Euchromatin

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 is composed of repeating subunits known as nucleosomes, reminiscent of an unfolded set of beads on a string, that are approximately 11 nm in diameter.

[2] Each core histone protein possesses a 'tail' structure, which can vary in several ways; it is thought that these variations act as "master control switches" through different methylation and acetylation states, which determine the overall arrangement of the chromatin.

[3] Nucleosomes along the strand are linked together via the histone, H1,[4] and a short space of open linker DNA, ranging from around 0-80 base pairs.

The unfolded structure allows gene regulatory proteins and RNA polymerase complexes to bind to the DNA sequence, which can then initiate the transcription process.

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.

[14] 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.

[21][22] Since the phosphate groups added to the structure will incorporate a negative charge, it will promote the more relaxed "open" form, similar to acetylation.