Heterochromatin

Heterochromatin is a tightly packed form of DNA or condensed DNA, which comes in multiple varieties. These varieties lie on a continuum between the two extremes of constitutive heterochromatin and facultative heterochromatin. Both play a role in the expression of genes. Because it is tightly packed, it was thought to be inaccessible to polymerases and therefore not transcribed; however, according to Volpe et al. (2002),[1] and many other papers since,[2] much of this DNA is in fact transcribed, but it is continuously turned over via RNA-induced transcriptional silencing (RITS). Recent studies with electron microscopy and OsO4 staining reveal that the dense packing is not due to the chromatin.[3]

Constitutive heterochromatin can affect the genes near itself (e.g. position-effect variegation). It is usually repetitive and forms structural functions such as centromeres or telomeres, in addition to acting as an attractor for other gene-expression or repression signals.

Facultative heterochromatin is the result of genes that are silenced through a mechanism such as histone deacetylation or Piwi-interacting RNA (piRNA) through RNAi. It is not repetitive and shares the compact structure of constitutive heterochromatin. However, under specific developmental or environmental signaling cues, it can lose its condensed structure and become transcriptionally active.[4]

Heterochromatin has been associated with the di- and tri -methylation of H3K9 in certain portions of the human genome.[5] H3K9me3-related methyltransferases appear to have a pivotal role in modifying heterochromatin during lineage commitment at the onset of organogenesis and in maintaining lineage fidelity.[6]

  1. ^ Volpe TA, Kidner C, Hall IM, Teng G, Grewal SI, Martienssen RA (September 2002). "Regulation of heterochromatic silencing and histone H3 lysine-9 methylation by RNAi". Science. 297 (5588): 1833–7. Bibcode:2002Sci...297.1833V. doi:10.1126/science.1074973. PMID 12193640. S2CID 2613813.
  2. ^ "What is the current evidence showing active transcription withinin..." www.researchgate.net. Retrieved 2016-04-30.
  3. ^ Ou HD, Phan S, Deerinck TJ, Thor A, Ellisman MH, O'Shea CC (July 2017). "ChromEMT: Visualizing 3D chromatin structure and compaction in interphase and mitotic cells". Science. 357 (6349): eaag0025. doi:10.1126/science.aag0025. PMC 5646685. PMID 28751582.
  4. ^ Oberdoerffer P, Sinclair DA (September 2007). "The role of nuclear architecture in genomic instability and ageing". Nature Reviews. Molecular Cell Biology. 8 (9): 692–702. doi:10.1038/nrm2238. PMID 17700626. S2CID 15674132.
  5. ^ Rosenfeld JA, Wang Z, Schones DE, Zhao K, DeSalle R, Zhang MQ (March 2009). "Determination of enriched histone modifications in non-genic portions of the human genome". BMC Genomics. 10 (1): 143. doi:10.1186/1471-2164-10-143. PMC 2667539. PMID 19335899.
  6. ^ Nicetto D, Donahue G, Jain T, Peng T, Sidoli S, Sheng L, et al. (January 2019). "H3K9me3-heterochromatin loss at protein-coding genes enables developmental lineage specification". Science. 363 (6424): 294–297. Bibcode:2019Sci...363..294N. doi:10.1126/science.aau0583. PMC 6664818. PMID 30606806.