Editing DNA (section)

=== Supercoiling ===
{{further|DNA supercoil}}

DNA can be twisted like a rope in a process called [[DNA supercoil]]ing. With DNA in its "relaxed" state, a strand usually circles the axis of the double helix once every 10.4 base pairs, but if the DNA is twisted the strands become more tightly or more loosely wound.<ref>{{cite journal | vauthors = Benham CJ, Mielke SP | s2cid = 1427671 | title = DNA mechanics | journal = Annual Review of Biomedical Engineering | volume = 7 | pages = 21–53 | year = 2005 | pmid = 16004565 | doi = 10.1146/annurev.bioeng.6.062403.132016 | url = http://pdfs.semanticscholar.org/ab63/d57290ebf9bc3536fd3f2257a2b509076fc1.pdf | archive-url = https://web.archive.org/web/20190301225243/http://pdfs.semanticscholar.org/ab63/d57290ebf9bc3536fd3f2257a2b509076fc1.pdf | url-status = dead | archive-date = 1 March 2019 }}</ref> If the DNA is twisted in the direction of the helix, this is positive supercoiling, and the bases are held more tightly together. If they are twisted in the opposite direction, this is negative supercoiling, and the bases come apart more easily. In nature, most DNA has slight negative supercoiling that is introduced by [[enzyme]]s called [[topoisomerase]]s.<ref name=Champoux>{{cite journal | vauthors = Champoux JJ | s2cid = 18144189 | title = DNA topoisomerases: structure, function, and mechanism | journal = Annual Review of Biochemistry | volume = 70 | pages = 369–413 | year = 2001 | pmid = 11395412 | doi = 10.1146/annurev.biochem.70.1.369 }}</ref> These enzymes are also needed to relieve the twisting stresses introduced into DNA strands during processes such as [[transcription (genetics)|transcription]] and [[DNA replication]].<ref name=Wang>{{cite journal | vauthors = Wang JC | title = Cellular roles of DNA topoisomerases: a molecular perspective | journal = Nature Reviews Molecular Cell Biology | volume = 3 | issue = 6 | pages = 430–40 | date = June 2002 | pmid = 12042765 | doi = 10.1038/nrm831 | s2cid = 205496065 }}</ref>