Heterokromatin

Heterochromatin is divided into constitutive and facultative types. Constitutive heterochromatin is repeat-rich, gene-poor, and permanently compacted, characterized by methylated histone tails and binding of Heterochromatin Protein 1. Facultative heterochromatin is repeat-poor and gene-silent but flexible—genes can be repressed in one cell yet expressed in another, often bound by Polycomb repressive complex 2.

Methylation of histone tails increases the affinity between histones and DNA, promoting chromatin compaction and blocking DNA access. Methylated histones attract Heterochromatin Protein 1, a nonhistone protein that facilitates further chromatin compaction and spreads constitutive heterochromatin across the region, maintaining gene repression.

Polycomb repressive complex 2 binds facultative heterochromatin regions and di- or tri-methylates H3 histones, contributing to transcriptional repression. PRC1 then binds to methylated nucleosomes and condenses chromatin into a compact structure. These complexes contain histone deacetylases and methyltransferases that inhibit transcription and maintain chromatin repression.

Heterochromatin appears darkly stained because its high compaction allows it to bind more DNA staining dye than loosely packed euchromatin. The dense, condensed structure of both constitutive and facultative heterochromatin increases dye uptake, making these regions visibly darker under microscopic observation.

A Barr body is an inactivated X chromosome visible as a dense, darkly-stained spot at the nucleus periphery in female mammalian cells. This represents facultative heterochromatin—one X chromosome becomes highly condensed, repressing all its genes. This ensures X-linked genes are expressed at equal levels in both males and females.

Constitutive heterochromatin is permanently gene-poor and repressed across all cell types. Facultative heterochromatin is flexible—genes within it can be repressed in one cell type but expressed in another at the same chromosomal locus. This reversibility makes facultative heterochromatin responsive to cellular differentiation and developmental signals.

Constitutive heterochromatin is highly concentrated at centromeres and telomeres, regions rich in repetitive DNA sequences. Human chromosomes 1, 9, 16, and the Y chromosome in males contain large portions of constitutive heterochromatin. These regions remain permanently compacted and transcriptionally silent across all cell types.