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]