Euchromatin
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2008-06-23T02:50:24Z
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[[Image:Diagram human cell nucleus.svg|thumb|350px]]
'''Euchromatin''' is a lightly packed form of [[chromatin]] that is rich in [[gene]] concentration, and is often (but not always) under active [[transcription (genetics)|transcription]]. Unlike [[heterochromatin]], it is found in both [[eukaryotes]] and [[prokaryotes]].
==Structure==
The structure of euchromatin is reminiscient of an unfolded set of beads along a string, where those beads represent [[nucleosomes]]. Nucleosomes consist of eight proteins known as [[histones]], with approximately 146 [[base pairs]] of [[DNA]] wound around them; in euchromatin this wrapping is loose so that the raw DNA may be accessed. Each core histone possesses a `tail' structure which can vary in several ways; it is thought that these variations act as "master control switches" which determine the overall arrangement of the chromatin. In particular, it is believed that the presence of methylated lysine 4 on the histone tails acts as a general marker for euchromatin.
==Appearance==
Euchromatin generally appears as light-colored bands when stained in [[GTG banding]] and observed under an [[Microscope|optical microscope]]; in contrast to [[heterochromatin]], which stains darkly. This lighter staining is due to the less compact structure of euchromatin. It should be noted that in [[prokaryotes]], euchromatin is the ''only'' form of chromatin present; this indicates that the heterochromatin structure evolved later along with the [[cell nucleus|nucleus]], possibly as a mechanism to handle increasing genome size and therefore a decrease in safety/manageability.
==Function==
Euchromatin participates in the active transcription of DNA to [[mRNA]] products. The unfolded structure allows gene regulatory proteins and [[RNA polymerase]] complexes to bind to the DNA sequence, which can then initiate the transcription process. Not all euchromatin is necessarily transcribed, but in general that which is not is transformed into [[heterochromatin]] to protect the genes while they are not in use. There is therefore a direct link to how actively productive a cell is and the amount of euchromatin that can be found in its nucleus. It is thought that the cell uses transformation from euchromatin into heterochromatin as a method of controlling gene expression and replication, since such processes behave differently on densely compacted chromatin- this is known as the `accessibility hypothesis'. One example of constitutive euchromatin that is 'always turned on' is housekeeping genes, which codes for the proteins needed for basic functions of cell survival.
==External links and references==
http://www.euchromatin.net/
*[http://chromatin.co.uk/Euchromatin.html Research news in Euchromatin]
* {{cite journal | author = Zheng C, Hayes J | title = Structures and interactions of the core histone tail domains. | journal = Biopolymers | volume = 68 | issue = 4 | pages = 539–46 | year = 2003 | pmid = 12666178 | doi = 10.1002/bip.10303 <!--Retrieved from CrossRef by DOI bot-->}}
* {{cite journal | author = Muegge K | title = Modifications of histone cores and tails in V(D)J recombination. | journal = Genome Biol | volume = 4 | issue = 4 | pages = 211 | year = 2003 | pmid = 12702201 | doi = 10.1186/gb-2003-4-4-211 <!--Retrieved from CrossRef by DOI bot-->}} [http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=154571 Article]
* {{BUHistology|20102loa}}
[[Category:Molecular genetics]]
{{Cell-biology-stub}}
{{Chromo}}
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