Euchromatin



Euchromatin is a lightly packed form of chromatin that is rich in gene concentration, and is often (but not always) under active 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 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 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'.

External links and references

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