Editing Heterochromatin (section)

== Structure ==
[[File:Heterochromatin vs. euchromatin.svg|thumb|Heterochromatin vs. euchromatin]]

[[Chromatin]] is found in two varieties: [[euchromatin]] and heterochromatin.<ref>
{{cite journal
| author = Elgin, S.C.
| title = Heterochromatin and gene regulation in ''Drosophila''
| year = 1996
| journal = [[Current Opinion in Genetics & Development]]
| issn = 0959-437X
| volume = 6
| pages = 193–202
| doi = 10.1016/S0959-437X(96)80050-5
| pmid = 8722176
| issue = 2| url = https://openscholarship.wustl.edu/cgi/viewcontent.cgi?article=1213&context=bio_facpubs
}}
</ref> Originally, the two forms were distinguished cytologically by how intensely they get stained – the euchromatin is less intense, while heterochromatin stains intensely, indicating tighter packing. Heterochromatin was given its name for this reason by botanist Emil Heitz who discovered that heterochromatin remained darkly stained throughout the entire cell cycle, unlike euchromatin whose stain disappeared during interphase.<ref>{{Cite journal |last1=Penagos-Puig |first1=Andrés |last2=Furlan-Magaril |first2=Mayra |date=2020-09-18 |title=Heterochromatin as an Important Driver of Genome Organization |journal=Frontiers in Cell and Developmental Biology |volume=8 |doi=10.3389/fcell.2020.579137 |doi-access=free |issn=2296-634X |pmc=7530337 |pmid=33072761}}</ref> Heterochromatin is usually localized to the periphery of the [[Cell nucleus|nucleus]].
Despite this early dichotomy, recent evidence in both animals<ref>
{{cite journal | vauthors = van Steensel B | title = Chromatin: constructing the big picture | journal = The EMBO Journal | volume = 30 | issue = 10 | pages = 1885–95 | date = May 2011 | pmid = 21527910 | pmc = 3098493 | doi = 10.1038/emboj.2011.135 }}
</ref> and plants<ref>
{{cite journal | vauthors = Roudier F, Ahmed I, Bérard C, Sarazin A, Mary-Huard T, Cortijo S, Bouyer D, Caillieux E, Duvernois-Berthet E, Al-Shikhley L, Giraut L, Després B, Drevensek S, Barneche F, Dèrozier S, Brunaud V, Aubourg S, Schnittger A, Bowler C, Martin-Magniette ML, Robin S, Caboche M, Colot V | display-authors = 6 | title = Integrative epigenomic mapping defines four main chromatin states in Arabidopsis | journal = The EMBO Journal | volume = 30 | issue = 10 | pages = 1928–38 | date = May 2011 | pmid = 21487388 | pmc = 3098477 | doi = 10.1038/emboj.2011.103 }}
</ref> has suggested that there are more than two distinct heterochromatin states, and it may in fact exist in four or five 'states', each marked by different combinations of [[epigenetic]] marks.

Heterochromatin mainly consists of genetically inactive [[satellite DNA|satellite sequences]],<ref>
{{cite journal | vauthors = Lohe AR, Hilliker AJ, Roberts PA | title = Mapping simple repeated DNA sequences in heterochromatin of Drosophila melanogaster | journal = Genetics | volume = 134 | issue = 4 | pages = 1149–74 | date = August 1993 | doi = 10.1093/genetics/134.4.1149 | pmid = 8375654 | pmc = 1205583 | url = http://www.genetics.org/cgi/content/full/134/4/1149 }}
</ref> and many genes are repressed to various extents, although some cannot be expressed in euchromatin at all.<ref>
{{cite journal | vauthors = Lu BY, Emtage PC, Duyf BJ, Hilliker AJ, Eissenberg JC | title = Heterochromatin protein 1 is required for the normal expression of two heterochromatin genes in Drosophila | journal = Genetics | volume = 155 | issue = 2 | pages = 699–708 | date = June 2000 | doi = 10.1093/genetics/155.2.699 | pmid = 10835392 | pmc = 1461102 | url = http://www.genetics.org/cgi/content/full/155/2/699 }}
</ref> Both [[centromere]]s and [[telomere]]s are heterochromatic, as is the [[Barr body]] of the second, inactivated [[X chromosome|X-chromosome]] in a female.