Editing Heterochromatin (section)

== Function ==
[[File:General model for duplication of heterochromatin during cell division.svg|thumb|General model for duplication of heterochromatin during cell division]]
[[File:Heterochromatic versus euchromatic nuclei.jpg|thumb|Microscopy of heterochromatic versus euchromatic nuclei ([[H&E stain]]).]]
Heterochromatin has been associated with several functions, from gene regulation to the protection of chromosome integrity;<ref>
{{cite journal | vauthors = Grewal SI, Jia S | title = Heterochromatin revisited | journal = Nature Reviews. Genetics | volume = 8 | issue = 1 | pages = 35–46 | date = January 2007 | pmid = 17173056 | doi = 10.1038/nrg2008 | s2cid = 31811880 | url = https://zenodo.org/record/1233527 | quote = An up-to-date account of the current understanding of repetitive DNA, which usually doesn't contain genetic information. If evolution makes sense only in the context of the regulatory control of genes, we propose that heterochromatin, which is the main form of chromatin in higher eukaryotes, is positioned to be a deeply effective target for evolutionary change. Future investigations into assembly, maintenance and the many other functions of heterochromatin will shed light on the processes of gene and chromosome regulation. }}
</ref> some of these roles can be attributed to the dense packing of DNA, which makes it less accessible to protein factors that usually bind DNA or its associated factors. For example, naked double-stranded DNA ends would usually be interpreted by the cell as damaged or viral DNA, triggering [[cell cycle]] arrest, [[DNA repair]] or destruction of the fragment, such as by [[endonuclease]]s in bacteria.

Some regions of chromatin are very densely packed with fibers that display a condition comparable to that of the chromosome at [[mitosis]]. Heterochromatin is generally clonally inherited; when a cell divides, the two daughter cells typically contain heterochromatin within the same regions of DNA, resulting in [[epigenetic inheritance]]. Variations cause heterochromatin to encroach on adjacent genes or recede from genes at the extremes of domains. Transcribable material may be repressed by being positioned (in ''cis'') at these boundary domains. This gives rise to expression levels that vary from cell to cell,<ref>
{{cite journal | vauthors = Fisher AG, Merkenschlager M | title = Gene silencing, cell fate and nuclear organisation | journal = Current Opinion in Genetics & Development | volume = 12 | issue = 2 | pages = 193–7 | date = April 2002 | pmid = 11893493 | doi = 10.1016/S0959-437X(02)00286-1 }}
</ref> which may be demonstrated by [[position-effect variegation]].<ref>
{{cite journal
|title=Cytogenetic and molecular aspects of position effect variegation in Drosophila melanogaster
|author=Zhimulev, I.F.
|date=December 1986
|journal=[[Chromosoma]]
|volume=94
|issue=6
|pages=492–504
|doi=10.1007/BF00292759
|s2cid=24439936
|issn=1432-0886|display-authors=etal|author-link=:ru:Жимулёв, Игорь Фёдорович
}}
</ref> [[Insulator (genetics)|Insulator]] sequences may act as a barrier in rare cases where constitutive heterochromatin and highly active genes are juxtaposed (e.g. the 5'HS4 insulator upstream of the chicken β-globin locus,<ref>
{{cite journal | vauthors = Burgess-Beusse B, Farrell C, Gaszner M, Litt M, Mutskov V, Recillas-Targa F, Simpson M, West A, Felsenfeld G | display-authors = 6 | title = The insulation of genes from external enhancers and silencing chromatin | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 99 | issue = Suppl 4 | pages = 16433–7 | date = December 2002 | pmid = 12154228 | pmc = 139905 | doi = 10.1073/pnas.162342499 | bibcode = 2002PNAS...9916433B | doi-access = free }}
</ref> and loci in two ''[[Saccharomyces]]'' spp.<ref>
{{cite journal | vauthors = Allis CD, Grewal SI | title = Transitions in distinct histone H3 methylation patterns at the heterochromatin domain boundaries | journal = Science | volume = 293 | issue = 5532 | pages = 1150–5 | date = August 2001 | pmid = 11498594 | doi = 10.1126/science.1064150 | s2cid = 26350729 }}
</ref><ref>
{{cite journal | vauthors = Donze D, Kamakaka RT | title = RNA polymerase III and RNA polymerase II promoter complexes are heterochromatin barriers in Saccharomyces cerevisiae | journal = The EMBO Journal | volume = 20 | issue = 3 | pages = 520–31 | date = February 2001 | pmid = 11157758 | pmc = 133458 | doi = 10.1093/emboj/20.3.520 }}
</ref>).