Heterochromatin

Chromatin is a tightly packed form of DNA- it is a complex of DNA, histone proteins and other nonhistone protein components. Chromatin structure impacts the regulation of gene expression. A chromatin unit, called the nucleosome, contains 146bp of DNA, which is wrapped around of an octamer of histones.
Nucleosomes are major obstacles to the basal transcription machinery accessing the template DNA. Nucleosomes, which are regularly spaced on the DNA are known as the 11nm DNA fiber. This 11nm fiber is still considered to be transcriptionally active, and therefore is described as euchromatin. Due to histone modification (i.e. histone hypoacetylation) and the action of histone-binding proteins this fiber is further on condensed. Histone H1 is also involved in chromatin compactation.The highest level of chromatin condensation in eukaryotic cells is observed during mitosis when chromosomes align at the metaphase plate. 
==History of discovery==
Nuclear components of eukaryotic cells were first visualized around 1840 using basophilic aniline dyes. These nuclear components are now known as chromosomes. Forty years later, Walther Flemming started ti call this colorable substance "chromatin". In 1904, T. Boveri discovered that chromatin transformes (condenses) into chromosomes during mitosis and decondenses again after cell division. Emil Heitz stained chromosomes and discovered that there are some chromosome regions in different individuals, that stain bring whereas other stain scarce. Heitz named these regions euchromatin (decondensed chromatin) and heterochromatin (condensed chromatin). He also proposed that genes in euchromatine are active, whereas genes in heterochromatin are passive. With subsequent improvements in staining methods and the development of electron microscopy, it became apparent that HC could be subdivided into constitutive (c) and facultative (f) HC.
==Function==
Heterochromatin has the ability to propagate and influence gene expression in a region-specific manner. It also represses recombination, which protects the genome integrity and the integrity of chromosomes. It is also a mean of epigenetic inheritance, since heterochromatin is usually clonally inherited.
==Types of heterochromatin==
===Facultative heterochromatin===
Facultative heterochromatin regions are condensed and transcriptionally silent. Nevertheless, these regions allow transcription:
*temporal: developmental stages, specific cell-cycle stages
*spacial: exogenous factors/signals, nuclear localization
*parental/heritable
​Facultative heterochromatin is activated by incorporation of specific/alternate chromatin components, chromatin modulation, action of trans-acting factors, and subnuclear localization.
===Constitutive heterochromatin===
Constitutive heterochromatin is a basic component of eukaryotic genomes. Constitutive heterochromatin regions share a strongly reduced level of meiotic remobination, low gene density, mosaic inactvation of the expression of euchromatic genes when moved nearby- position effect variegation, late replication during S phase, transcriptional inactivity and enrichment in highly repetitive satellite DNAs and transposable elements. Constitutive heterochromatin can be found in centromeres , duplications, genes and pseudogenes (in humans mostly on chromosomes 1,9,16 and X) and near telomeres .
==References==
Trojer P. and Reinberg D., Facultative Heterochromatin: Is There a Distinctive Molecular Signature? , Molecular Cell, 28: 1-13 (2007)
Dimitri P., Caizzi R., Giordano E., Accardo M.C., Lattanzi G. and Biamonti G., Constitutive heterochromatin: a surprising variety of expressed sequences , Chromosoma, 118: 419-435 (2009)
Grewal S.I.S. and Jia A., Heterochromatin revised , Nature Reviews Genetics, 8:35-46 (2007)