DNA topoisomerase I

DNA topoisomerases regulate the number of topological links between two DNA strands. Topoisomerase I is an ATP-independant enzyme that catalyses the reaction of supercoiled DNA, cleaving one strand and creating conformational freedom. Topoisomerase I will bind to a DNA molecule and cleave one strand, simultaenously generating a phosphoester bond between teh 5' free phosphate on the DNA and a tyrosine risdue found in the enzyme. The free 3' hydorxyl of the DNA is noncovalently held by the enzyme. The strand that had not been cleaves passes thorugh sinle strand break, this causes the resealing of the cleaved strand of the DNA. The newly resealed DNA strand has the exact chemical structure as it initially did prior form, the only difference that can be found it that teh DNA strand has now one less negatively supercoil. 

Topoisomerase has different effects in eukaryotes and prokaryotes. In eukaryotes topoisomerase I enzymes are able to remove both positive and negative supercoil, whilst in prokaryotes they are only able to remove negative supercoils.