Heterochromatine

Heterochromatin, also spelled hétérochromatine in some languages, refers to chromatin regions that are densely packed and largely transcriptionally inactive compared with euchromatin. It is characterized by compact structure, low gene density, repetitive DNA sequences, and distinctive epigenetic marks, such as DNA methylation and histone modifications that promote a repressive state. Heterochromatin tends to replicate late in S phase and is often positioned at the nuclear periphery or around the nucleolus, contributing to higher-order genome organization.

There are two major types of heterochromatin. Constitutive heterochromatin remains permanently compacted in most cell types and is enriched in repetitive sequences such as satellite DNA found at centromeres and telomeres. It is typically marked by H3K9me3 and binding of HP1 proteins, with high levels of DNA methylation contributing to long-term silencing of these regions. Facultative heterochromatin is developmentally regulated and can switch between repressed and active states. A well-known example is the inactive X chromosome in female mammals, which forms a Barr body; facultative heterochromatin often bears the H3K27me3 mark deposited by Polycomb group proteins.

Mechanistically, heterochromatin is established and maintained by a combination of DNA methylation, histone modifications, and chromatin-binding proteins that promote a compact, inaccessible structure. Its organization is linked to nuclear architecture through lamina-associated domains and nucleolar-associated regions, reinforcing transcriptional silencing.

Functions of heterochromatin include silencing repetitive elements and transposons to preserve genome integrity, regulating gene expression in development, and contributing to proper chromosome segregation and genome stability. Misregulation of heterochromatin is associated with aging and various diseases.