Euchromatineuchromatin vs heterochromatin, euchromatin
Euchromatin is a lightly packed form of chromatin DNA, RNA, and protein that is enriched in genes, 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
- 2 Appearance
- 3 Function
- 4 References
- 5 External links
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
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
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
- ^ "Finishing the euchromatic sequence of the human genome" Nature 431 7011: 931–45 21 October 2004 doi:101038/nature03001 PMID 15496913
- Research news in Euchromatin
- Zheng C, Hayes J 2003 "Structures and interactions of the core histone tail domains" Biopolymers 68 4: 539–46 doi:101002/bip10303 PMID 12666178
- Muegge K 2003 "Modifications of histone cores and tails in VDJ recombination" Genome Biol 4 4: 211 doi:101186/gb-2003-4-4-211 PMC 154571 PMID 12702201 Article
- Histology image: 20102loa – Histology Learning System at Boston University
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