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→Modifications: Added two sentences on the role of histone lysine methylation in DNA repair. |
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Well characterized modifications to histones include:<ref name="Strahl">{{cite journal |vauthors=Strahl B, Allis C |title=The language of covalent histone modifications |journal=Nature |volume=403 |issue=6765 |pages=41–5 |year=2000 |pmid=10638745 |doi=10.1038/47412}}</ref>
*[[Methylation]]
Both lysine and arginine residues are known to be methylated. Methylated lysines are the best understood marks of the histone code, as specific methylated lysine match well with gene expression states. Methylation of lysines H3K4 and H3K36 is correlated with transcriptional activation while demethylation of H3K4 is correlated with silencing of the genomic region. Methylation of lysines H3K9 and H3K27 is correlated with transcriptional repression.<ref name="Rosenfeld_2009">{{cite journal | last1 = Rosenfeld | first1 = Jeffrey A | last2 = Wang | first2 = Zhibin | last3 = Schones | first3 = Dustin | last4 = Zhao | first4=Keji | last5 = DeSalle | first5 = Rob | last6= Zhang | first6 = Michael Q | title = Determination of enriched histone modifications in non-genic portions of the human genome. | journal = BMC Genomics | volume = 10 | date = 31 March 2009 | pmid= 19335899 | doi = 10.1186/1471-2164-10-143 | pages = 143 | pmc = 2667539}}</ref> Particularly, H3K9me3 is highly correlated with constitutive heterochromatin.<ref name="Hublitz">{{cite journal | last1 = Hublitz | first1 = Philip | last2 = Albert | first2 = Mareike | last3 = Peters | first3 = Antoine | title = Mechanisms of Transcriptional Repression by Histone Lysine Methylation | journal = The International Journal of Developmental Biology | volume = 10 | issue = 1387 | pages = 335–354 | ___location = Basel | date = 28 April 2009 | issn =1696-3547}}</ref> Methylation of histone lysine also has a role in [[DNA repair]].<ref name="pmid29937925">{{cite journal |vauthors=Wei S, Li C, Yin Z, Wen J, Meng H, Xue L, Wang J |title=Histone methylation in DNA repair and clinical practice: new findings during the past 5-years |journal=J Cancer |volume=9 |issue=12 |pages=2072–2081 |date=2018 |pmid=29937925 |doi=10.7150/jca.23427 |url=}}</ref> For instance, [[H3K36me|H3K36me3]] is required for [[homologous recombination]]al repair of [[DNA damage (naturally occurring)|DNA double-strand breaks]], and H4K20me2 facilitates repair of such breaks by [[non-homologous end joining]].<ref name="pmid29937925" />
*[[Acetylation]]—by [[Histone acetyltransferase|HAT]] (histone acetyl transferase); deacetylation—by [[HDAC]] (histone deacetylase)
Acetylation tends to define the 'openness' of [[chromatin]] as acetylated histones cannot pack as well together as deacetylated histones.
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