotes, positive supercoils.
Type-II topoisomerase cleaves both strands, passes an intact duplex through the gap, and reseals it in an ATP-dependent reaction, changing the linking number by two and also removing catenanes and knots.
DNA helicase unwinds the duplex at replication or repair forks but does not alter linking number, and DNA ligase seals nicks after replication or repair without cleaving intact DNA, so neither can relax supercoils.
Hence, both Type-I topoisomerase and Type-II topoisomerase relax supercoiled DNA.
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