Chromatin

The chromatin is the form in which the DNA in the core is presented.

Chromatinienne fiber

The Nucléosome S are proteinic discs octamères, an assembly of 8 molecules of Histones H2A, H2B, H3, H4 and H1, around whose DNA is rolled up and constituted a structure in collar of pearls (extremely rare structure in the cell, nucléosomes in fact is piled up the ones on the others). This one is in its turn compacted in the form of fibers of 30nm of diameter, basic component of the chromatin . This fiber itself can be more or less condensed (on-rolled up). On the level Ultrastructure L, in electronic microscopy, one distinguishes the Euchromatine, not very dense, which contains the Gène S credits and the Hétérochromatine, dense. At the beginning of Mitosis, chromatin condenses in the form of Chromosome S.

Types of chromatin

Two types of chromatin can be distinguished:
  • the Euchromatine, which consists of active DNA, of overall décondensée structure likely to be expressed out of protein.
  • the Hétérochromatine, areas of condensed DNA which consists of mainly inactive DNA. It seems to be used for structural ends during the chromosomal phases. The hétérochromatine can in its turn be subdivided in two types:
    • the constitutive hétérochromatine , which is not expressed overall. It is located around the Centromère and of the télomère and consists in general of repetitive sequences.
    • the optional hétérochromatine , which generally contains genes silences. The transciptome of the cell is controlled by this struture, thus the cells in final stage of differentiation (which must thus express a number of genes restricted, just ensuring their metabolism and their function) present many areas of this hétérochromatine optional. The example most frequently given is the inactivation of one of two X chromosomes in the mammals.

Centromere . Length Microtubule S are attached At the centromere and two opposite ends off the concealment. During mitosis, the microtubules sweater the chromatids apart, so that each daughter concealment inherits one set off chromatids. Ounce the concealments cuts divided, the chromatids are uncoiled and edge function again ace chromatin. In off splashes to their appearance, chromosomes are highly structured (fig. 2). For example, constrained with similar functions are often kept closed together in the nucleus, even yew they are far apart one the chromosome. The shorts ARM off has chromosome edge Be extended by has Satellite ribosomal chromosome that contains codes for RNA. -->

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