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heterochromatin

Thursday 20 November 2003

Heterochromatin is one of the two forms of Chromatin, the assembly of DNA and DNA-associated proteins which forms chromosomes, with euchromatin being the other form.

Genes that lie within heterochromatin are generally silenced. DNA packaged in heterochromatin usually is densely compacted (which results in intense staining with DNA-binding dyes), late replicating within the cell cycle relative to euchromatin, and localized to the periphery of the nucleus.

A particular set of protein modifications and proteins differentiates heterochromatin from euchromatin.

Histones (the proteins around which DNA is wound) in heterochromatin display a pattern of post-translational modifications which varies between organisms, but generally includes the absence of acetylation, and the presence or absence of methylation, on specific amino acid residues within the histone protein sequence.

Specific non-histone proteins are contained within heterochromatin and are absent from euchromatin, and many of these proteins are essential for the stability of the heterochromatin. In some organisms heterochromatin can also be characterized by association with small RNA molecules and high levels of DNA methylation.

Heterochromatin is generally stable, so when a cell divides the two offspring cells will typically both contain heterochromatin along the same regions of DNA, resulting in epigenetic inheritance.

There are two types of heterochromatin: constitutive heterochromatin and facultative heterochromatin.

All cells of a given species will package the same regions of DNA in constitutive heterochromatin, and thus in all cells any genes contained within the constitutive heterochromatin will be poorly expressed.

For example, all human chromosomes 1, 9, 16, and the Y chromosome contain large regions of constitutive heterochromatin. In most organisms, constitutive heterochromatin occurs around the chromosome centromere and near telomeres.

The regions of DNA packaged in facultative heterochromatin will not be consistent within the cells of a species, and thus a sequence in one cell that is packaged in facultative heterochromatin (and the genes within poorly expressed) may be packaged in euchromatin in another cell (and the genes within no longer silenced).

However, the formation of facultative heterochromatin is regulated, and is often associated with morphogenesis or differentiation. An example of facultative heterochromatin is X-chromosome inactivation in female mammals: one X chromosome is packaged in facultative heterochromatin and silenced, while the other X chromosome in packaged in euchromatin and expressed.

See also

- heterochromatic genes

References

- Yasuhara JC, Wakimoto BT. Oxymoron no more: the expanding world of heterochromatic genes. Trends Genet. 2006 Jun;22(6):330-8. PMID: 16690158

- Horvath JE, Bailey JA, Locke DP, Eichler EE. Lessons from the human genome: transitions between euchromatin and heterochromatin. Hum Mol Genet. 2001 Oct 1;10(20):2215-23. PMID: 11673404