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 is the most active part of the genome within the cell nucleus . 92% of the human genome is euchromatic.
The structure of euchromatins is reminiscent of an unfolded set of beads on a string, in that 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 covering is loosened so that raw DNA can be accessed. Each core histone has a ‘tail’ structure, which can differ in several ways; These variations are believed to act as “master control switches”, which determine the overall arrangement of chromatin. In particular, it is believed that the presence of methylated lysine 4 on the histone tail serves as a general marker.
In general, euchromatins appears as a light-colored band when stained with G banding and viewed under an optical microscope , unlike heterochromatin , which is darker. This slight 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 later evolved with the nucleus , possibly as a mechanism to handle the increasing genome size.
Euchromatin participates in the active transcription of mRNA products of DNA. 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 what is not, is converted to heterochromatin to protect genes while they are not in use. Therefore there is a direct relationship between how actively a cell is producing and the amount of euchromatin found in its nucleus.
It is thought that the cell uses the change from euchromatin to heterochromatin as a method of controlling gene expression and replication, as such processes behave differently on densely compact chromatin, known as the ‘access hypothesis’. is referred to as. An example of constitutive euchromatin that is ‘always on’ is the housekeeping gene , which codes for proteins required for basic functions of cell survival.