Difference between revisions of "Heterochromatin"

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<p><strong>Heterochromatin</strong> is a tightly packed form of DNA. Its major  characteristic is that <a href="/wiki/Transcription_(genetics)" title="Transcription (genetics)">transcription</a> is limited. As such, it  is a means to control <a href="/wiki/Gene_expression" title="Gene expression">gene expression</a>, through regulation of the  transcription initiation.</p>
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<p><a name="Structure" id="Structure"></a></p>
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<h2><span class="editsection"></span><span class="mw-headline">Structure</span></h2>
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<p><a href="/wiki/Chromatin" title="Chromatin">Chromatin</a> is found in two  varieties: <a href="/wiki/Euchromatin" title="Euchromatin">euchromatin</a> and  heterochromatin.<sup id="cite_ref-0" class="reference"><a href="#cite_note-0" title=""><span>[</span>1<span>]</span></a></sup> Originally, the two  forms were distinguished cytologically by how intensely they stained - the  former is less intense, while the latter stains intensely, indicating tighter  packing. Heterochromatin is usually localized to the periphery of the <a href="/wiki/Cell_nucleus" title="Cell nucleus">nucleus</a>.</p>
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<p>Heterochromatin mainly consists of genetically inactive <a href="/wiki/Satellite_DNA" title="Satellite DNA">satellite sequences</a>,<sup id="cite_ref-1" class="reference"><a href="#cite_note-1" title=""><span>[</span>2<span>]</span></a></sup> and many genes are  repressed to various extents, although some cannot be expressed in euchromatin  at all.<sup id="cite_ref-2" class="reference"><a href="#cite_note-2" title=""><span>[</span>3<span>]</span></a></sup> Heterochromatin also  replicates later in S phase of the cell cycle than euchromatin, and is found  only in eukaryotes. Both <a href="/wiki/Centromere" title="Centromere">centromeres</a> and <a href="/wiki/Telomere" title="Telomere">telomeres</a> are heterochromatic, as is the <a href="/wiki/Barr_body" title="Barr body">Barr body</a> of the second inactivated  <a href="/wiki/X_chromosome" title="X chromosome">X chromosome</a> in a  female.</p>
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<p><a name="Function" id="Function"></a></p>
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<h2><span class="editsection"></span><span class="mw-headline">Function</span></h2>
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<p>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 <a href="/wiki/Cell_cycle" title="Cell cycle">cell cycle</a>  arrest and <a href="/wiki/DNA_repair" title="DNA repair">DNA repair</a>.<sup class="noprint Template-Fact"><span style="white-space: nowrap;" title="This claim needs references to reliable sources since June 2007">[<em><a href="/wiki/Wikipedia:Citation_needed" title="Wikipedia:Citation needed">citation needed</a></em>]</span></sup>  Some region of chromatin are very densly packed with fibres displaying a  condition comparable to that of the chromosome at mitosis. 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  <a href="/wiki/Epigenetic_inheritance" title="Epigenetic inheritance" class="mw-redirect">epigenetic inheritance</a>. 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 <em>cis</em>) at these boundary domains. This gives rise to different levels  of expression from cell to cell,<sup id="cite_ref-3" class="reference"><a href="#cite_note-3" title=""><span>[</span>4<span>]</span></a></sup> which may be  demonstrated by <a href="/wiki/Position-effect_variegation" title="Position-effect variegation">position-effect variegation</a>.<sup id="cite_ref-4" class="reference"><a href="#cite_note-4" title=""><span>[</span>5<span>]</span></a></sup> <a href="/wiki/Insulator" title="Insulator">Insulator</a> 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 &beta;-globin locus,<sup id="cite_ref-5" class="reference"><a href="#cite_note-5" title=""><span>[</span>6<span>]</span></a></sup> and loci in two  <em><a href="/wiki/Saccharomyces" title="Saccharomyces">Saccharomyces</a></em>  spp.<sup id="cite_ref-6" class="reference"><a href="#cite_note-6" title=""><span>[</span>7<span>]</span></a></sup><sup id="cite_ref-7" class="reference"><a href="#cite_note-7" title=""><span>[</span>8<span>]</span></a></sup>).</p>
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<p><a name="Constitutive_heterochromatin" id="Constitutive_heterochromatin"></a></p>
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<h2><span class="editsection"></span><span class="mw-headline">Constitutive heterochromatin</span></h2>
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<p>All cells of a given species will package the same regions of DNA in <a href="/wiki/Constitutive_heterochromatin" title="Constitutive heterochromatin">constitutive heterochromatin</a>, and  thus in all cells any genes contained within the constitutive heterochromatin  will be poorly <a href="/wiki/Gene_expression" title="Gene expression">expressed</a>. For example, all human chromosomes  <a href="/wiki/Chromosome_1_(human)" title="Chromosome 1 (human)">1</a>, <a href="/wiki/Chromosome_9_(human)" title="Chromosome 9 (human)">9</a>, <a href="/wiki/Chromosome_16_(human)" title="Chromosome 16 (human)">16</a>, and the  <a href="/wiki/Y_chromosome" title="Y chromosome">Y chromosome</a> contain large  regions of constitutive heterochromatin. In most organisms, constitutive  heterochromatin occurs around the chromosome centromere and near telomeres.</p>
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<p><a name="Facultative_heterochromatin" id="Facultative_heterochromatin"></a></p>
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<h2><span class="editsection"></span><span class="mw-headline">Facultative heterochromatin</span></h2>
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<p><a href="/wiki/Facultative_heterochromatin" title="Facultative heterochromatin" class="mw-redirect">Facultative heterochromatin</a> 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 <a href="/wiki/Morphogenesis" title="Morphogenesis">morphogenesis</a> or <a href="/wiki/Cellular_differentiation" title="Cellular differentiation">differentiation</a>. An example of  facultative heterochromatin is <a href="/wiki/X-inactivation" title="X-inactivation">X-chromosome inactivation</a> in female mammals: one  <a href="/wiki/X_chromosome" title="X chromosome">X chromosome</a> is packaged  in facultative heterochromatin and silenced, while the other X chromosome is  packaged in euchromatin and expressed.</p>
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<p>Among the molecular components that appear to regulate the spreading of  heterochromatin include the <a href="/wiki/Polycomb-group_proteins" title="Polycomb-group proteins">Polycomb-group proteins</a> and non-coding  genes such as <a href="/wiki/Xist" title="Xist">Xist</a>. The mechanism for such  spreading is still a matter of controversy.<sup id="cite_ref-8" class="reference"><a href="#cite_note-8" title=""><span>[</span>9<span>]</span></a></sup></p>
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<p><a name="Yeast_heterochromatin" id="Yeast_heterochromatin"></a></p>
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<h2><span class="editsection"></span><span class="mw-headline">Yeast heterochromatin</span></h2>
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<p><em><a href="/wiki/Saccharomyces_cerevisiae" title="Saccharomyces cerevisiae">Saccharomyces cerevisiae</a></em>, or  budding yeast, is a model <a href="/wiki/Eukaryote" title="Eukaryote">eukaryote</a> and its heterochromatin has been defined  thoroughly. Although most of its genome can be characterized as euchromatin,  <em>S. cerevisiae</em> 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 (<em><a href="/wiki/Schizosaccharomyces_pombe" title="Schizosaccharomyces pombe">Schizosaccharomyces pombe</a></em>) uses  another mechanism for heterochromatin formation at its centromeres. Gene  silencing at this location depends on components of the <a href="/wiki/RNAi" title="RNAi" class="mw-redirect">RNAi</a> pathway. Double-stranded RNA is believed  to result in silencing of the region through a series of steps.</p>
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<p>In the fission yeast <em><a href="/wiki/Schizosaccharomyces_pombe" title="Schizosaccharomyces pombe">Schizosaccharomyces pombe</a></em> 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 <a href="/wiki/SiRNA" title="SiRNA" class="mw-redirect">siRNA</a>-dependent manner on chromosomes, at the site of  heterochromatin assembly. <a href="/wiki/RNA_polymerase_II" title="RNA polymerase II">RNA polymerase II</a> 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 <em><a href="/wiki/Schizosaccharomyces_pombe" title="Schizosaccharomyces pombe">Schizosaccharomyces pombe</a></em> may  occur in other eukaryotes.<sup id="cite_ref-Vavasseur_9-0" class="reference"><a href="#cite_note-Vavasseur-9" title=""><span>[</span>10<span>]</span></a></sup></p>
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<p><a name="External_links" id="External_links"></a></p>
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<h2><span class="editsection"></span><span class="mw-headline">External links</span></h2>
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    <li><a href="/wiki/Boston_University" title="Boston University">Histology at  BU</a> <em><a rel="nofollow" href="http://www.bu.edu/histology/p/20102loa.htm" title="http://www.bu.edu/histology/p/20102loa.htm" class="external text">20102loa</a></em>  </li>
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<p><a name="References" id="References"></a></p>
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<h2><span class="editsection"></span><span class="mw-headline">References</span></h2>
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    <li id="cite_note-0"><strong><a href="#cite_ref-0" title="">^</a></strong> <cite style="font-style: normal;" id="CITEREFElgin.2C_S.C.1996" class="Journal">Elgin, S.C.  (1996). &quot;Heterochromatin and gene regulation in <em>Drosophila</em>&quot;. <em><a href="/wiki/Curr._Opin._Genet._Dev." title="Curr. Opin. Genet. Dev." class="mw-redirect">Curr. Opin. Genet. Dev.</a></em> <strong>6</strong>:  193&ndash;202. <a href="/wiki/Digital_object_identifier" title="Digital object identifier">doi</a>:<span class="neverexpand"><a rel="nofollow" href="http://dx.doi.org/10.1016%2FS0959-437X%2896%2980050-5" title="http://dx.doi.org/10.1016%2FS0959-437X%2896%2980050-5" class="external text">10.1016/S0959-437X(96)80050-5</a></span>. <a href="/wiki/International_Standard_Serial_Number" title="International Standard Serial Number">ISSN</a> <a rel="nofollow" href="http://worldcat.org/issn/0959-437X" title="http://worldcat.org/issn/0959-437X" class="external text">0959-437X</a>.</cite><span title="ctx_ver=Z39.88-2004&amp;rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Ajournal&amp;rft.genre=article&amp;rft.atitle=Heterochromatin+and+gene+regulation+in+%27%27Drosophila%27%27&amp;rft.jtitle=%5B%5BCurr.+Opin.+Genet.+Dev.%5D%5D&amp;rft.aulast=Elgin%2C+S.C.&amp;rft.au=Elgin%2C+S.C.&amp;rft.date=1996&amp;rft.volume=6&amp;rft.pages=193%E2%80%93202&amp;rft_id=info:doi/10.1016%2FS0959-437X%2896%2980050-5&amp;rft.issn=0959-437X&amp;rfr_id=info:sid/en.wikipedia.org:Heterochromatin" class="Z3988"><span style="display: none;">&nbsp;</span></span>  </li>
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    <li id="cite_note-1"><strong><a href="#cite_ref-1" title="">^</a></strong> <cite style="font-style: normal;" id="CITEREFLohe.2C_A.R..2C_.27.27et_al..27.271993" class="Journal">Lohe, A.R., <em>et al.</em> (1993). &quot;<a rel="nofollow" href="http://www.genetics.org/cgi/content/full/134/4/1149" title="http://www.genetics.org/cgi/content/full/134/4/1149" class="external text">Mapping  simple repeated DNA sequences in heterochromatin of <em>Drosophila  melanogaster</em></a>&quot;. <em><a href="/wiki/Genetics_(journal)" title="Genetics (journal)">Genetics</a></em> <strong>134</strong> (4): 1149&ndash;1174. <a href="/wiki/International_Standard_Serial_Number" title="International Standard Serial Number">ISSN</a> <a rel="nofollow" href="http://worldcat.org/issn/0016-6731" title="http://worldcat.org/issn/0016-6731" class="external text">0016-6731</a>. <a href="http://www.ncbi.nlm.nih.gov/pubmed/8375654" title="http://www.ncbi.nlm.nih.gov/pubmed/8375654" class="external">PMID 8375654</a><span class="printonly">. <a rel="nofollow" href="http://www.genetics.org/cgi/content/full/134/4/1149" title="http://www.genetics.org/cgi/content/full/134/4/1149" class="external free">http://www.genetics.org/cgi/content/full/134/4/1149</a></span>.</cite><span title="ctx_ver=Z39.88-2004&amp;rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Ajournal&amp;rft.genre=article&amp;rft.atitle=Mapping+simple+repeated+DNA+sequences+in+heterochromatin+of+%27%27Drosophila+melanogaster%27%27&amp;rft.jtitle=%5B%5BGenetics+%28journal%29%7CGenetics%5D%5D&amp;rft.aulast=Lohe%2C+A.R.%2C+%27%27et+al.%27%27&amp;rft.au=Lohe%2C+A.R.%2C+%27%27et+al.%27%27&amp;rft.date=1993&amp;rft.volume=134&amp;rft.issue=4&amp;rft.pages=1149%E2%80%931174&amp;rft_id=info:pmid/8375654&amp;rft.issn=0016-6731&amp;rft_id=http%3A%2F%2Fwww.genetics.org%2Fcgi%2Fcontent%2Ffull%2F134%2F4%2F1149&amp;rfr_id=info:sid/en.wikipedia.org:Heterochromatin" class="Z3988"><span style="display: none;">&nbsp;</span></span>  </li>
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    <li id="cite_note-2"><strong><a href="#cite_ref-2" title="">^</a></strong> <cite style="font-style: normal;" id="CITEREFLu.2C_B.Y..2C_.27.27et_al..27.272000" class="Journal">Lu, B.Y., <em>et al.</em> (2000). &quot;<a rel="nofollow" href="http://www.genetics.org/cgi/content/full/155/2/699" title="http://www.genetics.org/cgi/content/full/155/2/699" class="external text">Heterochromatin protein 1 is required for the normal expression of  two heterochromatin genes in Drosophila</a>&quot;. <em><a href="/wiki/Genetics_(journal)" title="Genetics (journal)">Genetics</a></em> <strong>155</strong> (2): 699&ndash;708. <a href="/wiki/International_Standard_Serial_Number" title="International Standard Serial Number">ISSN</a> <a rel="nofollow" href="http://worldcat.org/issn/0016-6731" title="http://worldcat.org/issn/0016-6731" class="external text">0016-6731</a>. <a href="http://www.ncbi.nlm.nih.gov/pubmed/10835392" title="http://www.ncbi.nlm.nih.gov/pubmed/10835392" class="external">PMID 10835392</a><span class="printonly">. <a rel="nofollow" href="http://www.genetics.org/cgi/content/full/155/2/699" title="http://www.genetics.org/cgi/content/full/155/2/699" class="external free">http://www.genetics.org/cgi/content/full/155/2/699</a></span>.</cite><span title="ctx_ver=Z39.88-2004&amp;rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Ajournal&amp;rft.genre=article&amp;rft.atitle=Heterochromatin+protein+1+is+required+for+the+normal+expression+of+two+heterochromatin+genes+in+Drosophila&amp;rft.jtitle=%5B%5BGenetics+%28journal%29%7CGenetics%5D%5D&amp;rft.aulast=Lu%2C+B.Y.%2C+%27%27et+al.%27%27&amp;rft.au=Lu%2C+B.Y.%2C+%27%27et+al.%27%27&amp;rft.date=2000&amp;rft.volume=155&amp;rft.issue=2&amp;rft.pages=699%E2%80%93708&amp;rft_id=info:pmid/10835392&amp;rft.issn=0016-6731&amp;rft_id=http%3A%2F%2Fwww.genetics.org%2Fcgi%2Fcontent%2Ffull%2F155%2F2%2F699&amp;rfr_id=info:sid/en.wikipedia.org:Heterochromatin" class="Z3988"><span style="display: none;">&nbsp;</span></span>  </li>
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    <li id="cite_note-3"><strong><a href="#cite_ref-3" title="">^</a></strong> <cite style="font-style: normal;" id="CITEREFFisherMatthias_Merkenschlager2002" class="Journal">Fisher, Amanda G.; Matthias Merkenschlager (April  2002). &quot;Gene silencing, cell fate and nuclear organisation&quot;. <em><a href="/wiki/Curr._Opin._Genet._Dev." title="Curr. Opin. Genet. Dev." class="mw-redirect">Curr. Opin. Genet. Dev.</a></em> <strong>12</strong>  (2): 193&ndash;197. <a href="/wiki/Digital_object_identifier" title="Digital object identifier">doi</a>:<span class="neverexpand"><a rel="nofollow" href="http://dx.doi.org/10.1016%2FS0959-437X%2802%2900286-1" title="http://dx.doi.org/10.1016%2FS0959-437X%2802%2900286-1" class="external text">10.1016/S0959-437X(02)00286-1</a></span>. <a href="/wiki/International_Standard_Serial_Number" title="International Standard Serial Number">ISSN</a> <a rel="nofollow" href="http://worldcat.org/issn/0959-437X" title="http://worldcat.org/issn/0959-437X" class="external text">0959-437X</a>.</cite><span title="ctx_ver=Z39.88-2004&amp;rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Ajournal&amp;rft.genre=article&amp;rft.atitle=Gene+silencing%2C+cell+fate+and+nuclear+organisation&amp;rft.jtitle=%5B%5BCurr.+Opin.+Genet.+Dev.%5D%5D&amp;rft.aulast=Fisher&amp;rft.aufirst=Amanda+G.&amp;rft.au=Fisher%2C+Amanda+G.&amp;rft.au=Matthias+Merkenschlager&amp;rft.date=April+2002&amp;rft.volume=12&amp;rft.issue=2&amp;rft.pages=193%E2%80%93197&amp;rft_id=info:doi/10.1016%2FS0959-437X%2802%2900286-1&amp;rft.issn=0959-437X&amp;rfr_id=info:sid/en.wikipedia.org:Heterochromatin" class="Z3988"><span style="display: none;">&nbsp;</span></span>  </li>
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    <li id="cite_note-4"><strong><a href="#cite_ref-4" title="">^</a></strong> <cite style="font-style: normal;" id="CITEREFZhimulev.2C_I.F..2C_.27.27et_al..27.271986" class="Journal">Zhimulev, I.F., <em>et al.</em> (December 1986).  &quot;<em>Cytogenetic and molecular aspects of position effect variegation in  Drosophila melanogaster</em>&quot;. <em><a href="/w/index.php?title=Chromosoma&amp;action=edit&amp;redlink=1" title="Chromosoma (page does not exist)" class="new">Chromosoma</a></em>  <strong>94</strong> (6): 492&ndash;504. <a href="/wiki/Digital_object_identifier" title="Digital object identifier">doi</a>:<span class="neverexpand"><a rel="nofollow" href="http://dx.doi.org/10.1007%2FBF00292759" title="http://dx.doi.org/10.1007%2FBF00292759" class="external text">10.1007/BF00292759</a></span>. <a href="/wiki/International_Standard_Serial_Number" title="International Standard Serial Number">ISSN</a> <a rel="nofollow" href="http://worldcat.org/issn/1432-0886" title="http://worldcat.org/issn/1432-0886" class="external text">1432-0886</a>.</cite><span title="ctx_ver=Z39.88-2004&amp;rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Ajournal&amp;rft.genre=article&amp;rft.atitle=%27%27Cytogenetic+and+molecular+aspects+of+position+effect+variegation+in+Drosophila+melanogaster%27%27&amp;rft.jtitle=%5B%5BChromosoma%5D%5D&amp;rft.aulast=Zhimulev%2C+I.F.%2C+%27%27et+al.%27%27&amp;rft.au=Zhimulev%2C+I.F.%2C+%27%27et+al.%27%27&amp;rft.date=December+1986&amp;rft.volume=94&amp;rft.issue=6&amp;rft.pages=492%E2%80%93504&amp;rft_id=info:doi/10.1007%2FBF00292759&amp;rft.issn=1432-0886&amp;rfr_id=info:sid/en.wikipedia.org:Heterochromatin" class="Z3988"><span style="display: none;">&nbsp;</span></span>  </li>
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    <li id="cite_note-5"><strong><a href="#cite_ref-5" title="">^</a></strong> <cite style="font-style: normal;" id="CITEREFBurgess-Beusse.2C_B.2C_.27.27et_al..27.272002" class="Journal">Burgess-Beusse, B, <em>et al.</em> (December 2002).  &quot;The insulation of genes from external enhancers and silencing chromatin&quot;. <em><a href="/wiki/Proc._Natl_Acad._Sci._USA" title="Proc. Natl Acad. Sci. USA" class="mw-redirect">Proc. Natl Acad. Sci. USA</a></em>  <strong>9</strong> (Suppl 4): 16433&ndash;16437. <a href="/wiki/Digital_object_identifier" title="Digital object identifier">doi</a>:<span class="neverexpand"><a rel="nofollow" href="http://dx.doi.org/10.1073%2Fpnas.162342499" title="http://dx.doi.org/10.1073%2Fpnas.162342499" class="external text">10.1073/pnas.162342499</a></span>. <a href="http://www.ncbi.nlm.nih.gov/pubmed/12154228" title="http://www.ncbi.nlm.nih.gov/pubmed/12154228" class="external">PMID  12154228</a>.</cite><span title="ctx_ver=Z39.88-2004&amp;rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Ajournal&amp;rft.genre=article&amp;rft.atitle=The+insulation+of+genes+from+external+enhancers+and+silencing+chromatin&amp;rft.jtitle=%5B%5BProc.+Natl+Acad.+Sci.+USA%5D%5D&amp;rft.aulast=Burgess-Beusse%2C+B%2C+%27%27et+al.%27%27&amp;rft.au=Burgess-Beusse%2C+B%2C+%27%27et+al.%27%27&amp;rft.date=December+2002&amp;rft.volume=9&amp;rft.issue=Suppl+4&amp;rft.pages=16433%E2%80%9316437&amp;rft_id=info:doi/10.1073%2Fpnas.162342499&amp;rft_id=info:pmid/12154228&amp;rfr_id=info:sid/en.wikipedia.org:Heterochromatin" class="Z3988"><span style="display: none;">&nbsp;</span></span>  </li>
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    <li id="cite_note-6"><strong><a href="#cite_ref-6" title="">^</a></strong> <cite style="font-style: normal;" id="CITEREFNoma.2C_K..2C_.27.27et_al..27.272001" class="Journal">Noma, K., <em>et al.</em> (August 2001). &quot;transitions  in distinct histone H3 methylation patterns at the heterochromatin domain  boundaries&quot;. <em><a href="/wiki/Science_(journal)" title="Science (journal)">Science</a></em> <strong>293</strong> (5532): 1150&ndash;1155. <a href="/wiki/Digital_object_identifier" title="Digital object identifier">doi</a>:<span class="neverexpand"><a rel="nofollow" href="http://dx.doi.org/10.1126%2Fscience.1064150" title="http://dx.doi.org/10.1126%2Fscience.1064150" class="external text">10.1126/science.1064150</a></span>. <a href="http://www.ncbi.nlm.nih.gov/pubmed/11498594" title="http://www.ncbi.nlm.nih.gov/pubmed/11498594" class="external">PMID  11498594</a>.</cite><span title="ctx_ver=Z39.88-2004&amp;rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Ajournal&amp;rft.genre=article&amp;rft.atitle=transitions+in+distinct+histone+H3+methylation+patterns+at+the+heterochromatin+domain+boundaries&amp;rft.jtitle=%5B%5BScience+%28journal%29%7CScience%5D%5D&amp;rft.aulast=Noma%2C+K.%2C+%27%27et+al.%27%27&amp;rft.au=Noma%2C+K.%2C+%27%27et+al.%27%27&amp;rft.date=August+2001&amp;rft.volume=293&amp;rft.issue=5532&amp;rft.pages=1150%E2%80%931155&amp;rft_id=info:doi/10.1126%2Fscience.1064150&amp;rft_id=info:pmid/11498594&amp;rfr_id=info:sid/en.wikipedia.org:Heterochromatin" class="Z3988"><span style="display: none;">&nbsp;</span></span>  </li>
 +
    <li id="cite_note-7"><strong><a href="#cite_ref-7" title="">^</a></strong> <cite style="font-style: normal;" id="CITEREFDonze.2C_D._.26_R.T._Kamakaka2000" class="Journal">Donze, D. &amp; R.T. Kamakaka (2000). &quot;RNA polymerase  III and RNA polymerase II promoter complexes are heterochromatin barriers in  <em>Saccharomyces cerevisiae</em>&quot;. <em><a href="/w/index.php?title=Embo_J.&amp;action=edit&amp;redlink=1" title="Embo J. (page does not exist)" class="new">Embo J.</a></em>  <strong>20</strong>: 520&ndash;31. <a href="/wiki/Digital_object_identifier" title="Digital object identifier">doi</a>:<span class="neverexpand"><a rel="nofollow" href="http://dx.doi.org/10.1093%2Femboj%2F20.3.520" title="http://dx.doi.org/10.1093%2Femboj%2F20.3.520" class="external text">10.1093/emboj/20.3.520</a></span>.</cite><span title="ctx_ver=Z39.88-2004&amp;rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Ajournal&amp;rft.genre=article&amp;rft.atitle=RNA+polymerase+III+and+RNA+polymerase+II+promoter+complexes+are+heterochromatin+barriers+in+%27%27Saccharomyces+cerevisiae%27%27&amp;rft.jtitle=%5B%5BEmbo+J.%5D%5D&amp;rft.aulast=Donze%2C+D.+%26+R.T.+Kamakaka&amp;rft.au=Donze%2C+D.+%26+R.T.+Kamakaka&amp;rft.date=2000&amp;rft.volume=20&amp;rft.pages=520%E2%80%9331&amp;rft_id=info:doi/10.1093%2Femboj%2F20.3.520&amp;rfr_id=info:sid/en.wikipedia.org:Heterochromatin" class="Z3988"><span style="display: none;">&nbsp;</span></span>  </li>
 +
    <li id="cite_note-8"><strong><a href="#cite_ref-8" title="">^</a></strong> Talbert PB,  Henikoff S. Spreading of silent chromatin: inaction at a distance. Nat Rev  Genet. 2006 Oct;7(10):793-803.  </li>
 +
    <li id="cite_note-Vavasseur-9"><strong><a href="#cite_ref-Vavasseur_9-0" title="">^</a></strong> <cite style="font-style: normal;" id="CITEREFVavasseur_et_al.2008" class="book">Vavasseur et al.  (2008). <a rel="nofollow" href="http://www.horizonpress.com/rnareg" title="http://www.horizonpress.com/rnareg" class="external text">&quot;Heterochromatin Assembly  and Transcriptional Gene Silencing under the Control of Nuclear RNAi: Lessons  from Fission Yeast&quot;</a>. <em>RNA and the Regulation of Gene Expression: A Hidden  Layer of Complexity</em>. Caister Academic Press. <a rel="nofollow" href="http://www.horizonpress.com/rnareg" title="http://www.horizonpress.com/rnareg" class="external text">ISBN  978-1-904455-25-7</a>.</cite><span title="ctx_ver=Z39.88-2004&amp;rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Abook&amp;rft.genre=bookitem&amp;rft.btitle=Heterochromatin+Assembly+and+Transcriptional+Gene+Silencing+under+the+Control+of+Nuclear+RNAi%3A+Lessons+from+Fission+Yeast&amp;rft.atitle=RNA+and+the+Regulation+of+Gene+Expression%3A+A+Hidden+Layer+of+Complexity&amp;rft.aulast=Vavasseur+et+al.&amp;rft.au=Vavasseur+et+al.&amp;rft.date=2008&amp;rft.pub=Caister+Academic+Press&amp;rfr_id=info:sid/en.wikipedia.org:Heterochromatin" class="Z3988"><span style="display: none;">&nbsp;</span></span> </li>
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</ol>
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</div>
 +
<ul>
 +
    <li>Z. Avramova <em>Heterochromatin in Animals and Plants. Similarities and  Differences</em>. <a rel="nofollow" href="http://www.plantphysiol.org/cgi/content/full/129/1/40" title="http://www.plantphysiol.org/cgi/content/full/129/1/40" class="external text">Plant  Physiology</a> May 2002, Vol. 129, pp. 40-49.  </li>
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    <li><cite style="font-style: normal;" id="CITEREFCaron.2C_H..2C_.27.27et_al..27.272001" class="Journal">Caron, H., <em>et al.</em> (2001). &quot;The Human  Transcriptome Map: Clustering of Highly Expressed Genes in Chromosomal Domains&quot;.  <em><a href="/wiki/Science_(journal)" title="Science (journal)">Science</a></em>  <strong>291</strong> (5507): 1289&ndash;1292. <a href="/wiki/Digital_object_identifier" title="Digital object identifier">doi</a>:<span class="neverexpand"><a rel="nofollow" href="http://dx.doi.org/10.1126%2Fscience.1056794" title="http://dx.doi.org/10.1126%2Fscience.1056794" class="external text">10.1126/science.1056794</a></span>. <a href="http://www.ncbi.nlm.nih.gov/pubmed/11181992" title="http://www.ncbi.nlm.nih.gov/pubmed/11181992" class="external">PMID  11181992</a>.</cite><span title="ctx_ver=Z39.88-2004&amp;rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Ajournal&amp;rft.genre=article&amp;rft.atitle=The+Human+Transcriptome+Map%3A+Clustering+of+Highly+Expressed+Genes+in+Chromosomal+Domains&amp;rft.jtitle=%5B%5BScience+%28journal%29%7CScience%5D%5D&amp;rft.aulast=Caron%2C+H.%2C+%27%27et+al.%27%27&amp;rft.au=Caron%2C+H.%2C+%27%27et+al.%27%27&amp;rft.date=2001&amp;rft.volume=291&amp;rft.issue=5507&amp;rft.pages=1289%E2%80%931292&amp;rft_id=info:doi/10.1126%2Fscience.1056794&amp;rft_id=info:pmid/11181992&amp;rfr_id=info:sid/en.wikipedia.org:Heterochromatin" class="Z3988"><span style="display: none;">&nbsp;</span></span> </li>
 +
</ul>
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<p><br />
 +
</p>
 
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<p>동의어</p>
 
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Revision as of 22:53, 10 January 2009

Heterochromatin is a tightly packed form of DNA. Its major characteristic is that 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.[1] Originally, the two forms were distinguished cytologically by how intensely they stained - the former is less intense, while the latter stains intensely, indicating tighter packing. Heterochromatin is usually localized to the periphery of the nucleus.

Heterochromatin mainly consists of genetically inactive satellite sequences,[2] and many genes are repressed to various extents, although some cannot be expressed in euchromatin at all.[3] Heterochromatin also replicates later in S phase of the cell cycle than euchromatin, and is found only in eukaryotes. Both centromeres and telomeres 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.[citation needed] Some region of chromatin are very densly packed with fibres displaying a condition comparable to that of the chromosome at mitosis. 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,[4] which may be demonstrated by position-effect variegation.[5] 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,[6] and loci in two Saccharomyces spp.[7][8]).

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 expressed. For example, all human chromosomes 1, 9, 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 differentiation. An example of facultative heterochromatin is 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.[9]

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.[10]

External links

References

  1. ^ Elgin, S.C. (1996). "Heterochromatin and gene regulation in Drosophila". Curr. Opin. Genet. Dev. 6: 193–202. doi:10.1016/S0959-437X(96)80050-5. ISSN 0959-437X. 
  2. ^ Lohe, A.R., et al. (1993). "Mapping simple repeated DNA sequences in heterochromatin of Drosophila melanogaster". Genetics 134 (4): 1149–1174. ISSN 0016-6731. PMID 8375654. http://www.genetics.org/cgi/content/full/134/4/1149. 
  3. ^ Lu, B.Y., et al. (2000). "Heterochromatin protein 1 is required for the normal expression of two heterochromatin genes in Drosophila". Genetics 155 (2): 699–708. ISSN 0016-6731. PMID 10835392. http://www.genetics.org/cgi/content/full/155/2/699. 
  4. ^ Fisher, Amanda G.; Matthias Merkenschlager (April 2002). "Gene silencing, cell fate and nuclear organisation". Curr. Opin. Genet. Dev. 12 (2): 193–197. doi:10.1016/S0959-437X(02)00286-1. ISSN 0959-437X. 
  5. ^ Zhimulev, I.F., et al. (December 1986). "Cytogenetic and molecular aspects of position effect variegation in Drosophila melanogaster". Chromosoma 94 (6): 492–504. doi:10.1007/BF00292759. ISSN 1432-0886. 
  6. ^ Burgess-Beusse, B, et al. (December 2002). "The insulation of genes from external enhancers and silencing chromatin". Proc. Natl Acad. Sci. USA 9 (Suppl 4): 16433–16437. doi:10.1073/pnas.162342499. PMID 12154228. 
  7. ^ Noma, K., et al. (August 2001). "transitions in distinct histone H3 methylation patterns at the heterochromatin domain boundaries". Science 293 (5532): 1150–1155. doi:10.1126/science.1064150. PMID 11498594. 
  8. ^ Donze, D. & R.T. Kamakaka (2000). "RNA polymerase III and RNA polymerase II promoter complexes are heterochromatin barriers in Saccharomyces cerevisiae". Embo J. 20: 520–31. doi:10.1093/emboj/20.3.520. 
  9. ^ Talbert PB, Henikoff S. Spreading of silent chromatin: inaction at a distance. Nat Rev Genet. 2006 Oct;7(10):793-803.
  10. ^ Vavasseur et al. (2008). "Heterochromatin Assembly and Transcriptional Gene Silencing under the Control of Nuclear RNAi: Lessons from Fission Yeast". RNA and the Regulation of Gene Expression: A Hidden Layer of Complexity. Caister Academic Press. ISBN 978-1-904455-25-7. 
  • Z. Avramova Heterochromatin in Animals and Plants. Similarities and Differences. Plant Physiology May 2002, Vol. 129, pp. 40-49.
  • Caron, H., et al. (2001). "The Human Transcriptome Map: Clustering of Highly Expressed Genes in Chromosomal Domains". Science 291 (5507): 1289–1292. doi:10.1126/science.1056794. PMID 11181992. 


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