Chromatin

In the observation of live or fixed cells detected within the nucleus zone of dense matter, which are receptive to different dyes, especially basic. Thanks to this ability is well painted and the core component called "chromatin" (from the Greek. Chroma - color, paint). The composition includes chromatin DNA complexed with protein. Have the same properties and chromosomes, which are clearly visible during mitotic cell division. In non-dividing (interphase) cells chromatin seen in a light microscope, can be more or less evenly fill the volume of the nucleus, or have a separate clumps.

Chromatin is an interphase nuclei chromosomes, which, however, at this time lose its compact form, disintegrate, dekondensiruyutsya. The extent of such decondensation of chromosomes may be different. Zone full decondensation their sites morphology called euchromatin (euchromatinum). With a partial loosening of chromosomes in the interphase nucleus visible areas of condensed chromatin, sometimes called heterochromatin (heterochromatinum).

Degree decondensation of chromosomal material - chromatin in interphase may reflect the functional load of the structure. Than "diffusive" distributed chromatin in the interphase nucleus (ie, more than euchromatin), the more intense it synthetic processes.

Maximally condensed chromatin during mitotic cell division, as it is found in solid form chromosomes. During this period the chromosome does not perform any synthetic functions, they do not occur inclusion precursors of DNA and RNA.

Thus, the chromosomes of cells can exist in two structural-functional States: active, desktop, partially or fully decondensation when they participate in the interphase nucleus of the processes of transcription and reduplication, and inactive, in a state of metabolic dormancy at the maximum of their kondensirovannoi when they perform the function of distribution and transfer of genetic material to daughter cells.

Observation of chromatin structure using an electron microscope showed that both drugs selected interphase chromatin or dedicated mitotic chromosomes and the structure of the nucleus on ultrathin sections are always visible elementary chromosomal fibers of a thickness of 20-25 nm.

Chemically, chromatin fibrils are complex systems deoxyribonucleoprotein (DNP), which are composed of DNA and specific chromosomal proteins, histone and nonhistone. In the structure of chromatin is found also RNA. Quantitative relationships of DNA, protein and RNA are 1:1,3:0,2. Found that the length of the individual linear DNA molecules can reach hundreds of micrometers or even centimeters. Among human chromosomes largest chromosome contains DNA with a total length of 7 cm to the total length of DNA molecules in all chromosomes in a single human cell is about 170 cm, which corresponds to 6 • 10^12.

Proteins of chromatin 60-70% of its dry weight. These include the so-called histones and nonhistone proteins. Nonhistone proteins constitute 20% of the number of histones. The histone - alkaline proteins enriched in basic amino acids (mainly lysine and arginine). Obvious structural role of histones, which not only provide specific styling chromosomal DNA, but are important in the regulation of transcription. Histones are located along the length of the DNA molecules are not uniformly, but in blocks. In one such block consists of 8 histone molecules, forming a so-called nucleosome. The size of the nucleosome about 10 nm. In the formation of nucleosome compaction occurs, the supercoiling of DNA, which leads to shortening the length of the chromosomal fibers about 5 times. The very same chromosomal fiber has the form of beads or rosary beads, where each bead is a nucleosome (see Fig. 17). Such fibers have a thickness of 10 nm advanced longitudinally condense and form the basic elemental chromatin fiber thickness of 25 nm.

Nonhistone proteins interphase nuclei form within the nucleus of the structural network, which is called the nuclear protein matrix, which is the basis that determines the morphology and metabolism of the nucleus.

In nuclei, in addition to chromatin sites and matrix, there are perichromatin fibrils, perichromatin and interchromatin granules. They contain RNA and are found in almost all active cores represent the RNA associated with the protein - ribonucleoprotein (informality). Matrices for the synthesis of these RNAS are different genes, scattered decondensation chromosomal sites (more precisely, chromatin) fibrils.

A special type of matrix DNA, and the DNA for the synthesis of ribosomal RNA, usually assembled in several compact areas included in the nuclei of interphase nuclei.

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