The inactivation of gene transcription as a result of condensation of chromosomal DNA into heterochromatin (see chromatin). A key mechanism for maintaining DNA in a silent state is chemical modification of the histone proteins that form the scaffolding around which the DNA is coiled. Methylation of certain amino acids in the ‘tails’ of the histones, which protrude from the coiled chromatin structure, is associated with silencing of genes, whereas acetylation tends to activate gene transcription. Chromatin-binding proteins, such as heterochromatin protein 1 (HP1), bind to the methylated histones and reinforce the silenced state by recruiting histone methyltransferases, which add more methyl groups to the histones. It is now known that small RNA molecules are also involved in chromatin silencing, for example in inactivation of one or more X chromosomes in female mammals (see X inactivation).
Subjects: Biological Sciences.