Chromatin

Chromatin, or nuclear material, is a complex of DNA and proteins that together form a nucleohistone (chromosome) thread. In the nucleus, where nuclear division does not take place, chromatin is found in 2 forms, heterochromatin and euchromatin.

Heterochromatin and Euchromatin
If we stained the nuclear mass with nuclear dyes, "euchromatin" appears as a lighter, i.e. more transcriptionally active part (the fiber is more relaxed) when stained with nuclear dyes. The parts that turn darker are called heterochromatin, they are places where transcription is not active. Heterochromatin is further divided into:
 * constitutive – permanently inactive sections of DNA (typical are large heterochromatin blocks on chromosomes 1, 9, 16 and Y );
 * optional - currently inactive sections (under certain conditions they can resume their function) e.g. developmental genesy.

Structure of Chromatin

 * DNA
 * Specific proteins - Histones and non-histone proteins

The DNA double helix is ​​wrapped around the histone octamers H2A, H2B, H3, H4. Histone H1 forms a link between these formations. This whole thing together (DNA, histone octamer and histone H1) forms a 'nucleosome.

Nucleosome
It represents the basic building block of the chromosome (nucleohistone) fiber. Several nucleosomes in a row form formations similar to beads on a thread - so-called polynucleosomes.

Histones
In addition to the construction function, they also have a regulatory function. It forms the histone code. They are involved in the regulation of gene expression as one of many epigenetic modifications.

E.g.:
 * methylation of a lysine residue on H3 stops expression
 * acetylation of a lysine residue on H3 initiates transcription
 * and others: phosphorylation, ubiquitinization, ADP ribosylation affecting expression regulation as well as chromatin condensation and decondensation

Links
ws:Chromatin

Related Articles

 * Chromosomes
 * Histones
 * Transcript
 * Epigenetics