Editing DNA (section)

==== Topoisomerases and helicases ====
[[Topoisomerase]]s are enzymes with both nuclease and ligase activity. These proteins change the amount of [[DNA supercoil|supercoiling]] in DNA. Some of these enzymes work by cutting the DNA helix and allowing one section to rotate, thereby reducing its level of supercoiling; the enzyme then seals the DNA break.<ref name=Champoux /> Other types of these enzymes are capable of cutting one DNA helix and then passing a second strand of DNA through this break, before rejoining the helix.<ref>{{cite journal | vauthors = Schoeffler AJ, Berger JM | title = Recent advances in understanding structure-function relationships in the type II topoisomerase mechanism | journal = Biochemical Society Transactions | volume = 33 | issue = Pt 6 | pages = 1465–70 | date = December 2005 | pmid = 16246147 | doi = 10.1042/BST0331465 }}</ref> Topoisomerases are required for many processes involving DNA, such as DNA replication and transcription.<ref name=Wang />

[[Helicase]]s are proteins that are a type of [[molecular motor]]. They use the chemical energy in [[nucleoside triphosphate]]s, predominantly [[adenosine triphosphate]] (ATP), to break hydrogen bonds between bases and unwind the DNA double helix into single strands.<ref>{{cite journal | vauthors = Tuteja N, Tuteja R | title = Unraveling DNA helicases. Motif, structure, mechanism and function | journal = European Journal of Biochemistry | volume = 271 | issue = 10 | pages = 1849–63 | date = May 2004 | pmid = 15128295 | doi = 10.1111/j.1432-1033.2004.04094.x | url = http://repository.ias.ac.in/52775/1/40-pub.pdf | doi-access = free }}</ref> These enzymes are essential for most processes where enzymes need to access the DNA bases.