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Heterochromatin is a tightly packed form of DNA, which comes in multiple 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. Constitutive heterochromatin can affect the genes near them (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. Heterochromatin has been associated with the di- and tri-methylation of H3K9 in certain portions of the genome. ==Structure== Chromatin is found in two varieties: euchromatin and heterochromatin.〔 〕 Originally, the two forms were distinguished cytologically by how intensely they stained – the euchromatin is less intense, while heterochromatin stains intensely, indicating tighter packing. Heterochromatin is usually localized to the periphery of the nucleus. Despite this early dichotomy, recent evidence in both animals〔 〕 and plants〔 〕 has suggested that there are more than two distinct heterochromatin states, and it may in fact exist in four or five 'states', each marked by different combinations of epigenetic marks. Heterochromatin mainly consists of genetically inactive satellite sequences,〔 〕 and many genes are repressed to various extents, although some cannot be expressed in euchromatin at all.〔 〕 Both centromeres and telomeres are heterochromatic, as is the Barr body of the second, inactivated X-chromosome in a female. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Heterochromatin」の詳細全文を読む スポンサード リンク
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