Everything about Euchromatin totally explained
Euchromatin is a lightly packed form of
chromatin that's rich in
gene concentration, and is often (but not always) under active
transcription. Unlike
heterochromatin, it's 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's thought that these variations act as "master control switches" which determine the overall arrangement of the chromatin. In particular, it's 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 isn't is transformed into
heterochromatin to protect the genes while they're 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's 'always turned on' is housekeeping genes, which codes for the proteins needed for basic functions of cell survival.
External links and references
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