heterochromatin

Heterochromatin is a form of chromatin that remains densely packed and is largely transcriptionally inactive. It contrasts with euchromatin, which is more open and permissive to gene expression. Heterochromatin is found throughout the genome, often in regions rich in repetitive DNA and at the nuclear periphery, and it is characterized by specific histone modifications, DNA methylation, and a compact higher-order structure that reduces accessibility to transcriptional machinery.

Constitutive heterochromatin consists of permanently silent regions, typically rich in repetitive sequences such as satellite DNA at centromeres and pericentromeric regions, and often including telomeres. It is usually late-replicating and densely stained in microscopy. This form is marked by histone H3 lysine 9 methylation (H3K9me2/3) and binding of HP1 proteins, contributing to stable transcriptional repression and genome stability.

Facultative heterochromatin refers to regions that can transition between silent and active states during development or in response to cellular conditions. It is commonly associated with histone modifications such as H3K27me3 placed by Polycomb repressive complexes. A notable example is X-chromosome inactivation in female mammals, where one X chromosome becomes transcriptionally silent yet remains within the nucleus as a facultatively heterochromatic structure.

Molecular organization of heterochromatin often involves interactions with the nuclear lamina and formation of chromocenters, along with DNA methylation and, in some organisms, RNA interference pathways that help establish and maintain the silent state. Functions of heterochromatin include silencing transposable elements, maintaining genome stability, regulating gene expression through spatial genome organization, and ensuring proper chromosome segregation during cell division. Misregulation of heterochromatin is associated with aging and disease.