Editing Endonuclease (section)

=== DNA repair ===
Endonucleases play a role in DNA repair. [[AP endonuclease]], specifically, catalyzes the incision of DNA exclusively at AP sites, and therefore prepares DNA for subsequent excision, repair synthesis and DNA ligation. For example, when depurination occurs, this lesion leaves a deoxyribose sugar with a missing base.<ref name="isbn1-55581-319-4">{{cite book |title=DNA repair and mutagenesis |vauthors=Ellenberger T, Friedberg EC, Walker GS, Wolfram S, Wood RJ, Schultz R |publisher=ASM Press |year=2006 |isbn=978-1-55581-319-2 |location=Washington, D.C.}}</ref> The AP endonuclease recognizes this sugar and essentially cuts the DNA at this site and then allows for DNA repair to continue.<ref name="isbn0-8153-3218-1">{{cite book |author=Alberts B |title=Molecular biology of the cell |publisher=Garland Science |year=2002 |isbn=978-0-8153-3218-3 |location=New York}}</ref> ''E. coli'' cells contain two AP endonucleases: endonuclease IV (endoIV) and exonuclease III (exoIII) while in eukaryotes, there is only one AP endonuclease.<ref name="pmid12483517">{{cite journal |vauthors=Nishino T, Morikawa K |date=December 2002 |title=Structure and function of nucleases in DNA repair: shape, grip and blade of the DNA scissors |journal=Oncogene |volume=21 |issue=58 |pages=9022–32 |doi=10.1038/sj.onc.1206135 |pmid=12483517 |doi-access=free}}</ref>

[[File:APEndonucleasecartoon.gif|APEndonucleasecartoon]]

==== DNA crosslink repair ====
[[DNA repair|Repair of DNA]] in which the two complementary strands are joined by an [[crosslinking of DNA|interstrand covalent crosslink]] requires multiple incisions in order to disengage the strands and remove the damage.  Incisions are required on both sides of the crosslink and on both strands of the duplex DNA.  In mouse embryonic stem cells, an intermediate stage of crosslink repair involves production of double-strand breaks.<ref name="Hanada2006">{{cite journal |last1=Hanada |first1=K. |last2=Budzowska |first2=M. |last3=Modesti |first3=M. |last4=Maas |first4=A. |last5=Wyman |first5=C. |last6=Essers |first6=J. |last7=Kanaar |first7=R. |date=2006 |title=The structure-specific endonuclease Mus81-Eme1 promotes conversion of interstrand DNA crosslinks into double-strands breaks |journal=The EMBO Journal |volume=25 |issue=20 |pages=4921–4932 |doi=10.1038/sj.emboj.7601344 |pmc=1618088 |pmid=17036055}}</ref>   [[MUS81]]/[[EME1]] is a structure specific endonuclease involved in converting interstrand crosslinks to double-strand breaks in a DNA replication-dependent manner.<ref name="Hanada2006" />  After introduction of a double-strand break, further steps are required to complete the repair process.  If a crosslink is not properly repaired it can block [[DNA replication]].{{citation needed|date=January 2023}}

==== Thymine dimer repair ====
Exposure of [[Escherichia virus T4|bacteriophage (phage) T4]] to [[ultraviolet]] irradiation induces [[pyrimidine dimer|thymine dimers]] in the phage DNA.  The phage T4 ''denV'' gene encodes [[endonuclease V]] that catalyzes the initial steps in the repair of these UV-induced thymine dimers.<ref>{{Cite journal |last1=Bernstein |first1=C. |date=1981 |title=Deoxyribonucleic acid repair in bacteriophage |journal=Microbiological Reviews |volume=45 |issue=1 |pages=72–98 |doi=10.1128/mr.45.1.72-98.1981 |pmc=281499 |pmid=6261109}}</ref>  Endonuclease V first cleaves the glycosylic bond on the 5’ side of a pyrimidine dimer and then catalyzes cleavage of the DNA phosphodiester bond that originally linked the two nucleotides of the dimer. Subsequent steps in the repair process involve removal of the dimer remnants and repair synthesis to fill in the resulting single-strand gap using the undamaged strand as template.{{citation needed|date=January 2023}}