Heterochromatin
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2008-06-25T21:44:19Z
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[[Image:Diagram human cell nucleus.svg|thumb|350px|The nucleus of a human cell showing the location of heterochromatin]]
'''Heterochromatin''' is a tightly packed form of DNA. Its major characteristic is that [[transcription (genetics)|transcription]] is limited. As such, it is a means to control [[gene expression]], through regulation of the transcription initiation.
==Structure==
[[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 = [[Curr. Opin. Genet. Dev.]]
| issn = 0959-437X
| volume = 6
| pages = 193–202
| doi = 10.1016/S0959-437X(96)80050-5}}
</ref> Originally, the two forms were distinguished cytologically by how darkly they stained - the former is lighter, while the latter stains darkly, indicating tighter packing. Heterochromatin is usually localized to the periphery of the [[Cell nucleus|nucleus]].
Heterochromatin mainly consists of genetically inactive [[satellite DNA|satellite sequences]],<ref>
{{cite journal
| author = Lohe, A.R., ''et al.''
| title = Mapping simple repeated DNA sequences in heterochromatin of ''Drosophila melanogaster''
| year = 1993
| journal = [[Genetics (journal)|Genetics]]
| volume = 134
| issue = 4
| pages = 1149–1174
| issn = 0016-6731
| url = http://www.genetics.org/cgi/content/full/134/4/1149
| pmid = 8375654}}
</ref> and many genes are repressed to various extents, although some cannot be expressed in euchromatin at all.<ref>
{{cite journal
| author = Lu, B.Y., ''et al.''
| year = 2000
| title = Heterochromatin protein 1 is required for the normal expression of two heterochromatin genes in Drosophila
| journal = [[Genetics (journal)|Genetics]]
| volume = 155
| issue = 2
| pages = 699–708
| url = http://www.genetics.org/cgi/content/full/155/2/699
| issn = 0016-6731
| pmid = 10835392}}
</ref> Heterochromatin also replicates later in S phase of the cell cycle than euchromatin, and is found only in eukaryotes. Both [[centromere]]s and [[telomere]]s are heterochromatic, as is the [[Barr body]] of the second inactivated [[X chromosome]] in a female.
==Function==
Heterochromatin is believed to serve several functions, from gene regulation to the protection of the integrity of chromosomes; all of these roles can be attributed to the dense packing of DNA, which makes it less accessible to protein factors that bind DNA or its associated factors. For example, naked double-stranded DNA ends would usually be interpreted by the cell as damaged DNA, triggering [[cell cycle]] arrest and [[DNA repair]].{{Fact|date=June 2007}}
Heterochromatin is generally clonally inherited; when a cell divides the two daughter cells will 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 different levels of expression from cell to cell,<ref>
{{cite journal
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|title=Gene silencing, cell fate and nuclear organisation
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</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., ''et al.''
|year=1986
|month=Dec
|journal=[[Chromosoma]]
|volume=94
|issue=6
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</ref> [[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
|author=Burgess-Beusse, B, ''et al.''
|month=Dec
|year=2002
|title=The insulation of genes from external enhancers and silencing chromatin
|journal=[[Proc. Natl Acad. Sci. USA]]
|volume=9
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|pmid=12154228}}
</ref> and loci in two ''[[Saccharomyces]]'' spp.<ref>
{{cite journal
|author=Noma, K., ''et al.''
|month=Aug
|year=2001
|title=transitions in distinct histone H3 methylation patterns at the heterochromatin domain boundaries
|journal=[[Science (journal)|Science]]
|volume=293
|issue=5532
|pages=1150–1155
|doi=10.1126/science.1064150
|pmid=11498594}}
</ref><ref>
{{cite journal
|author=Donze, D. & R.T. Kamakaka
|year=2000
|title=RNA polymerase III and RNA polymerase II promoter complexes are heterochromatin barriers in ''Saccharomyces cerevisiae''
|journal=[[Embo J.]]
|volume=20
|pages=520–31
|doi=10.1093/emboj/20.3.520}}
</ref>).
==Constitutive heterochromatin==
All cells of a given species will package the same regions of DNA in [[constitutive heterochromatin]], and thus in all cells any genes contained within the constitutive heterochromatin will be poorly [[Gene expression|expressed]]. For example, all human chromosomes [[Chromosome 1 (human)|1]], [[Chromosome 9 (human)|9]], [[Chromosome 16 (human)|16]], and the [[Y chromosome]] contain large regions of constitutive heterochromatin. In most organisms, constitutive heterochromatin occurs around the chromosome centromere and near telomeres.
==Facultative heterochromatin==
[[Facultative heterochromatin]] The regions of DNA packaged in facultative heterochromatin will not be consistent within the cell types of a species, and thus a sequence in one cell that is packaged in facultative heterochromatin (and the genes within poorly expressed) may be packaged in euchromatin in another cell (and the genes within no longer silenced). However, the formation of facultative heterochromatin is regulated, and is often associated with [[morphogenesis]] or [[Cellular differentiation|differentiation]]. An example of facultative heterochromatin is [[X-inactivation|X-chromosome inactivation]] in female mammals: one [[X chromosome]] is packaged in facultative heterochromatin and silenced, while the other X chromosome is packaged in euchromatin and expressed.
Among the molecular components that appear to regulate the spreading of heterochromatin include the [[Polycomb-group proteins]] and non-coding genes such as [[Xist]]. The mechanism for such spreading is still a matter of controversy.<ref>Talbert PB, Henikoff S. Spreading of silent chromatin: inaction at a distance. Nat Rev Genet. 2006 Oct;7(10):793-803.
</ref>
==Yeast heterochromatin==
''[[Saccharomyces cerevisiae]]'', or budding yeast, is a model [[eukaryote]] and its heterochromatin has been defined thoroughly. Although most of its genome can be characterized as euchromatin, ''S. cerevisiae'' has regions of DNA that are transcribed very poorly. These loci are the so-called silent mating type loci (HML and HMR), the rDNA (encoding ribosomal RNA), and the sub-telomeric regions.
Fission yeast (''[[Schizosaccharomyces pombe]]'') uses another mechanism for heterochromatin formation at its centromeres. Gene silencing at this location depends on components of the [[RNAi]] pathway. Double-stranded RNA is believed to result in silencing of the region through a series of steps.
In the fission yeast ''[[Schizosaccharomyces pombe]]'' two RNAi complexes, the RNAi-induced transcriptional gene silencing (RITS) complex and the RNA-directed RNA polymerase complex (RDRC), are part of a RNAi machinery involved in the initiation, propagation and maintenance of heterochromatin assembly. These two complexes localize in a [[siRNA]]-dependent manner on chromosomes, at the site of heterochromatin assembly. [[RNA polymerase II]] synthesizes a transcript that serves as a platform to recruit RITS, RDRC and possibly other complexes required for heterochromatin assembly. Both RNAi and an exosome-dependent RNA degradation process contribute to heterochromatic gene silencing. These mechanisms of ''[[Schizosaccharomyces pombe]]'' may occur in other eukaryotes.<ref name= Vavasseur>{{cite book |chapterurl=http://www.horizonpress.com/rnareg|author= Vavasseur et al. |year=2008|chapter=Heterochromatin Assembly and Transcriptional Gene Silencing under the Control of Nuclear RNAi: Lessons from Fission Yeast|title=RNA and the Regulation of Gene Expression: A Hidden Layer of Complexity|publisher=Caister Academic Press|id=[http://www.horizonpress.com/rnareg ISBN 978-1-904455-25-7]}}</ref>
==External links==
* {{BUHistology|20102loa}}
==References==
{{reflist}}
* Z. Avramova ''Heterochromatin in Animals and Plants. Similarities and Differences''. [http://www.plantphysiol.org/cgi/content/full/129/1/40 Plant Physiology] May 2002, Vol. 129, pp. 40-49.
* {{cite journal
| author = Caron, H., ''et al.''
| title = The Human Transcriptome Map: Clustering of Highly Expressed Genes in Chromosomal Domains
| journal = [[Science (journal)|Science]]
| year = 2001
| volume = 291
| issue = 5507
| pages = 1289–1292
| doi = 10.1126/science.1056794
| pmid = 11181992}}
{{Chromo}}
[[Category:Molecular genetics]]
[[ar:كروماتين متغاير]]
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[[fr:Hétérochromatine]]
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[[ru:Гетерохроматин]]
[[bar:heterochromatin]]
[[ja:ヘテロクロマチン ]]
[[zh:異染色質]]