What is the main function of the nucleoli. Cell nucleus: functions and structure

Nucleolus (plasmosome)- dense formation detected in the interphase nuclei of eukaryotic cells, which is formed at certain loci of chromosomes (nucleolar organizer). The nucleolus is a derivative of the chromosome, one of its loci, actively functioning in the interphase. The cell usually contains 1-2 R., sometimes more than 2. The main function of the R. is the synthesis of ribosomes; it contains factors involved in the transcription of ribosomal genes, pre-rRNA processing, and assembly of preribosomal particles. Some R. proteins are polyfunctional and are involved in a number of other processes in the cell, such as apoptosis, regulation of the cell cycle, etc.

The nucleolus is a highly organized structure within the nucleus. The nucleolus contains large DNA loops containing pRNA genes, which are transcribed at an unusually high rate by RNA polymerase I. These loops are called "nucleolar organizers".

Unlike cytoplasmic organelles, the nucleolus does not have a membrane that would surround its contents. It appears to be formed by immature ribosome precursors specifically linked to each other in an unknown way. (fig. nucleolus) The size of the nucleolus reflects the degree of its functional activity, which varies widely in different cells and can change in an individual cell.

In the nucleolus, transcription of ribosomal genes, processing of rRNA precursors, and assembly of preribosomal particles from ribosomal proteins and rRNA take place. The mechanisms of nucleolus formation are not clear. According to one of the hypotheses, the nucleolus is considered as a nucleoprotein complex that spontaneously appears as a result of the association of regulatory protein-nucleic acid complexes that arise on repetitive rDNA sequences during their transcription. Indeed, human rRNA genes are organized into 250 44 kb long tandem repeat sequences. each, which, together with their associated proteins, form the core of the nucleolus. It is filled with other components during rRNA processing and assembly of ribosomal subunits.

Morphologically, three main zones are distinguished in the nucleolus: fibrillar centersurrounded by dense fibrillar and granular regions.

In an electron micrograph of the nucleolus, these three discrete zones can be distinguished:

1) a weakly colored component containing DNA from the region of the nucleolar organizer of the chromosome,

2) a dense fibrillar component, consisting of many thin (5 nm) ribonucleoprotein fibrils, which are RNA transcripts and

3) a granular component, which includes particles with a diameter of 15 nm, representing the most mature precursors of ribosomal particles.

Using specific antibodies and hybridization probes, it was found that the rRNA, RNA polymerase I, transcription factor UBF, and topoisomerase I genes are localized in the fibrillar center of the nucleolus. It is believed that the fibrillar center of the nucleolus is the site of assembly of regulatory nucleoprotein complexes necessary for the transcription of rRNA genes. The dense fibrillar component surrounding the center of the nucleolus is represented by growing chains of rRNA precursors and their associated proteins involved in processing. In the granular region of the nucleolus, mature 28S and 18S rRNAs, partially processed RNAs, and ribosomal subunit assembly products are found. Ribosome assembly intermediates are represented by particles with a diameter of 15–20 nm. The transfer of preribosomal subparticles to the cytoplasm seems to be provided by specific proteins that move from the nucleolus to the nuclear envelope. Due to the hierarchy in the structural and functional organization of the nucleolus in the form of separate morphologically distinct compartments, it is often used as a model for the functional compartmentalization of mRNA synthesis, its processing, and export to the cytoplasm.

The observed “highly ordered” spatial structure of the nucleolus may simply be a consequence of the functioning of a large number of rRNA genes organized into tandem repeats, which is accompanied by the accumulation of RNA polymerase I transcripts and their processing products in the vicinity of actively working genes. The structure of the nucleolus is dynamic, and its spatial arrangement and structural features depend on the intranuclear localization and the level of activity of the corresponding rRNA genes.

Even the yeast genome contains ~200 tandemly repeated rRNA genes. At the same time, not all genes are functionally identical: only half of the rDNA sequences are transcribed, and only ~20% of the available replication origin regions are involved in their reproduction. The transfer of genes into the rDNA region is often accompanied by their repression, which is believed to be a consequence of the functioning of the mechanism of suppression of homologous recombination in genome regions containing tandem repeats. Mutational disruption of this mechanism is accompanied by the formation of hundreds of extrachromosomal circular rDNA, which are unevenly distributed between daughter cells during mitosis. The accumulation of extrachromosomal rDNA by mother cells leads to a decrease in the ability of cells to divide. This phenomenon has been called "cell aging" (cellular aging). In addition, the nucleolus can regulate the entry of cells into meiosis, as well as the activity of Cdc 14 phosphatase, which controls the passage of the telophase of mitosis. Data have been obtained that the nucleolus rDNA repeating sequences serve as an assembly site for the RENT (regulator of nucleolar silencing and telophase exit) regulatory protein complex, which includes phosphatase and three other proteins that provide the regulatory functions of the nucleolus.

45S rRNA transcripts first form large complexes by binding to a large number of different proteins imported from the cytoplasm, where all cellular proteins are synthesized. Most of the 70 different polypeptide chains that form the ribosome, as well as 5S rRNA, are switched on at this stage.

Other molecules are also needed for the assembly process to proceed correctly. For example, other RNA-binding proteins are present in the nucleolus, as well as certain small ribonucleoprotein particles (including U3-snRNP) that are thought to catalyze ribosome assembly. These components remain in the nucleolus, while the finished ribosome subunits are transported to the cytoplasm. A particularly prominent component of the nucleolus is nucleolin, a well-studied protein that is present in large amounts and appears to only bind to ribosomal RNA transcripts. Nucleolin stains with silver in a special way. Such staining also characterizes the entire nucleolus as a whole.

During 45S-RNA processing, this giant ribonucleoprotein complex gradually loses some of its proteins and RNA sequences and then specifically cleaves to form independent precursors of large and small ribosomal subunits.

Thirty minutes after the introduction of the radioactive label, the first mature small subunits of ribosomes containing labeled 18S rRNA leave the nucleolus and appear in the cytoplasm.

The assembly of large ribosomal subunits containing 28S-RNA, 5,8S-RNA, and 5S-RNA requires somewhat more time (about 1 h), so much more unfinished large subunits accumulate in the nucleolus than small ones.

The final stages of ribosome maturation are carried out only after the release of ribosomal subunits from the nucleus into the cytoplasm. This achieves isolation of functioning ribosomes from immature nuclear transcripts.

There are data indicating the participation of the nucleolus in the regulation of the cell cycle.

The cell is the elementary unit of living organisms on Earth and has a complex chemical organization structures called organelles. These include the nucleolus, the structure and functions of which we will study in this article.

Features of eukaryotic nuclei

Nucleated cells in their composition contain non-membrane rounded organelles, denser than karyoplasm, and called nucleoli or nucleoli. They were discovered in the 19th century. Now nucleoli are quite fully studied thanks to electron microscopy. Almost until the 50s of the 20th century, the functions of the nucleoli were not determined, and scientists considered this organelle, rather, as a reservoir of reserve substances used during mitosis.

Modern studies have established that the organoid includes granules of a nucleoprotein nature. Moreover, biochemical experiments have confirmed that the organelle contains a large amount of proteins. They are the ones who cause it. high density. In addition to proteins, the nucleolus contains RNA and a small amount of DNA.

cell cycle

Interestingly, in the life of a cell, which consists of a period of rest (interphase) and division (meiosis - in the sex, mitosis - in the nucleolus, they are not permanently preserved. So, in the interphase, the nucleus with the nucleolus, whose functions are to preserve the genome and form protein-synthesizing organelles, are necessarily present At the beginning of cell division, namely in prophase, they disappear and are re-formed only at the end of telophase, remaining in the cell until the next division or until apoptosis - its death.

nucleolar organizer

In the 30s of the last century, scientists found that the formation of nucleoli is controlled by certain sections of some chromosomes. They contain genes that store information about the structure and functions of the nucleolus in the cell. There is a correlation between the number of nucleolar organizers and the organelles themselves. For example, it contains two nucleolus-forming chromosomes in its karyotype and, accordingly, there are two nucleoli in the nuclei of its somatic cells.

Since the functions of the nucleolus, as well as its presence, are closely related to the formation of ribosomes, the organelles themselves are absent in highly specialized brain tissues, blood, and also in the blastomeres of the crushing zygote.

Nucleol amplification

In the synthetic stage of interphase, along with DNA self-duplication, there is an excessive replication of the number of rRNA genes. Since the main functions of the nucleolus are the production of ribosomes, the number of these organelles sharply increases due to the oversynthesis of DNA loci that carry information about RNA. Nucleoproteins that are not associated with chromosomes begin to function autonomously. As a result, many nucleoli are formed in the nucleus, distancing themselves from the nucleolus-forming chromosomes. This phenomenon is called rRNA gene amplification. Continuing to study the functions of the nucleolus in the cell, we note that their most active synthesis occurs in the prophase of the reduction division of meiosis, as a result of which first-order oocytes can contain several hundred nucleoli.

The biological significance of this phenomenon becomes clear if we take into account that at the early stages of embryogenesis: crushing and blastulation, a huge number of ribosomes are needed to synthesize the main building material - protein. Amplification is a fairly common process; it occurs in the oogenesis of plants, insects, amphibians, yeasts, and also in some protists.

Histochemical composition of the organelle

Let us continue the study of their structures, and consider the nucleolus, the structure and functions of which are interconnected. It is established that it contains three types of elements:

1. Nucleonema (filamentous formations). They are heterogeneous and contain fibrils and lumps. Being part of both plant and nucleonemes form fibrillar centers. The cytochemical structure and functions of the nucleolus also depend on the presence of a matrix in it - a network of supporting protein molecules of a tertiary structure.
2. Vacuoles (light areas).
3. Granular granules (nucleolins).

From point of view chemical analysis, this organoid consists almost entirely of RNA and protein, and DNA is located only on its periphery, forming a ring-shaped structure - perinucleolar chromatin.

So, we have established that the nucleolus consists of five formations: fibrillar and granular centers, chromatin, protein reticulum and a dense fibrillar component.

Types of nucleoli

The biochemical structure of these organelles depends on where they are present, as well as on the characteristics of their metabolism. There are 5 main structural types of nucleolus. The first - reticular, is the most common and is characterized by an abundance of dense fibrillar material, lumps of nucleoproteins and nucleone. The process of rewriting information from the nucleolar organizers is very active, so the fibrillar centers are poorly visible in the field of view of the microscope.

Since the main functions of the nucleolus in the cell are the synthesis of ribosomal subunits, from which protein-synthesizing organelles are formed, the reticular type of organization is inherent in both plant and animal cells. The ring-shaped type of nucleoli is found in connective tissue cells: lymphocytes and endotheliocytes, in which rRNA genes are practically not transcribed. Residual nucleoli occur in cells that have completely lost the ability to transcribe, such as normoblasts and enterocytes.

The segregated appearance is inherent in cells that have experienced intoxication with carcinogens and antibiotics. And, finally, the compact type of the nucleolus is characterized by many fibrillar centers and a small number of nucleonemes.

Protein nucleolar matrix

Let's continue studying internal structure structures of the nucleus and determine what are the functions of the nucleolus in cell metabolism. It is known that about 60% of the dry mass of this organelle is accounted for by the proteins that make up the chromatin, ribosomal particles, and also by the nucleolar proteins themselves. Let's dwell on them in more detail. Some of the proteins are involved in processing - the formation of mature ribosomal RNA. These include RNA polymerase 1 and nuclease, which remove extra triplets from the ends of the rRNA molecule. The fibrillarin protein is located in the dense fibrillar component and, like the nuclease, carries out processing. Another protein is nucleolin. Together with fibrillarin, it is found in the PFC and PC of the nucleoli and in the nucleolar organizers of the chromosomes of the prophase of mitosis.

A polypeptide such as nucleophosin is located in the granular zone and the dense fibrillar component; it is involved in the formation of ribosomes from 40 S and 60 S subunits.

What is the function of the nucleolus

The synthesis of ribosomal RNA is the main task that the nucleolus must perform. At this time, transcription occurs on its surface (namely, in fibrillar centers) with the participation of the RNA polymerase enzyme. On this nucleolar organizer, hundreds of pre-ribosomes, called ribonucleoprotein globules, are synthesized. They form ribosomal subunits, which leave the karyoplasm through and end up in the cytoplasm of the cell. The 40S small subunit binds to the messenger RNA, and only then does the 40S large subunit attach to them. A mature ribosome is formed, capable of carrying out translation - the synthesis of cellular proteins.

In this article, we studied the structure and functions of the nucleolus in plant and animal cells.

The main function of the nucleolus is the synthesis of ribosomal RNA and ribosomes, on which the synthesis of polypeptide chains is carried out in the cytoplasm. There are special regions in the cell genome called nucleolar organizers containing ribosomal RNA (rRNA) genes, around which the nucleoli are formed. In the nucleolus, rRNA is synthesized by RNA polymerase I, its maturation, and the assembly of ribosomal subunits. The nucleolus contains proteins involved in these processes. Some of these proteins have a special sequence called the nucleolar localization signal (NoLS). N ucle o lus L ocalization S ignal). It should be noted that the highest concentration of protein in the cell is observed in the nucleolus. About 600 types of different proteins were localized in these structures, and it is believed that only a small part of them are really necessary for the implementation of nucleolar functions, while the rest get there nonspecifically.

Electron microscopy reveals two main components in the nucleolus: granular(along the periphery) - maturing subunits of ribosomes and fibrillar(center) - ribonucleoprotein strands of ribosome precursors. So called fibrillar centers surrounded by plots dense fibrillar component where rRNA synthesis takes place. Outside of the dense fibrillar component is located granular component, which is an accumulation of maturing ribosomal subunits.

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See what "Nuclear" is in other dictionaries:

- (nucleolus) dense body inside the cell nucleus. Consists mainly of ribonucleoproteins; participates in the formation of ribosomes. Usually there is one nucleolus in the cell, less often several or many ... Big Encyclopedic Dictionary

Nucleol, nucleus Dictionary of Russian synonyms. nucleolus n., number of synonyms: 2 nucleolus (1) nucleus ... Synonym dictionary

NUCLEUS, nucleolus, pl. kernels, kernels, kernels, cf. reduce to the core in 1 and 5 digits. Dictionary Ushakov. D.N. Ushakov. 1935 1940 ... Explanatory Dictionary of Ushakov

NUCLEUS, a, cf. 1. see core. 2. Dense body inside the cell nucleus (spec.). Explanatory dictionary of Ozhegov. S.I. Ozhegov, N.Yu. Shvedova. 1949 1992 ... Explanatory dictionary of Ozhegov

Nucleol (nucleolus), a dense body inside the nucleus of most eukaryotic cells. Consists of ribonucleoproteins (RNP) precursors of ribosomes. Usually there is one egg in the nucleus, less often several or many (for example, in the nuclei of growing fish eggs). I.… … Biological encyclopedic dictionary

nucleolus- nucleolus, a, pl. hours shki, shek ... Russian spelling dictionary

nucleolus- A rounded mass in the cell nucleus containing ribonucleoproteins Biotechnology Topics EN nucleolus … Technical Translator's Handbook

nucleolus- * nucleolus * nucleolus or plasmosome spherical or globular semi-nuclear (subnuclear) organelle associated with the nucleolar organizer (see) of the chromosome. I. consists mainly of primary rDNA transcripts, ribosomal proteins and a set of other proteins ... Genetics. encyclopedic Dictionary

BUT; pl. genus. shek, dat. shkam; cf. 1. to the Core (1, 4 digits). 2. Biol. A small spherical dense body located in the nucleus of plant and animal cells. * * * Nucleolus (nucleolus), a dense body inside the cell nucleus. Consists mainly of... encyclopedic Dictionary

Nucleole, a dense light-refracting body within the cell nucleus (See Nucleus) of eukaryotic organisms; consists mainly of complexes of ribonucleic acids with ribonucleoprotein proteins (RNPs). Number I. 1 3 (see figures 2 4); less often there are a lot of them ... Great Soviet Encyclopedia

nucleolus- a spherical formation (1-5 microns in diameter), present in almost all living cells of eukaryotic organisms. In the nucleus, one or more usually rounded bodies that strongly refract light are visible - this is the nucleolus, or nucleolus (nucleolus). The nucleolus well perceives the main dyes and is located among the chromatin. Basophilia of the nucleolus is determined by the fact that the nucleoli are rich in RNA. The nucleolus, the densest structure of the nucleus, is a derivative of the chromosome, one of its loci with the highest concentration and activity of RNA synthesis in interphase. The formation of nucleoli and their number are associated with the activity and number of certain sections of chromosomes - nucleolar organizers, which are located mostly in the zones of secondary constrictions, it is not an independent structure or organelle. In humans, such sites are in the 13th, 14th, 15th, 21st and 22nd pairs of chromosomes.

The function of the nucleoli is the synthesis of rRNA and the formation of ribosome subunits.

The nucleolus is heterogeneous in its structure: in a light microscope one can see its fine-fibrous organization. In an electron microscope, two main components are revealed: granular and fibrillar. The diameter of the granules is about 15-20nm, the thickness of the fibrils is 6-8nm. Granules are maturing subunits of ribosomes.

Granular component localized in the peripheral part of the nucleolus and is an accumulation of ribosome subunits.

fibrillar component is localized in the central part of the nucleolus and is a thread of ribonucleoprotein precursors of ribosomes.

The ultrastructure of the nucleoli depends on the activity of RNA synthesis: at a high level of rRNA synthesis in the nucleolus, big number granules, when the synthesis is stopped, the number of granules decreases, the nucleoli turn into dense fibrillar bodies of a basophilic nature.

The scheme of participation of nucleoli in the synthesis of cytoplasmic proteins can be represented as follows:

Picture? - SCHEME OF RIBOSOMES SYNTHESIS IN EUKARYOTIC CELLS

Scheme of ribosome synthesis in eukaryotic cells.
1. Synthesis of mRNA of ribosomal proteins by RNA polymerase II. 2. Export of mRNA from the nucleus. 3. Recognition of mRNA by the ribosome and 4. synthesis of ribosomal proteins. 5. Synthesis of rRNA precursor (45S - precursor) by RNA polymerase I. 6. Synthesis of 5S pRNA by RNA polymerase III. 7. Assembly of a large ribonucleoprotein particle, including the 45S precursor, ribosomal proteins imported from the cytoplasm, as well as special nucleolar proteins and RNA involved in the maturation of ribosomal subparticles. 8. Attachment of 5S rRNA, cutting of the precursor and separation of the small ribosomal subunit. 9. Maturation of the large subunit, release of nucleolar proteins and RNA. 10. Release of ribosomal subparticles from the nucleus. 11. Involving them in the broadcast.

Micrographs of the nucleolus (according to electron microscopy)

Picture? – Electron micrograph of the nucleus with nucleolus

1- Fibrillar component; 2- granular component; 3 - perinucleolar heterochromatin; 4-karyoplasm; 5-nuclear membrane.

Picture? – RNA in the cytoplasm and nucleoli of submandibular gland cells.

Coloring according to Brachet, X400

1 – cytoplasm; 2 – nucleoli. Both of these structures are rich in RNA (mainly due to rRNA - free or as part of ribosomes) and therefore, when stained according to Brachet, they are stained crimson.

The nucleoli that make up the nucleus were first described by Fontana in 1774. Nucleoli are found in almost all nuclei of eukaryotic cells. This is a denser structure against the background of diffuse chromatin organization. The main component of the nucleolus is protein. It accounts for up to 80%. In addition to protein, the nucleolus contains nucleic acids. RNA 5-14% and DNA 2-12%. In the 30s of the 20th century, it was shown that the emergence of nucleoli is always tied to certain zones. Scientists McClinton, Nates and Navashin called these zones nucleolar organizers. In other words, it is the location of ribosome genes. The nucleolar organizers are not some kind of point locus; they are a formation that is multiple in structure and contains several identical gene regions, each of which is responsible for the formation of the nucleolus. In the genomes of eukaryotes, ribosomal genes are represented by thousands of units. They belong to moderately repetitive DNA sequences. Often nucleolar organizers are localized in secondary chromosome constrictions. In humans, nucleolar organizers are located on the short arms of some chromosomes. But the nucleolus is formed one.

The maximum number of nucleoli is also determined by the number of nucleolar organizers. Increases according to the ploidy of the nucleus.

It is characteristic that a small number of nucleoli predominate in cells of different tissues and taxonomic affiliation. Most often, the number of nucleoli is less than the number of organizers. This is due to the fact that during the neoplasm of the nucleolus, the nucleolar organizers merge into one overall structure. They unite in the space of the interphase nucleus, forming one nucleolus from different chromosomes.

In oocytes, the number of nucleoli reaches several hundred. This is the phenomenon of ribosomal RNA gene amplification. Overpopulation. Usually in somatic cells, the number of genes in ribosomal RNA is constant. It does not change depending on the level of transcription of these genes. During DNA replication in the S-period, the number of ribosomal RNA genes is also doubled, and in germ cells these genes undergo excessive replication in order to provide a large number of ribosomes. As a result of oversynthesis of ribosomal RNA genes, their copies become free circular molecules or extrachromosomal. They can function independently, and as a result, a mass of free additional nucleoli is formed, which are no longer structurally associated with nucleolus-forming chromosomes. And the number of ribosomal RNA genes becomes almost 3,000 times greater than the amount of haploid ribosomal RNA.

The biological meaning is to provide a large number of spare products that are used on early stages embryogenesis and which in the cell can be synthesized only on additional matrices of amplified nucleoli, since the embryo does not have its own synthesis of ribosomal genes.

After a period of oocyte maturation, the additional nucleoli are destroyed. Therefore, the replication of ribosomal genes is a temporary phenomenon.

The following components are distinguished in the structure of the nucleolus:

1) Granular component;

2) Fibrillar component (represented by a fibrillar center and a dense component);

3) Chromatin;

4) Protein matrix.

The nucleoli are built from a granular and fibrillar component and their mutual arrangement differs. Most often, the granular component is located along the periphery of the nucleolus, and the fibrillar component forms nucleolar filaments, about 100–200 nm thick. They are sometimes called nucleolonemes. They are not homogeneous in their structure; in addition to granules, they include many new fibrils, which form separate thickenings in nucleolonemes.

It turned out that the structure of the diffuse fibrillar component is also heterogeneous. Fibrillar centers have been found to occur in the nucleoli. These are areas of accumulation of fibrils with low electron density, surrounded by a zone of fibrils of higher electron density. This zone is called the dense component.

Nucleolar chromatin is perinucleolar chromatin that can adjoin the nucleolus and even completely surround it. Often, 30nm chromatin fibrils extend between the nucleolonemal regions.

On sections, we cannot isolate the protein matrix as a separate component.

In addition to varying degrees of severity, there are other variants of the structural organization of the nucleolus.

Several types of nucleolus: 1) reticular or nucleolonemal 2) compact 3) annular 4) residual or resting 5) segregated.

Reticular characteristic of most cells. It has a typical nucleolonemic structure. Fibrillar centers appear poorly because the level of transcription is very high. This type of nucleolus is found in animal and plant cells and is typical of dipteran polytene chromosomes.

Compact the type is characterized by a lesser severity of nucleolonema, a greater frequency of occurrence in fibrillar centers. It occurs in actively proliferating cells, in plant meristem cells, in tissue culture cells. It is assumed that the first type can be transferred and vice versa.

annular nucleoli are characteristic of animals. They have the shape of a ring, which is a fibrillar center surrounded by fibrils and grana. The size of such nucleoli is about 1 µm. Typical ring-shaped nucleoli are characteristic of endocytes, endoeleocytes, i.e. for cells with a low level of transcription.

Residual– characteristic of cells that have lost the ability to synthesize rRNA.

Segregated nucleoli are cells that are exposed to various chemical substances that cause cessation of rRNA synthesis.

The term is used in connection with the fact that there is a separation of different components of the nucleoli, accompanied by a progressive decrease in its volume. In an inactive form, the nucleolar organizer of chromosomes is presented as one large fibrillar center, which includes a compactly folded part of chromosomal DNA, in which the following ribosomal genes are located one after another. At the beginning of nucleolus activation, decondensation of ribosomal genes occurs at the periphery of the fibrillar center. These genes begin to be transcribed and RNP transcripts are formed on them. These transcripts, upon maturation, give rise to ribosome precursors that accumulate around the periphery of the activated nucleolus. As transcription intensifies, a single fibrillar center breaks up into a number of smaller structures connected to each other by completely decondensed DNA regions. The higher the transcriptional activity of the nucleolus, the greater the number of small interconnected fibrillar structures surrounded by a dense fibrillar component containing precursors of 45 S ribosomal genes. When the nucleolus is fully activated, all small fibrillar centers decondense, and in this case, the zones of the dense component contain all ribosomal RNA , which is active. In the case of inactivation of the nucleolus, gradual condensation of ribosomal DNA occurs, fibrillar centers are formed again. They combine with each other and their value grows in parallel with the decrease in the fractions of the dense component. This inactivated state of the nucleolus is structurally similar to the nucleolar organizer of mitotic chromosomes.

The nucleolus is a non-permanent structure in the cell. It changes its properties and structure during the cell cycle. At the beginning of mitosis, the structures of the nucleolus are slightly compacted, and after the rupture of the nuclear membrane, on the contrary, they lose their density, loosen, disintegrate into their structural components and spread between the condensed chromosomes in the form of nucleolar material. And therefore, in metaphase and anaphase, the nucleoli as such are absent in the cell. They are in the form of a matrix of mitotic chromosomes. The first signs of new nucleoli appear in the middle telophase, simultaneously with almost decondensed chromosomes and with cells that have a new nuclear membrane, in the form of dense rings, which are called prenucleoli. Their number is usually large. In the G1 period of the cell cycle, the prenucleoli grow, unite with each other, their total number decreases, and the total volume increases. In G2 and S periods, the total volume of the nucleolus doubles.

Thus, after division, protein components and enzymes are transferred to new daughter nuclei, which creates the conditions necessary for the resumption of synthesis and maturation of both ribosomes and rRNA synthesis. The mitotic chromosome transfers to the daughter nucleus not only genetic information in the form of DNA chromatin, but also required amount a synthetic apparatus ready to activate transcription in a new cell cycle. And these necessary components are in the form of a matrix on mitotic chromosomes.

Functions of the nucleolus:

1) rRNA synthesis;

2) Participation in the maturation of messenger RNA;

3) Participation in the maturation of transport RNAs;

4) In the nucleoli, RNA types are formed that are part of the srp-particle of ribosomes;

5) In the nucleolus, the synthesis of the proton carrier nicotinamide adenine dinucleotide is carried out.