Heterochromatin - chromatin areas that are in a condensed (compact) state during the cell cycle. A feature of heterochromatic DNA is extremely low transcribable .
Opening History
In 1907, the German cytologist S. Gutherts ( S. Gutherz ) discovered that some fragments of chromosomes or chromosomes during the whole cell division were intensely stained and looked more condensed compared to poorly stained areas. This phenomenon has been called heteropicnosis, but the term has not yet taken root [1] . In the nuclei of cells in the interphase , areas intensively stained with chromatin binding dyes were found, such areas were called chromocenters. S. Guthertz showed that heteropypicotic segments of chromosomes become noticeable at the beginning of the prophase, that is, at the beginning of the condensation of chromosomes, differing from the “normal” areas in more intense coloration; Differences in coloration intensity decrease as they condense and become practically indistinguishable at the end of metaphase.
Another German cytologist , analyzing the ratio of chromocenters and geteropiknoticheskih chromosomal regions observed during mitosis in cells of moss, came to the conclusion that chromocenters detected in interphase, associated with highly condensed and intense stained geteropiknoticheskimi regions of the chromosomes observed during The mitotic cycle, that is, the chromocenters and heteropyknotic sites are the same sites of chromosomes that are not subject to decondensation in telophase.
In 1928, Heutz proposed the terms “ euchromatin ” for chromosome regions that undergo a process of compaction – decompactation in the process of mitosis and “heterochromatin” for sites that remain permanently in a condensed state. Heutz believed that the heterochromatic regions of chromosomes are genetically inert [1] .
Optional and constitutive (structural) heterochromatin
The main functional difference of the optional heterochromatin from the constitutive is the possibility of transition to the euchromatin state in which DNA becomes transcriptionally active and, accordingly, expression of genes localized in this region of the chromosome occurs.
The optional heterochromatin contains coding and, therefore, relatively conservative DNA; Constitutive heterochromatin DNA is predominantly non-coding and, therefore, highly polymorphic and variable.
In the early stages of ontogenesis, in many cases, the content of heterochromatin in metaphase chromosomes is significantly lower than in the later stages and in the cells of an adult organism — the metaphase chromosomes of the blastomeres of many vertebrates are highly compacted; in the interphase nuclei, heterochromatic formations are not detected.
The optional and constitutive heterochromatins are also detected by the difference in staining: if the optional heterochromatin undergoes G-staining according to Romanowsky-Giemsa under standard conditions, then the coloring with the same dye after denaturation of DNA-rataturation selectively stains the constitutive heterochromatin. Such a selective method is called coloration for constitutive (C) heterochromatin, or C-staining .
Optional heterochromatin
Usually, optional heterochromatic regions are present only on one of the homologous chromosomes. A typical example of an optional heterochromatin is an inactive sex chromosome with a homogametic karyotype , for example, an inactive X chromosome in female mammals that is deactivated to a condensed heterochromatin state; such heterochromatic X -chromosome is observed in the interphase as the Barr body . At the same time, during gametogenesis and in the early stages of embryogenesis, both X- chromosomes are euchromatin and transcriptionally active.
Another example of the formation of the optional heterochromatin is the pachytic stage of meiotic division of heterogametic gametocyte, which is accompanied by spermatogenesis in mammals during the formation of the XY chromosome heterochromatin complex - the genital vesicle. The formation of such a heterochromatic complex is temporary and reversible, necessary for deactivating the sex chromosomes at this stage of meiosis : when the X and Y chromosomes remain active at this stage, there is an imbalance between the products of autosome expression and sex chromosomes, which leads to death cells.
The optional heterochromatin also causes the “silence” of tissue-specific genes that become euchromatin and are expressed only in differentiated cells of certain tissues: ~ 10% of the genes are active in such cells - the rest of the genes are inactivated and are part of the optional heterochromatin.
Constitutive heterochromatin
Constitutive (structural) heterochromatin is contained in both homologous chromosomes and is localized predominantly in the exposed sites — the centromere , telomeres , and nucleolar organizer . Constitutive heterochromatin DNA is predominantly satellite DNA consisting of tandem repeats (for example, HS1 (Human Satellite 1), HS2, HS3, alpha satellite and other human satellites). In the interphase nucleus, the constitutive heterochromatin forms chromocenters on the inner side of the nuclear membrane, as well as in regions of nucleolar organizers. The question of the functional role of structural heterochromatin in a eukaryotic cell remains open.
Features of the structure and composition of heterochromatin
Chromatin is a nucleoprotein - a complex of DNA with histones . Chromatin condensation into heterochromatin is accompanied by both the modification of histones and the complication of the composition of the nucleoprotein complex due to the participation of heterochromatin HP1 proteins (Heterochromatin Protein 1) in it.
The histones of the heterochromatin complex are characterized by a low degree of acetylation of lysine residues, which increases their basic properties and, accordingly, their binding to acidic phosphate groups of DNA, which contributes to the compactification of the complex. Another feature leading to the formation of heterochromatin is the methylation of the 27th lysine residue of the histone H3 by Polycomb-complex 2 (PRC2) proteins and the 9th lysine residue of the histone H3 by the histone methyltransferase Suv39h. Methylation of the 9th lysine residue of histone H3 leads to the formation of a high affinity binding site of histone H3 and the heterochromatin protein HP1. In Drosophila Suv39h methyltransferase functionally associated with a histone deacetylase in a way that the acetylated and methylated state of the 9th lysine residue of histone H3 are mutually exclusive, that is, provides a single mechanism for the deacetylation and methylation of histone H3, leading to increased binding of histone both DNA and protein heterochromatin HP1.
See also
- Polycomb group proteins
- CAF-1
Notes
- ↑ 1 2 Koryakov D. E., Zhimulev I. F. Chromosomes. Structure and functions / Ed. Dr.Sc. L.V. Vysotskaya. - Novosibirsk: Publishing House of the Siberian Branch of the Russian Academy of Sciences, 2009. - 258 p. - ISBN 978-8-7692-1045-7 . - p. 51-60.
Literature
- Zhimulev I.F. Heterochromatin and the effect of gene position. - Novosibirsk: Science, 1993.
- Prokof'eva-Belgovskaya A.A. Heterochromatic regions of chromosomes. - M .: Science, 1986. - 430 p.
- Marie-Geneviève Mattei, Judith Luciani. Heterochromatin, from Chromosome to Protein // Atlas of Genetics and Cytogenetics in Oncology and Haematology
Links
- Elena Naimark. The participation of heterochromatin in speciation proven . // Elements: a popular site about fundamental science (11/03/2009). The appeal date is June 13, 2012. Archived August 6, 2012.