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Euchromatin
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<h2 id="structure">Structure</h2> <p>Euchromatin is composed of repeating subunits known as <a href="/facts/Nucleosome/HX5DR9bn">nucleosomes</a>, reminiscent of an unfolded set of beads on a string, that are approximately 11 nm in diameter.<a class="footnote-ref" id="fnref:2" href="#fn:2"><sup>2</sup></a> At the core of these nucleosomes are a set of four <a href="/facts/Histone/krb5VJfp">histone</a> protein pairs: <a href="/facts/Histone_H3/MXc7C8v2">H3</a>, <a href="/facts/Histone_H4/lcGq1Xpv">H4</a>, <a href="/facts/Histone_H2A/mE8C347j">H2A</a>, and <a href="/facts/Histone_H2B/h8gyE0fp">H2B</a>.<a class="footnote-ref" id="fnref:3" href="#fn:3"><sup>3</sup></a> Each core histone protein possesses a 'tail' structure, which can vary in several ways; it is thought that these variations act as "master control switches" through different <a href="/facts/Methylation/EvDMWVW5">methylation</a> and <a href="/facts/Acetylation/PNMzRM4r">acetylation</a> states, which determine the overall arrangement of the chromatin.<a class="footnote-ref" id="fnref:4" href="#fn:4"><sup>4</sup></a> Approximately 147 base pairs of <a href="/facts/DNA/nB89Iauz">DNA</a> are wound around the histone octamers, or a little less than 2 turns of the helix.<a class="footnote-ref" id="fnref:5" href="#fn:5"><sup>5</sup></a> Nucleosomes along the strand are linked together via the histone, <a href="/facts/Histone_H1/vok0flPo">H1</a>,<a class="footnote-ref" id="fnref:6" href="#fn:6"><sup>6</sup></a> and a short space of open <a href="/facts/Linker_DNA/J8ebo485">linker DNA</a>, ranging from around 0-80 base pairs. The key distinction between the structure of euchromatin and <a href="/facts/Heterochromatin/frJ8HL41">heterochromatin</a> is that the nucleosomes in euchromatin are much more widely spaced, which allows for easier access of different protein complexes to the DNA strand and thus increased gene <a href="/facts/Transcription_(biology)/bDgp9HDN">transcription</a>.<a class="footnote-ref" id="fnref:7" href="#fn:7"><sup>7</sup></a> </p> <h2 id="appearance">Appearance</h2> <p>Euchromatin resembles a set of beads on a string at large magnifications.<a class="footnote-ref" id="fnref:8" href="#fn:8"><sup>8</sup></a> From farther away, it can resemble a ball of tangled thread, such as in some <a href="/facts/Micrograph/M6fk5VED">electron microscope</a> visualizations.<a class="footnote-ref" id="fnref:9" href="#fn:9"><sup>9</sup></a> In both optical and electron microscopic visualizations, euchromatin appears lighter in color than <a href="/facts/Heterochromatin/frJ8HL41">heterochromatin</a> - which is also present in the <a href="/facts/Cell_nucleus/D7sFRa3c">nucleus</a> and appears darkly<a class="footnote-ref" id="fnref:10" href="#fn:10"><sup>10</sup></a> - due to its less compact structure.<a class="footnote-ref" id="fnref:11" href="#fn:11"><sup>11</sup></a> When visualizing <a href="/facts/Chromosome/FRGf11FC">chromosomes</a>, such as in a <a href="/facts/Karyotype/UM8j41yA">karyogram</a>, <a href="/facts/Cytogenetics/Rj22VPvy">cytogenetic banding</a> is used to stain the chromosomes. Cytogenetic banding allows us to see which parts of the chromosome are made up of euchromatin or heterochromatin in order to differentiate chromosomal subsections, irregularities or rearrangements.<a class="footnote-ref" id="fnref:12" href="#fn:12"><sup>12</sup></a> One such example is <a href="/facts/G_banding/dEuKPThJ">G banding</a>, otherwise known as <a href="/facts/Giemsa_stain/qkA81qvW">Giemsa staining</a> where euchromatin appears lighter than heterochromatin.<a class="footnote-ref" id="fnref:13" href="#fn:13"><sup>13</sup></a> </p> Appearance of Heterochromatin and Euchromatin Under Various Visualization Techniques<a class="footnote-ref" id="fnref:14" href="#fn:14"><sup>14</sup></a><a class="footnote-ref" id="fnref:15" href="#fn:15"><sup>15</sup></a><a class="footnote-ref" id="fnref:16" href="#fn:16"><sup>16</sup></a><a class="footnote-ref" id="fnref:17" href="#fn:17"><sup>17</sup></a><a class="footnote-ref" id="fnref:18" href="#fn:18"><sup>18</sup></a><a class="footnote-ref" id="fnref:19" href="#fn:19"><sup>19</sup></a><table><tbody><tr><th></th><th>Giemsa (G-) Banding</th><th>Reverse (R-) Banding</th><th>Constitutive Heterochromatin (C-) banding</th><th>Quinacrine (Q-) banding</th><th>Telomeric R (T-) banding</th></tr><tr><td>Euchromatin</td><td>Lighter</td><td>Darker</td><td>Lighter</td><td>Dull</td><td>Light</td></tr><tr><td>Heterochromatin</td><td>Darker</td><td>Lighter</td><td>Darker</td><td>Bright (Fluorescent)</td><td>Darker (Faint)</td></tr></tbody></table> <h2 id="function">Function</h2> <h3>Transcription</h3> <p>Euchromatin participates in the active <a href="/facts/Transcription_(biology)/bDgp9HDN">transcription</a> of <a href="/facts/DNA/nB89Iauz">DNA</a> to <a href="/facts/MRNA/oUL6qxQn">mRNA</a> products. The unfolded structure allows gene regulatory proteins and <a href="/facts/RNA_polymerase/gHd0ET9R">RNA polymerase</a> complexes to bind to the DNA sequence, which can then initiate the transcription process.<a class="footnote-ref" id="fnref:20" href="#fn:20"><sup>20</sup></a> While not all euchromatin is necessarily transcribed, as the euchromatin is divided into transcriptionally active and inactive domains,<a class="footnote-ref" id="fnref:21" href="#fn:21"><sup>21</sup></a> euchromatin is still generally associated with active gene transcription. 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. </p><p>It is thought that the cell uses transformation from euchromatin into heterochromatin as a method of controlling <a href="/facts/Gene_expression/alCPohLR">gene expression</a> and <a href="/facts/DNA_replication/wrU6RuR3">replication</a>, since such processes behave differently on densely compacted chromatin. This is known as the 'accessibility hypothesis'.<a class="footnote-ref" id="fnref:22" href="#fn:22"><sup>22</sup></a> One example of constitutive euchromatin that is 'always turned on' is <a href="/facts/Housekeeping_genes/N9yVF3lR">housekeeping genes</a>, which code for the proteins needed for basic functions of cell survival.<a class="footnote-ref" id="fnref:23" href="#fn:23"><sup>23</sup></a> </p> <h3>Epigenetics</h3> <p><a href="/facts/Epigenetics/UfAAERcn">Epigenetics</a> involves changes in the <a href="/facts/Phenotype/vxWXKGLj">phenotype</a> that can be inherited without changing the DNA sequence. This can occur through many types of environmental interactions.<a class="footnote-ref" id="fnref:24" href="#fn:24"><sup>24</sup></a> Regarding euchromatin, post-translational modifications of the histones can alter the structure of chromatin, resulting in altered gene expression without changing the DNA.<a class="footnote-ref" id="fnref:25" href="#fn:25"><sup>25</sup></a> Additionally, a loss of heterochromatin and increase in euchromatin has been shown to correlate with an accelerated <a href="/facts/Ageing/AxL4qCW4">aging process</a>, especially in <a href="/facts/Progeroid_syndromes/XupMdWW4">diseases known to resemble premature aging</a>.<a class="footnote-ref" id="fnref:26" href="#fn:26"><sup>26</sup></a> Research has shown epigenetic markers on histones for a number of additional diseases.<a class="footnote-ref" id="fnref:27" href="#fn:27"><sup>27</sup></a><a class="footnote-ref" id="fnref:28" href="#fn:28"><sup>28</sup></a> </p> <h2 id="regulation">Regulation</h2> <p>Euchromatin is primarily regulated by <a href="/facts/Post-translational_modification/KMUayTIc">post-translational modifications</a> to its nucleosomes' <a href="/facts/Histone/krb5VJfp">histones</a>, conducted by many <a href="/facts/Histone-modifying_enzymes/JC9zSUvu">histone-modifying enzymes</a>. These modifications occur on the histones' <a href="/facts/N-terminus/yKYpuaBJ">N-terminal</a> tails that protrude from the nucleosome structure, and are thought of to recruit enzymes to either keep the chromatin in its open form, as euchromatin, or in its closed form, as <a href="/facts/Heterochromatin/frJ8HL41">heterochromatin</a>.<a class="footnote-ref" id="fnref:29" href="#fn:29"><sup>29</sup></a> <a href="/facts/Histone_acetylation_and_deacetylation/jafcW82T">Histone acetylation</a>, for instance, is typically associated with euchromatin structure, whereas <a href="/facts/Histone_methylation/GP0gj9Av">histone methylation</a> promotes heterochromatin remodeling.<a class="footnote-ref" id="fnref:30" href="#fn:30"><sup>30</sup></a> Acetylation makes the histone group more negatively charged, which in turn disrupts its interactions with the DNA strand, essentially "opening" the strand for easier access.<a class="footnote-ref" id="fnref:31" href="#fn:31"><sup>31</sup></a> Acetylation can occur on multiple <a href="/facts/Lysine/CEetgA9L">lysine</a> residues of a histone's <a href="/facts/N-terminus/yKYpuaBJ">N-terminal</a> tail and in different histones of the same nucleosome, which is thought to further increase DNA accessibility for <a href="/facts/Transcription_factor/wro0DPHe">transcription factors</a>.<a class="footnote-ref" id="fnref:32" href="#fn:32"><sup>32</sup></a> </p><p><a href="/facts/Phosphorylation/LXUEEeIL">Phosphorylation</a> of histones is another method by which euchromatin is regulated.<a class="footnote-ref" id="fnref:33" href="#fn:33"><sup>33</sup></a> This tends to occur on the N-terminal tails of the histones, however some sites are present in the core.<a class="footnote-ref" id="fnref:34" href="#fn:34"><sup>34</sup></a> Phosphorylation is controlled by <a href="/facts/Kinases/dhlHpjnD">kinases</a> and <a href="/facts/Phosphatase/LBJs1fXx">phosphatases</a>, which add and remove the phosphate groups respectively. This can occur at <a href="/facts/Serine/4duVl30B">serine</a>, <a href="/facts/Threonine/7zffFQ6W">threonine</a>, or <a href="/facts/Tyrosine/RJhEgE4C">tyrosine</a> residues present in euchromatin.<a class="footnote-ref" id="fnref:35" href="#fn:35"><sup>35</sup></a><a class="footnote-ref" id="fnref:36" href="#fn:36"><sup>36</sup></a> Since the phosphate groups added to the structure will incorporate a negative charge, it will promote the more relaxed "open" form, similar to acetylation.<a class="footnote-ref" id="fnref:37" href="#fn:37"><sup>37</sup></a> In regards to functionality, histone phosphorylation is involved with gene expression, DNA damage repair, and <a href="/facts/Chromatin_remodeling/ukC9TjoG">chromatin remodeling</a>.<a class="footnote-ref" id="fnref:38" href="#fn:38"><sup>38</sup></a> </p><p>Another method of regulation that incorporates a negative charge, thereby favoring the "open" form, is <a href="/facts/ADP-ribosylation/rv6QqIqv">ADP ribosylation</a>.<a class="footnote-ref" id="fnref:39" href="#fn:39"><sup>39</sup></a> This process adds one or more <a href="/facts/Adenosine_diphosphate_ribose/fmaoRzeG">ADP-ribose</a> units to the histone, and is involved in the <a href="/facts/DNA-damage_response/hsURuYYq">DNA damage response</a> pathway.<a class="footnote-ref" id="fnref:40" href="#fn:40"><sup>40</sup></a> </p> <h2 id="see-also">See also</h2> <ul><li><a href="/facts/Histone-modifying_enzymes/JC9zSUvu">Histone Modifying Enzymes</a></li> <li><a href="/facts/Constitutive_heterochromatin/JdSVe3Tp">Constitutive Heterochromatin</a></li></ul> <h2 id="further-reading">Further reading</h2> <ul><li>Heterochromatin formation involves changes in histone modifications over multiple cell generations – Katan-Khaykovich Y, Struhl K (June 2005). <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1150886">"Heterochromatin formation involves changes in histone modifications over multiple cell generations"</a>. <i>The EMBO Journal</i>. 24 (12): 2138–2149. <a href="/facts/Doi_(identifier)/muM9Etpq">doi</a>:<a href="https://doi.org/10.1038%2Fsj.emboj.7600692">10.1038/sj.emboj.7600692</a>. <a href="/facts/PMC_(identifier)/dX1zMt71">PMC</a> <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1150886">1150886</a>. <a href="/facts/PMID_(identifier)/JlHAvMHt">PMID</a> <a href="https://pubmed.ncbi.nlm.nih.gov/15920479">15920479</a>.</li> <li>Chromatin Velocity reveals epigenetic dynamics by single-cell profiling of heterochromatin and euchromatin – Tedesco M, Giannese F, Lazarević D, Giansanti V, Rosano D, Monzani S, et al. (October 2021). "Chromatin Velocity reveals epigenetic dynamics by single-cell profiling of heterochromatin and euchromatin". <i>Nature Biotechnology</i>. 40 (2): 235–244. <a href="/facts/Doi_(identifier)/muM9Etpq">doi</a>:<a href="https://doi.org/10.1038%2Fs41587-021-01031-1">10.1038/s41587-021-01031-1</a>. <a href="/facts/Hdl_(identifier)/rdebSxmC">hdl</a>:<a href="https://hdl.handle.net/11368%2F3007419">11368/3007419</a>. <a href="/facts/PMID_(identifier)/JlHAvMHt">PMID</a> <a href="https://pubmed.ncbi.nlm.nih.gov/34635836">34635836</a>. <a href="/facts/S2CID_(identifier)/ldJsHa2Y">S2CID</a> <a href="https://api.semanticscholar.org/CorpusID:238637962">238637962</a>.</li> <li>Epigenetic inheritance and the missing heritability – Trerotola M, Relli V, Simeone P, Alberti S (July 2015). <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4517414">"Epigenetic inheritance and the missing heritability"</a>. <i>Human Genomics</i>. 9 (1): 17. <a href="/facts/Doi_(identifier)/muM9Etpq">doi</a>:<a href="https://doi.org/10.1186%2Fs40246-015-0041-3">10.1186/s40246-015-0041-3</a>. <a href="/facts/PMC_(identifier)/dX1zMt71">PMC</a> <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4517414">4517414</a>. <a href="/facts/PMID_(identifier)/JlHAvMHt">PMID</a> <a href="https://pubmed.ncbi.nlm.nih.gov/26216216">26216216</a>.</li> <li>Histone epigenetic marks in heterochromatin and euchromatin of the Chagas' disease vector, <i>Triatoma infestans</i> – Alvarenga EM, Rodrigues VL, Moraes AS, Naves LS, Mondin M, Felisbino MB, Mello ML (May 2016). "Histone epigenetic marks in heterochromatin and euchromatin of the Chagas' disease vector, Triatoma infestans". <i>Acta Histochemica</i>. 118 (4): 401–412. <a href="/facts/Doi_(identifier)/muM9Etpq">doi</a>:<a href="https://doi.org/10.1016%2Fj.acthis.2016.04.002">10.1016/j.acthis.2016.04.002</a>. <a href="/facts/PMID_(identifier)/JlHAvMHt">PMID</a> <a href="https://pubmed.ncbi.nlm.nih.gov/27079857">27079857</a>.</li></ul> <h2 id="references">References</h2> <ol> <li id="fn:1"><p>International Human Genome Sequencing Consortium (October 2004). "Finishing the euchromatic sequence of the human genome". Nature. 431 (7011): 931–945. Bibcode:2004Natur.431..931H. doi:10.1038/nature03001. PMID 15496913. S2CID 186242248. <a href="https://doi.org/10.1038%2Fnature03001" target="_blank">https://doi.org/10.1038%2Fnature03001</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></p></li> <li id="fn:2"><p>Babu A, Verma RS (January 1987). Bourne GH, Jeon KW, Friedlander M (eds.). "Chromosome structure: euchromatin and heterochromatin". International Review of Cytology. 108. Academic Press: 1–60. doi:10.1016/s0074-7696(08)61435-7. ISBN 978-0-12-364508-1. PMID 2822591. <a href="978-0-12-364508-1" target="_blank">978-0-12-364508-1</a> <a href="#fnref:2" class="footnote-back-ref">↩</a></p></li> <li id="fn:3"><p>Babu A, Verma RS (January 1987). Bourne GH, Jeon KW, Friedlander M (eds.). "Chromosome structure: euchromatin and heterochromatin". International Review of Cytology. 108. Academic Press: 1–60. doi:10.1016/s0074-7696(08)61435-7. ISBN 978-0-12-364508-1. PMID 2822591. <a href="978-0-12-364508-1" target="_blank">978-0-12-364508-1</a> <a href="#fnref:3" class="footnote-back-ref">↩</a></p></li> <li id="fn:4"><p>Babu A, Verma RS (January 1987). Bourne GH, Jeon KW, Friedlander M (eds.). "Chromosome structure: euchromatin and heterochromatin". International Review of Cytology. 108. Academic Press: 1–60. doi:10.1016/s0074-7696(08)61435-7. ISBN 978-0-12-364508-1. PMID 2822591. <a href="978-0-12-364508-1" target="_blank">978-0-12-364508-1</a> <a href="#fnref:4" class="footnote-back-ref">↩</a></p></li> <li id="fn:5"><p>"Definition: nucleosome/nucleosomes". Scitable Nature Education. Retrieved 2021-10-06. <a href="https://www.nature.com/scitable/definition/nucleosome-nucleosomes-30/" target="_blank">https://www.nature.com/scitable/definition/nucleosome-nucleosomes-30/</a> <a href="#fnref:5" class="footnote-back-ref">↩</a></p></li> <li id="fn:6"><p>Mobley AS (January 2019). "Chapter 4 - Induced Pluripotent Stem Cells". In Mobley AS (ed.). Neural Stem Cells and Adult Neurogenesis. Academic Press. pp. 67–94. ISBN 978-0-12-811014-0. <a href="978-0-12-811014-0" target="_blank">978-0-12-811014-0</a> <a href="#fnref:6" class="footnote-back-ref">↩</a></p></li> <li id="fn:7"><p>Babu A, Verma RS (January 1987). Bourne GH, Jeon KW, Friedlander M (eds.). "Chromosome structure: euchromatin and heterochromatin". International Review of Cytology. 108. Academic Press: 1–60. doi:10.1016/s0074-7696(08)61435-7. ISBN 978-0-12-364508-1. PMID 2822591. <a href="978-0-12-364508-1" target="_blank">978-0-12-364508-1</a> <a href="#fnref:7" class="footnote-back-ref">↩</a></p></li> <li id="fn:8"><p>Babu A, Verma RS (January 1987). Bourne GH, Jeon KW, Friedlander M (eds.). "Chromosome structure: euchromatin and heterochromatin". International Review of Cytology. 108. Academic Press: 1–60. doi:10.1016/s0074-7696(08)61435-7. ISBN 978-0-12-364508-1. PMID 2822591. <a href="978-0-12-364508-1" target="_blank">978-0-12-364508-1</a> <a href="#fnref:8" class="footnote-back-ref">↩</a></p></li> <li id="fn:9"><p>"The cell. 4. Nucleus. Chromatin. Atlas of plant and animal histology". mmegias.webs.uvigo.es. Retrieved 2021-12-02. <a href="https://mmegias.webs.uvigo.es/02-english/5-celulas/4-cromatina.php" target="_blank">https://mmegias.webs.uvigo.es/02-english/5-celulas/4-cromatina.php</a> <a href="#fnref:9" class="footnote-back-ref">↩</a></p></li> <li id="fn:10"><p>Enukashvily NI (January 2013). "Chapter Two - Mammalian Satellite DNA: A Speaking Dumb". In Donev R, Ponomartsev NV (eds.). Advances in Protein Chemistry and Structural Biology. Organisation of Chromosomes. Vol. 90. Academic Press. pp. 31–65. doi:10.1016/B978-0-12-410523-2.00002-X. ISBN 978-0-12-410523-2. PMID 23582201. <a href="978-0-12-410523-2" target="_blank">978-0-12-410523-2</a> <a href="#fnref:10" class="footnote-back-ref">↩</a></p></li> <li id="fn:11"><p>"The cell. 4. Nucleus. Chromatin. Atlas of plant and animal histology". mmegias.webs.uvigo.es. Retrieved 2021-12-02. <a href="https://mmegias.webs.uvigo.es/02-english/5-celulas/4-cromatina.php" target="_blank">https://mmegias.webs.uvigo.es/02-english/5-celulas/4-cromatina.php</a> <a href="#fnref:11" class="footnote-back-ref">↩</a></p></li> <li id="fn:12"><p>Shen CH (January 2019). "Chapter 13 - Molecular Diagnosis of Chromosomal Disorders". In Shen CH (ed.). Diagnostic Molecular Biology. Academic Press. pp. 331–358. doi:10.1016/B978-0-12-802823-0.00013-4. ISBN 978-0-12-802823-0. S2CID 131915096. <a href="978-0-12-802823-0" target="_blank">978-0-12-802823-0</a> <a href="#fnref:12" class="footnote-back-ref">↩</a></p></li> <li id="fn:13"><p>"Giemsa banding". Biology Articles, Tutorials & Dictionary Online. 2019-10-07. Retrieved 2021-12-02. <a href="https://www.biologyonline.com/dictionary/giemsa-banding" target="_blank">https://www.biologyonline.com/dictionary/giemsa-banding</a> <a href="#fnref:13" class="footnote-back-ref">↩</a></p></li> <li id="fn:14"><p>"Giemsa banding". Biology Articles, Tutorials & Dictionary Online. 2019-10-07. Retrieved 2021-12-02. <a href="https://www.biologyonline.com/dictionary/giemsa-banding" target="_blank">https://www.biologyonline.com/dictionary/giemsa-banding</a> <a href="#fnref:14" class="footnote-back-ref">↩</a></p></li> <li id="fn:15"><p>"Reverse banding - Definition and Examples - Biology Online Dictionary". Biology Articles, Tutorials & Dictionary Online. 2020-09-18. Retrieved 2021-12-02. <a href="https://www.biologyonline.com/dictionary/reverse-banding" target="_blank">https://www.biologyonline.com/dictionary/reverse-banding</a> <a href="#fnref:15" class="footnote-back-ref">↩</a></p></li> <li id="fn:16"><p>"Constitutive heterochromatin banding". Biology Articles, Tutorials & Dictionary Online. 2019-10-07. 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