Heterochromatin

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 heterochromatin and facultative heterochromatin. Both play a role in the expression of genes. Because it is tightly packed, it was thought to be inaccessible to polymerases and therefore not transcribed, however, it is now known to be dynamically modified and is related to gene silencing.

Constitutive Heterochromatin

Constitutive heterochromatin can affect the genes near itself. It is usually repetitive and forms around the centromere and telomeres of a chromosome. This type of heterochromatin is present at the same locations within the chromosome, in all cells of an organism.

Facultative Heterochromatin

Facultative heterochromatin is the result of genes that are silenced through a process called methylation. This type of heterochromatin is not present in the same locations within the chromosome in different types of cells.

Role in Gene Regulation

Heterochromatin has been associated with the regulation of gene expression. When a section of a chromosome is organized into heterochromatin, the genes located there are less likely to be transcribed into mRNA. Therefore, the formation of heterochromatin can help to determine which genes are expressed.

See Also

• Euchromatin
• Epigenetics
• Histone

References

• 
  Heterochromatin vs. euchromatin
• 
  General model for duplication of heterochromatin during cell division
• 
  Heterochromatic versus euchromatic nuclei
• Error creating thumbnail: File with dimensions greater than 12.5 MP
  Human karyotype with bands and sub-bands