KERNEL (nucleolus) — the component of a kernel of a cell representing optically the dense, strongly refracting light little body. (See) a kernel admits modern cytology the place of synthesis and accumulation of all ribosomal RNA (RRNK), except 5S-PHK (see Ribosomes).
I. it is for the first time described in 1838 — 1839 by M. Shleyden in vegetable and T. Shvann — in zooblasts.
Number Ya., their sizes and a form vary depending on a type of cells. Most often meet I. spherical shape. I. are capable to merge with each other therefore can be present at a kernel or several small I., or one large, or several I. different size. In cells with low level of synthesis of protein Ya. are small or do not come to light. Activation of protein synthesis is accompanied by increase in total amount of kernels. In many cases total amount I. also correlates with number of chromosomal complements of a cell (see. Chromosomal set).
I. has no cover and it is surrounded with a layer of the condensed chromatin (see) — so-called okoloyadryshkovy, or perinukleolyarny, heterochromatin. By means of cytochemical methods in I. reveal RNA and proteins, acid and the main. Proteins Ya. include the enzymes participating in synthesis of ribosomal RNA. During the coloring of drugs Ya. are, as a rule, painted over by the main dye. In ova of nek-ry worms, mollusks and arthropods meet difficult I. (amfinukleola), two-piece, one of to-rykh is painted by the main dye, another (a proteinaceous little body) — acid. At the termination of synthesis of RRNK at the beginning of a mitosis (see) I. disappear (an exception make I. nek-ry protozoa), and at recovery of synthesis of RRNK in telophase of a mitosis form on sites of the chromosomes (see) called by organizers of a kernel again. In cells of the person organizers of a kernel are localized in the field of secondary banners of short shoulders of chromosomes 13, 14, 15, 21 and 22. At active synthesis of protein a cell organizers of a kernel reduplitsirutsya usually, and the quantity them reaches several hundred copies. In oocytes of animals (e.g., amphibians) such copies can come off chromosomes and create multiple regional kernels of ova.
Organizers of a kernel consist of the repeating blocks of the transcribed sequences of DNA including the genes 5,8S-PHK, 28S-RNK and 18S-pPHK divided by two sites which are not coding RRNK. The transcribed sequences of DNA alternate with is not present-ranskribiruyemymi the sequences (spacers). Synthesis of RRNK, or transcription (see), is carried out by special enzyme — a RNA polymerase of I. Huge molecules 45S-PHK are originally synthesized; during maturing (processing) all three types of RRNK are formed of these molecules by means of special enzymes; this process proceeds in several stages. The excessive, not being a part of RRNK sites 45S-PHK break up in a kernel, and mature RRNK are transported in cytoplasm where molecules 5,8S-pPHK and 28S-pPHK together with synthesized in a kernel out of I. a molecule 5S-pPHK and additional proteins create big unit of a ribosome, and the molecule 18S-pPHK is a part of its small subunit. According to modern representations rr the Tax Code and their predecessors at all stages of processing are present at a kernel in the form of complexes with proteins — ribonucleoproteins. Accession of proteins to a molecule 45 S-PHK happens in process of its synthesis so by the time of completion of synthesis a molecule already
Fig. Diffraction pattern of a kernel of cell NER-2: 1 — a granular component; 2 — a fibrillar component (nukleolone-ma); z — the fibrillar center; 4 — an amorphous matrix; X 70 Ltd companies.
represents a ribonucleoprotein.
Ultrastructure I. reflects consecutive stages of synthesis of RRNK in matrixes of organizers of a kernel. On diffraction patterns in I. distinguish a fibrillar component (nukleolony), a granular component and an amorphous matrix (fig). Nukleolonema represents filamentous structure 150 — 200 nanometers thick; it consists of granules diameter apprx. 15 nanometers and rykhlo the located fibrilla 4 — 8 nanometers thick. On cuts of a nukleolonema rather light sites — the so-called fibrillar centers are visible. Assume that these centers are formed by not transcribed DNA areas of organizers of a kernel which are in a complex with argento-filny proteins. The fibrillar centers are surrounded with loops of the transcribed chains of DNA with the ribonucleoproteins which are synthesized on them 45S-PHK. Probably, the last also come to light on diffraction patterns in the form of fibrilla.
Granular component Ya. contains the granules of ribonucleoproteins representing various products of processing of RRNK. Among them sometimes it is possible to distinguish dark granules of the ribonukleopro-teidny predecessor 28S-pPHK (32S-pPHK) and lighter grains containing mature 28S-pPHK. Amorphous matrix I. practically does not differ from nuclear sap (see the Kernel of a cell).
Thus, the kernel represents the dynamic, constantly renewed structure. It is a zone of a kernel of a cell where RRNK are synthesized and ripen and from where they are transported in cytoplasm.
Ways of an exit of ribonucleoproteins from I. in cytoplasm are studied insufficiently. Consider that they pass through porosoma of a nuclear envelope (see the Kernel of a cell) or through sites of its local destruction. Bonds I. with a cover of a kernel in cells of different types are carried out both in the form of direct contacts, and by means of the channels which are formed owing to invagination of a cover of a kernel. Through similar bonds there is also a metabolism between I. and cytoplasm.
At patol. processes note various changes Ya. Tak, at a malignancy of cells increase in number and the sizes Ya is observed., at the expressed dystrophic processes in a cell — a so-called segregation of kernels. At a segregation there is a redistribution of granular and fibrillar components. At the expressed segregation I. the nukleolonema can disappear, and in a granular component dark and light zones — so-called hats, or caps are formed. These structural changes reflect disturbances of synthesis, process of maturing and intra nucleolar transport of RRNK.
See also RNA.
Bibliogrzavarzin A. A. and Ha-razova A. D. Fundamentals of the general cytology, page 183, D., 1982; Chentsov Yu. S. General cytology, M., 1984; Chen
of ts Yu. S. and Polyakov V. Yu, Ultrastructure of a cellular kernel, page 50, M., 1974; In about u t e i 1 1 e M. a. D and-puy-Go in A. M 3-dimensional analysis of the interphase nucleus, Biol. Cell, v. 45, p. 455, 1982; Busch H.
Smetana K. The nucleolus, N. Y. — L., 1970; Hadjiolov A. A. The nucleolus and ribosome biogenesis, Wien — N. Y., 1985, bibliogr.
I. E. Hesinonim