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出典(authority):フリー百科事典『ウィキペディア（Wikipedia）』「2013/02/01 14:12:40」(JST)

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Euchromatin is a lightly packed form of chromatin (DNA, RNA and protein) that is rich in gene concentration, and is often (but not always) under active transcription. Euchromatin comprises the most active portion of the genome within the cell nucleus. 92% of the human genome is euchromatic.^[1]

Structure

The structure of euchromatin is reminiscent of an unfolded set of beads along a string, wherein those beads represent nucleosomes. Nucleosomes consist of eight proteins known as histones, with approximately 147 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

In general, euchromatin appears as light-colored bands when stained in G banding and observed under an optical microscope, in contrast to heterochromatin, which stains darkly. This lighter staining is due to the less compact structure of euchromatin. The basic structure of euchromatin is an elongated, open, 10 nm microfibril, as noted by electron microscopy. In prokaryotes, euchromatin is the only form of chromatin present; this indicates that the heterochromatin structure evolved later along with the nucleus, possibly as a mechanism to handle increasing genome size.

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, known as the `accessibility hypothesis'. One example of constitutive euchromatin that is 'always turned on' is housekeeping genes, which code for the proteins needed for basic functions of cell survival.

References

^ http://www.nature.com/nature/journal/v431/n7011/full/nature03001.html

External links

http://www.euchromatin.net/
Research news in Euchromatin
Zheng C, Hayes J (2003). "Structures and interactions of the core histone tail domains.". Biopolymers 68 (4): 539–46. doi:10.1002/bip.10303. PMID 12666178.
Muegge K (2003). "Modifications of histone cores and tails in V(D)J recombination". Genome Biol 4 (4): 211. doi:10.1186/gb-2003-4-4-211. PMC 154571. PMID 12702201. //www.ncbi.nlm.nih.gov/pmc/articles/PMC154571/. Article
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English Journal

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Microanatomical study of testis in juvenile ostrich (Struthio camelus).

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Histone-lysine methyltransferase EHMT2 is involved in proliferation, apoptosis, cell invasion, and DNA methylation of human neuroblastoma cells.

Lu Z, Tian Y, Salwen HR, Chlenski A, Godley LA, Raj JU, Yang Q.SourceDepartment of Pediatrics, College of Medicine, University of Illinois at Chicago, Chicago, Illinois 60612, USA.
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Japanese Journal

Expression of the rice microRNA miR820 is associated with epigenetic modifications at its own locus

Nosaka Misuzu,Ono Akemi,Ishiwata Aiko [他],SHIMIZU SATO Sae,ISHIMOTO Kiyoe,NODA Yusaku,SATO Yutaka
Genes & genetic systems 88(2), 105-112, 2013-04-25
… MIRNA genes are usually located in transcriptionally active euchromatic regions. …
NAID 10031183176

Metaphase karyotypes of four species of Calliphoridae (Diptera)

Agrawal Uma Rani,Bajpai Neelam,Kurahashi Hiromu [他],TEWARI Raghav Ram
Chromosome science 13(3), 49-52, 2011-08-01
… The chromosome complement of all the species comprises 2n=12, with five pairs of large euchromatic autosomes and a pair of heteromorphic sex chromosomes-XX in females and XY in males. …
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Boundaries of transcriptionally silent chromatin in Saccharomyces cerevisiae

Sun Jing-Qian,Hatanaka Akira,Oki Masaya
Genes & genetic systems 86(2), 73-81, 2011-04-25
… Many assays show that specific DNA regulatory elements separate a euchromatic locus from a neighboring heterochromatic domain and thereby function as a boundary. …
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