Chromatin
Chromatin is the material that makes up the chromosomes. It is a complex of DNA and specific proteins, of which about half of histones. The name comes from the Greek chroma ( color), because chromatin be stained with basic dyes core. In the light microscope, it appears as a visible filament scaffold in the nucleus of a eukaryotic cell. In a functional sense, all that can be found again during the division of the cell nucleus (mitosis or meiosis) in the chromosomes applies as chromatin - except for some structural proteins. Chromatin is in addition to the nucleoli of the core matrix and the core shell is an important structural component of the nucleus (nucleus ).
Chromatin is comprised of DNA, which is wound around the histones, as well as other proteins, which bind to the DNA. DNA and histones form nucleosomes that are chain-like strung together. The nucleosomes are closely packed with the help of non-histone proteins. Chromatin is thus the product of interactions of eukaryotic DNA with different DNA-binding proteins that form a compact filamentous complex, the so-called Desoxyribonucleoprotein complex, also referred to as chromatin or chromatin (English: chromatin fibers ). Due to the complex formation, the long chromosomal DNA strands are in length by the 10,000 - to 50,000 -fold reduced ( condensed), so that they fit into the cell nucleus. Despite the dense packing of the DNA chromosomes are still in a form which allows access to the DNA regulatory proteins such that the biosynthesis of RNA and protein from the genetic information ( gene expression ) or the duplication of the chromosomal DNA ( replication ) is possible.
During mitosis and meiosis, the chromosomes condense, so that they are visible in the light microscope.
The understanding of chromatin structure and its contribution to regulation of the genes is the content of epigenetics.
Chromatin make rods in nocturnal mammals sensitive because they affect the propagation of light. In non- mammals, the phenomenon (as of 2010 ) has not yet been investigated.
Chromatin types
There are two types of chromatin:
- Euchromatin, the DNA is active, ie it can be expressed into proteins. The euchromatic segments of the chromosome do not exhibit differences in their structure, no matter to what degree of condensation, a chromosome is.
- Heterochromatin, mainly composed of an inactive DNA. It seems to structural features in the different condensation stages exercise. The heterochromatic segments of the chromosome in the interphase have the same degree of condensation on how to metaphase, ie it is condensed in the interphase nucleus and occurs in the form of dense chromocenters in appearance. Heterochromatin can be divided into two types: Constitutive heterochromatin, which is never expressed. It can be found in the area of the centromere and usually consists of repetitive ( repetitive ) DNA sequences.
- Facultative heterochromatin, which is sometimes expressed.
Another conceptual distinction can therefore be made even after the nuclear division phase: Here the Interphasechromatin against the Metaphasechromatin with its very compact chromosomes is greatly loosened.
Prokaryotes have a circular DNA structure in contrast to eukaryotes. The eukaryotic chromosomes have which form the structure of the DNA.