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<p><strong>Euchromatin</strong> is a lightly packed form of <a title="Chromatin" href="http://en.wikipedia.org/wiki/Chromatin"><font color="#000080">chromatin</font></a> that is rich in <a title="Gene" href="http://en.wikipedia.org/wiki/Gene"><font color="#000080">gene</font></a> concentration, and is often (but not always) under active <a title="Transcription (genetics)" href="http://en.wikipedia.org/wiki/Transcription_(genetics)"><font color="#000080">transcription</font></a>. Unlike <a title="Heterochromatin" href="http://en.wikipedia.org/wiki/Heterochromatin"><font color="#000080">heterochromatin</font></a>, it is found in both <a class="mw-redirect" title="Eukaryotes" href="http://en.wikipedia.org/wiki/Eukaryotes"><font color="#000080">eukaryotes</font></a> and <a class="mw-redirect" title="Prokaryotes" href="http://en.wikipedia.org/wiki/Prokaryotes"><font color="#000080">prokaryotes</font></a>. Euchromatin comprises the most active portion of the genome within the cell nucleus.</p>
<p><a id="Structure" name="Structure"></a></p>
<h2><span class="mw-headline">Structure</span></h2>
<p>The structure of euchromatin is reminiscient of an unfolded set of beads along a string, where those beads represent <a class="mw-redirect" title="Nucleosomes" href="http://en.wikipedia.org/wiki/Nucleosomes"><font color="#000080">nucleosomes</font></a>. Nucleosomes consist of eight proteins known as <a class="mw-redirect" title="Histones" href="http://en.wikipedia.org/wiki/Histones"><font color="#000080">histones</font></a>, with approximately 147 <a class="mw-redirect" title="Base pairs" href="http://en.wikipedia.org/wiki/Base_pairs"><font color="#000080">base pairs</font></a> of <a title="DNA" href="http://en.wikipedia.org/wiki/DNA"><font color="#000080">DNA</font></a> wound around them; in euchromatin this wrapping is loose so that the raw DNA may be accessed. Each core histone possesses a `tail' structure which can vary in several ways; it is thought that these variations act as "master control switches" which determine the overall arrangement of the chromatin. In particular, it is believed that the presence of methylated lysine 4 on the histone tails acts as a general marker for euchromatin.</p>
<p><a id="Appearance" name="Appearance"></a></p>
<h2><span class="mw-headline">Appearance</span></h2>
<p>Euchromatin generally appears as light-colored bands when stained in <a class="new" title="GTG banding (page does not exist)" href="http://en.wikipedia.org/w/index.php?title=GTG_banding&action=edit&redlink=1"><font color="#000080">GTG banding</font></a> and observed under an <a title="Microscope" href="http://en.wikipedia.org/wiki/Microscope"><font color="#000080">optical microscope</font></a>; in contrast to <a title="Heterochromatin" href="http://en.wikipedia.org/wiki/Heterochromatin"><font color="#000080">heterochromatin</font></a>, which stains darkly. This lighter staining is due to the less compact structure of euchromatin. It should be noted that in <a class="mw-redirect" title="Prokaryotes" href="http://en.wikipedia.org/wiki/Prokaryotes"><font color="#000080">prokaryotes</font></a>, euchromatin is the <em>only</em> form of chromatin present; this indicates that the heterochromatin structure evolved later along with the <a title="Cell nucleus" href="http://en.wikipedia.org/wiki/Cell_nucleus"><font color="#000080">nucleus</font></a>, possibly as a mechanism to handle increasing genome size</p>
<p><a id="Function" name="Function"></a></p>
<h2><span class="mw-headline">Function</span></h2>
<p>Euchromatin participates in the active transcription of DNA to <a class="mw-redirect" title="MRNA" href="http://en.wikipedia.org/wiki/MRNA"><font color="#000080">mRNA</font></a> products. The unfolded structure allows gene regulatory proteins and <a title="RNA polymerase" href="http://en.wikipedia.org/wiki/RNA_polymerase"><font color="#000080">RNA polymerase</font></a> complexes to bind to the DNA sequence, which can then initiate the transcription process. Not all euchromatin is necessarily transcribed, but in general that which is not is transformed into <a title="Heterochromatin" href="http://en.wikipedia.org/wiki/Heterochromatin"><font color="#000080">heterochromatin</font></a> to protect the genes while they are not in use. There is therefore a direct link to how actively productive a cell is and the amount of euchromatin that can be found in its nucleus. It is thought that the cell uses transformation from euchromatin into heterochromatin as a method of controlling gene expression and replication, since such processes behave differently on densely compacted chromatin- this is known as the `accessibility hypothesis'. One example of constitutive euchromatin that is 'always turned on' is <a class="mw-redirect" title="Housekeeping genes" href="http://en.wikipedia.org/wiki/Housekeeping_genes"><font color="#000080">housekeeping genes</font></a>, which codes for the proteins needed for basic functions of cell survival.</p>
<p><a id="External_links_and_references" name="External_links_and_references"></a></p>
<h2><span class="mw-headline">External links and references</span></h2>
<p><a class="external free" title="http://www.euchromatin.net/" href="http://www.euchromatin.net/" rel="nofollow"><font color="#000080">http://www.euchromatin.net/</font></a></p>
<ul>
<li><a class="external text" title="http://chromatin.co.uk/Euchromatin.html" href="http://chromatin.co.uk/Euchromatin.html" rel="nofollow"><font color="#000080">Research news in Euchromatin</font></a> </li>
<li><cite class="Journal" id="CITEREFZheng_C.2C_Hayes_J2003" style="FONT-STYLE: normal">Zheng C, Hayes J (2003). "Structures and interactions of the core histone tail domains.". <em>Biopolymers</em> <strong>68</strong> (4): 539–46. <a title="Digital object identifier" href="http://en.wikipedia.org/wiki/Digital_object_identifier"><font color="#000080">doi</font></a>:<span class="neverexpand"><a class="external text" title="http://dx.doi.org/10.1002%2Fbip.10303" href="http://dx.doi.org/10.1002%2Fbip.10303" rel="nofollow"><font color="#000080">10.1002/bip.10303</font></a></span>. <a class="external" title="http://www.ncbi.nlm.nih.gov/pubmed/12666178" href="http://www.ncbi.nlm.nih.gov/pubmed/12666178"><font color="#000080">PMID 12666178</font></a>.</cite><span class="Z3988" title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Ajournal&rft.genre=article&rft.atitle=Structures+and+interactions+of+the+core+histone+tail+domains.&rft.jtitle=Biopolymers&rft.aulast=Zheng+C%2C+Hayes+J&rft.au=Zheng+C%2C+Hayes+J&rft.date=2003&rft.volume=68&rft.issue=4&rft.pages=539%E2%80%9346&rft_id=info:doi/10.1002%2Fbip.10303&rft_id=info:pmid/12666178&rfr_id=info:sid/en.wikipedia.org:Euchromatin"><span style="DISPLAY: none"> </span></span> </li>
<li><cite class="Journal" id="CITEREFMuegge_K2003" style="FONT-STYLE: normal">Muegge K (2003). "Modifications of histone cores and tails in V(D)J recombination.". <em>Genome Biol</em> <strong>4</strong> (4): 211. <a title="Digital object identifier" href="http://en.wikipedia.org/wiki/Digital_object_identifier"><font color="#000080">doi</font></a>:<span class="neverexpand"><a class="external text" title="http://dx.doi.org/10.1186%2Fgb-2003-4-4-211" href="http://dx.doi.org/10.1186%2Fgb-2003-4-4-211" rel="nofollow"><font color="#000080">10.1186/gb-2003-4-4-211</font></a></span>. <a class="external" title="http://www.ncbi.nlm.nih.gov/pubmed/12702201" href="http://www.ncbi.nlm.nih.gov/pubmed/12702201"><font color="#000080">PMID 12702201</font></a>.</cite><span class="Z3988" title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Ajournal&rft.genre=article&rft.atitle=Modifications+of+histone+cores+and+tails+in+V%28D%29J+recombination.&rft.jtitle=Genome+Biol&rft.aulast=Muegge+K&rft.au=Muegge+K&rft.date=2003&rft.volume=4&rft.issue=4&rft.pages=211&rft_id=info:doi/10.1186%2Fgb-2003-4-4-211&rft_id=info:pmid/12702201&rfr_id=info:sid/en.wikipedia.org:Euchromatin"><span style="DISPLAY: none"> </span></span> <a class="external text" title="http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=154571" href="http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=154571" rel="nofollow"><font color="#000080">Article</font></a> </li>
<li><a title="Boston University" href="http://en.wikipedia.org/wiki/Boston_University"><font color="#000080">Histology at BU</font></a> <em><a class="external text" title="http://www.bu.edu/histology/p/20102loa.htm" href="http://www.bu.edu/histology/p/20102loa.htm" rel="nofollow"><font color="#000080">20102loa</font></a></em> </li>
</ul>
<p> </p>
<p><a id="Structure" name="Structure"></a></p>
<h2><span class="mw-headline">Structure</span></h2>
<p>The structure of euchromatin is reminiscient of an unfolded set of beads along a string, where those beads represent <a class="mw-redirect" title="Nucleosomes" href="http://en.wikipedia.org/wiki/Nucleosomes"><font color="#000080">nucleosomes</font></a>. Nucleosomes consist of eight proteins known as <a class="mw-redirect" title="Histones" href="http://en.wikipedia.org/wiki/Histones"><font color="#000080">histones</font></a>, with approximately 147 <a class="mw-redirect" title="Base pairs" href="http://en.wikipedia.org/wiki/Base_pairs"><font color="#000080">base pairs</font></a> of <a title="DNA" href="http://en.wikipedia.org/wiki/DNA"><font color="#000080">DNA</font></a> wound around them; in euchromatin this wrapping is loose so that the raw DNA may be accessed. Each core histone possesses a `tail' structure which can vary in several ways; it is thought that these variations act as "master control switches" which determine the overall arrangement of the chromatin. In particular, it is believed that the presence of methylated lysine 4 on the histone tails acts as a general marker for euchromatin.</p>
<p><a id="Appearance" name="Appearance"></a></p>
<h2><span class="mw-headline">Appearance</span></h2>
<p>Euchromatin generally appears as light-colored bands when stained in <a class="new" title="GTG banding (page does not exist)" href="http://en.wikipedia.org/w/index.php?title=GTG_banding&action=edit&redlink=1"><font color="#000080">GTG banding</font></a> and observed under an <a title="Microscope" href="http://en.wikipedia.org/wiki/Microscope"><font color="#000080">optical microscope</font></a>; in contrast to <a title="Heterochromatin" href="http://en.wikipedia.org/wiki/Heterochromatin"><font color="#000080">heterochromatin</font></a>, which stains darkly. This lighter staining is due to the less compact structure of euchromatin. It should be noted that in <a class="mw-redirect" title="Prokaryotes" href="http://en.wikipedia.org/wiki/Prokaryotes"><font color="#000080">prokaryotes</font></a>, euchromatin is the <em>only</em> form of chromatin present; this indicates that the heterochromatin structure evolved later along with the <a title="Cell nucleus" href="http://en.wikipedia.org/wiki/Cell_nucleus"><font color="#000080">nucleus</font></a>, possibly as a mechanism to handle increasing genome size</p>
<p><a id="Function" name="Function"></a></p>
<h2><span class="mw-headline">Function</span></h2>
<p>Euchromatin participates in the active transcription of DNA to <a class="mw-redirect" title="MRNA" href="http://en.wikipedia.org/wiki/MRNA"><font color="#000080">mRNA</font></a> products. The unfolded structure allows gene regulatory proteins and <a title="RNA polymerase" href="http://en.wikipedia.org/wiki/RNA_polymerase"><font color="#000080">RNA polymerase</font></a> complexes to bind to the DNA sequence, which can then initiate the transcription process. Not all euchromatin is necessarily transcribed, but in general that which is not is transformed into <a title="Heterochromatin" href="http://en.wikipedia.org/wiki/Heterochromatin"><font color="#000080">heterochromatin</font></a> to protect the genes while they are not in use. There is therefore a direct link to how actively productive a cell is and the amount of euchromatin that can be found in its nucleus. It is thought that the cell uses transformation from euchromatin into heterochromatin as a method of controlling gene expression and replication, since such processes behave differently on densely compacted chromatin- this is known as the `accessibility hypothesis'. One example of constitutive euchromatin that is 'always turned on' is <a class="mw-redirect" title="Housekeeping genes" href="http://en.wikipedia.org/wiki/Housekeeping_genes"><font color="#000080">housekeeping genes</font></a>, which codes for the proteins needed for basic functions of cell survival.</p>
<p><a id="External_links_and_references" name="External_links_and_references"></a></p>
<h2><span class="mw-headline">External links and references</span></h2>
<p><a class="external free" title="http://www.euchromatin.net/" href="http://www.euchromatin.net/" rel="nofollow"><font color="#000080">http://www.euchromatin.net/</font></a></p>
<ul>
<li><a class="external text" title="http://chromatin.co.uk/Euchromatin.html" href="http://chromatin.co.uk/Euchromatin.html" rel="nofollow"><font color="#000080">Research news in Euchromatin</font></a> </li>
<li><cite class="Journal" id="CITEREFZheng_C.2C_Hayes_J2003" style="FONT-STYLE: normal">Zheng C, Hayes J (2003). "Structures and interactions of the core histone tail domains.". <em>Biopolymers</em> <strong>68</strong> (4): 539–46. <a title="Digital object identifier" href="http://en.wikipedia.org/wiki/Digital_object_identifier"><font color="#000080">doi</font></a>:<span class="neverexpand"><a class="external text" title="http://dx.doi.org/10.1002%2Fbip.10303" href="http://dx.doi.org/10.1002%2Fbip.10303" rel="nofollow"><font color="#000080">10.1002/bip.10303</font></a></span>. <a class="external" title="http://www.ncbi.nlm.nih.gov/pubmed/12666178" href="http://www.ncbi.nlm.nih.gov/pubmed/12666178"><font color="#000080">PMID 12666178</font></a>.</cite><span class="Z3988" title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Ajournal&rft.genre=article&rft.atitle=Structures+and+interactions+of+the+core+histone+tail+domains.&rft.jtitle=Biopolymers&rft.aulast=Zheng+C%2C+Hayes+J&rft.au=Zheng+C%2C+Hayes+J&rft.date=2003&rft.volume=68&rft.issue=4&rft.pages=539%E2%80%9346&rft_id=info:doi/10.1002%2Fbip.10303&rft_id=info:pmid/12666178&rfr_id=info:sid/en.wikipedia.org:Euchromatin"><span style="DISPLAY: none"> </span></span> </li>
<li><cite class="Journal" id="CITEREFMuegge_K2003" style="FONT-STYLE: normal">Muegge K (2003). "Modifications of histone cores and tails in V(D)J recombination.". <em>Genome Biol</em> <strong>4</strong> (4): 211. <a title="Digital object identifier" href="http://en.wikipedia.org/wiki/Digital_object_identifier"><font color="#000080">doi</font></a>:<span class="neverexpand"><a class="external text" title="http://dx.doi.org/10.1186%2Fgb-2003-4-4-211" href="http://dx.doi.org/10.1186%2Fgb-2003-4-4-211" rel="nofollow"><font color="#000080">10.1186/gb-2003-4-4-211</font></a></span>. <a class="external" title="http://www.ncbi.nlm.nih.gov/pubmed/12702201" href="http://www.ncbi.nlm.nih.gov/pubmed/12702201"><font color="#000080">PMID 12702201</font></a>.</cite><span class="Z3988" title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Ajournal&rft.genre=article&rft.atitle=Modifications+of+histone+cores+and+tails+in+V%28D%29J+recombination.&rft.jtitle=Genome+Biol&rft.aulast=Muegge+K&rft.au=Muegge+K&rft.date=2003&rft.volume=4&rft.issue=4&rft.pages=211&rft_id=info:doi/10.1186%2Fgb-2003-4-4-211&rft_id=info:pmid/12702201&rfr_id=info:sid/en.wikipedia.org:Euchromatin"><span style="DISPLAY: none"> </span></span> <a class="external text" title="http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=154571" href="http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=154571" rel="nofollow"><font color="#000080">Article</font></a> </li>
<li><a title="Boston University" href="http://en.wikipedia.org/wiki/Boston_University"><font color="#000080">Histology at BU</font></a> <em><a class="external text" title="http://www.bu.edu/histology/p/20102loa.htm" href="http://www.bu.edu/histology/p/20102loa.htm" rel="nofollow"><font color="#000080">20102loa</font></a></em> </li>
</ul>
<p> </p>