Heterochromatin

Heterochromatin is a tightly packed form of DNA, which comes in different varieties. These varieties lie on a continuum between the two extremes of constitutive and facultative heterochromatin. Both play a role in the expression of genes, where constitutive heterochromatin can affect the genes near them (position-effect variegation) and where facultative heterochromatin is the result of genes that are silenced through a mechanism such as histone methylation or siRNA through RNAi. Constitutive heterochromatin 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 not repetitive and although it shares the compact structure of constitutive heterochromatin, facultative heterochromatin can, under specific developmental or environmental signaling cues, lose its condensed structure and become transcriptionally active. Heterochromatin is often associated with the di and tri-methylation of H3K9.