Heterochromatin
As heterochromatin condensed chromatin is called the nucleus, which can be stained well. Here, the deoxyribonucleic acid (DNA ) is present in strongly bound to histone and non-histone proteins form. Thus, the genetic information remains largely inactive. The term heterochromatin is used in contrast to euchromatin, which is easily accessible. Heterochromatin is different staining techniques of light microscopy (eg Feulgen reaction, Giemsa staining) stronger ( positive heteropyknotisch ) colored than euchromatin. Also in the transmission electron microscope, it appears dark because of its strong electron absorption.
Types of heterochromatin
- Constitutive heterochromatin is often near the centromeres and is then known as centromerisches heterochromatin. Here are located directly behind each other numerous copies Repetitive DNA but no genes. Constitutive heterochromatin is formed in all cells of an organism to the same sections of chromosomes, hence the name.
- In contrast, facultative heterochromatin forms only in some cells of an organism. The best known example is the Barr body, one of the two X chromosomes in the cells of female mammals. It is closed to the gene dosage of a female mammal a male X, Y align individual, and is characterized heterochromatic. In female organisms with an extra X chromosome ( trisomy X) two X chromosomes will be decommissioned heterochromatic so often hardly any symptoms occur.
The classic primary marker for heterochromatin is the highly conserved heterochromatin protein 1 ( HP1 ). Another characteristic feature of the hypoacetylation heterochromatic regions (the absence of acetyl groups ), and methylation of certain amino acids of the histones.
Aggregates of heterochromatin is known as chromocenters.
History
With the technical terms heterochromatin and euchromatin clarified Emil Heitz proposed by Walther Flemming word Chromatin: The non- heterochromatic Chromatinbereiche are euchromatic. Moreover Heitz first observed in Drosophila virilis that in endoreplizierenden nuclei multiplies the ( constitutive ) heterochromatin far less than the euchromatin or not at all.