Editing DNA (section)

==== Nucleases and ligases ====
[[File:EcoRV 1RVA.png|thumb|left|upright=1.1|The [[restriction enzyme]] [[EcoRV]] (green) in a complex with its substrate DNA<ref>{{Cite web| vauthors = Kostrewa D, Winkler FK |title=RCSB PDB – 1RVA: Mg2+ binding to the active site of EcoRV endonuclease: a crystallographic study of complexes with substrate and product DNA at 2 Å resolution |url=https://www.rcsb.org/structure/1RVA|access-date=2023-03-27|website=www.rcsb.org|language=en-US}}</ref>]]
[[Nuclease]]s are [[enzyme]]s that cut DNA strands by catalyzing the [[hydrolysis]] of the [[phosphodiester bond]]s. Nucleases that hydrolyse nucleotides from the ends of DNA strands are called [[exonuclease]]s, while [[endonuclease]]s cut within strands. The most frequently used nucleases in [[molecular biology]] are the [[restriction enzyme|restriction endonucleases]], which cut DNA at specific sequences. For instance, the EcoRV enzyme shown to the left recognizes the 6-base sequence 5′-GATATC-3′ and makes a cut at the horizontal line. In nature, these enzymes protect [[bacteria]] against [[Bacteriophage|phage]] infection by digesting the phage DNA when it enters the bacterial cell, acting as part of the [[restriction modification system]].<ref>{{cite journal | vauthors = Bickle TA, Krüger DH | title = Biology of DNA restriction | journal = Microbiological Reviews | volume = 57 | issue = 2 | pages = 434–50 | date = June 1993 | pmid = 8336674 | pmc = 372918 | doi = 10.1128/MMBR.57.2.434-450.1993 }}</ref> In technology, these sequence-specific nucleases are used in [[molecular cloning]] and [[Genetic fingerprinting|DNA fingerprinting]].

Enzymes called [[DNA ligase]]s can rejoin cut or broken DNA strands.<ref name=Doherty>{{cite journal | vauthors = Doherty AJ, Suh SW | title = Structural and mechanistic conservation in DNA ligases | journal = Nucleic Acids Research | volume = 28 | issue = 21 | pages = 4051–58 | date = November 2000 | pmid = 11058099 | pmc = 113121 | doi = 10.1093/nar/28.21.4051 }}</ref> Ligases are particularly important in [[Replication fork|lagging strand]] DNA replication, as they join the short segments of DNA produced at the [[replication fork]] into a complete copy of the DNA template. They are also used in [[DNA repair]] and [[genetic recombination]].<ref name=Doherty />