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, a large number of granules are detected in the nucleolus, when synthesis is stopped, the number of granules decreases, and 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.
Typically, a eukaryotic cell has one nucleus, but there are binuclear (ciliates) and multinuclear cells (opaline). Some highly specialized cells lose their nucleus for the second time (mammalian erythrocytes, angiosperm sieve tubes).
The shape of the nucleus is spherical, elliptical, less often lobed, bean-shaped, etc. The diameter of the nucleus is usually from 3 to 10 microns.
Core structure:
1 - outer membrane; 2 - inner membrane; 3 - pores; 4 - nucleolus; 5 - heterochromatin; 6 - euchromatin.
The nucleus is delimited from the cytoplasm by two membranes (each of them has a typical structure). Between the membranes is a narrow gap filled with a semi-liquid substance. In some places, the membranes merge with each other, forming pores (3), through which the exchange of substances between the nucleus and the cytoplasm takes place. The outer nuclear (1) membrane from the side facing the cytoplasm is covered with ribosomes, giving it a roughness, the inner (2) membrane is smooth. Nuclear membranes are part of the cell membrane system: outgrowths of the outer nuclear membrane are connected to the channels of the endoplasmic reticulum, forming a single system of communicating channels.
Karyoplasm (nuclear sap, nucleoplasm)- the internal contents of the nucleus, in which chromatin and one or more nucleoli are located. The composition of the nuclear juice includes various proteins (including nuclear enzymes), free nucleotides.
nucleolus(4) is a rounded dense body immersed in nuclear juice. The number of nucleoli depends on the functional state of the nucleus and varies from 1 to 7 or more. Nucleoli are found only in non-dividing nuclei; during mitosis they disappear. The nucleolus is formed on certain regions of chromosomes that carry information about the structure of rRNA. Such regions are called the nucleolar organizer and contain numerous copies of the rRNA-coding genes. Ribosome subunits are formed from rRNA and proteins coming from the cytoplasm. Thus, the nucleolus is an accumulation of rRNA and ribosomal subunits at different stages of their formation.
Chromatin- internal nucleoprotein structures of the nucleus, stained with some dyes and differing in shape from the nucleolus. Chromatin has the form of lumps, granules and threads. The chemical composition of chromatin: 1) DNA (30–45%), 2) histone proteins (30–50%), 3) non-histone proteins (4–33%), therefore, chromatin is a deoxyribonucleoprotein complex (DNP). Depending on the functional state of chromatin, there are: heterochromatin(5) and euchromatin(6). Euchromatin - genetically active, heterochromatin - genetically inactive sections of chromatin. Euchromatin is not distinguishable under light microscopy, is weakly stained and represents decondensed (despiralized, untwisted) sections of chromatin. Under a light microscope, heterochromatin looks like lumps or granules, is intensely stained and is a condensed (spiralized, compacted) sections of chromatin. Chromatin is a form of existence of genetic material in interphase cells. During cell division (mitosis, meiosis), chromatin is converted into chromosomes.
Kernel functions: 1) storage of hereditary information and its transfer to daughter cells in the process of division, 2) regulation of cell vital activity by regulating the synthesis of various proteins, 3) the place of formation of ribosome subunits.
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Chromosomes
Chromosomes- These are cytological rod-shaped structures, which are condensed chromatin and appear in the cell during mitosis or meiosis. Chromosomes and chromatin are different forms of spatial organization of the deoxyribonucleoprotein complex corresponding to different phases of the cell life cycle. The chemical composition of chromosomes is the same as that of chromatin: 1) DNA (30–45%), 2) histone proteins (30–50%), 3) non-histone proteins (4–33%).
The basis of the chromosome is one continuous double-stranded DNA molecule; the length of the DNA of one chromosome can reach several centimeters. It is clear that a molecule of such a length cannot be located in a cell in an elongated form, but is folded, acquiring a certain three-dimensional structure, or conformation. The following levels of spatial packing of DNA and DNP can be distinguished: 1) nucleosomal (wrapping DNA around protein globules), 2) nucleomeric, 3) chromomeric, 4) chromonemic, 5) chromosomal.
In the process of transformation of chromatin into chromosomes, DNP forms not only spirals and supercoils, but also loops and superloops. Therefore, the process of chromosome formation, which occurs in the prophase of mitosis or prophase 1 of meiosis, is better called not spiralization, but condensation of chromosomes.
Chromosomes: 1 - metacentric; 2 - submetacentric; 3, 4 - acrocentric. The structure of the chromosome: 5 - centromere; 6 - secondary constriction; 7 - satellite; 8 - chromatids; 9 - telomeres.
The metaphase chromosome (chromosomes are studied in the metaphase of mitosis) consists of two chromatids (8). Every chromosome has primary constriction (centromere)(5), which divides the chromosome into arms. Some chromosomes have secondary constriction(6) and satellite(7). Satellite - a section of a short arm, separated by a secondary constriction. Chromosomes that have a satellite are called satellite (3). The ends of chromosomes are called telomeres(9). Depending on the position of the centromere, there are: a) metacentric(equilateral) (1), b) submetacentric(moderately unequal) (2), c) acrocentric(sharply unequal) chromosomes (3, 4).
Somatic cells contain diploid(double - 2n) set of chromosomes, sex cells - haploid(single - n). The diploid set of roundworm is 2, Drosophila - 8, chimpanzee - 48, crayfish - 196. The chromosomes of the diploid set are divided into pairs; chromosomes of one pair have the same structure, size, set of genes and are called homologous.
Karyotype- a set of information about the number, size and structure of metaphase chromosomes. An idiogram is a graphic representation of a karyotype. Representatives of different species have different karyotypes, the same species are the same. autosomes- chromosomes are the same for male and female karyotypes. sex chromosomes Chromosomes in which the male karyotype differs from the female.
The human chromosome set (2n = 46, n = 23) contains 22 pairs of autosomes and 1 pair of sex chromosomes. Autosomes are grouped and numbered:
Sex chromosomes do not belong to any of the groups and do not have a number. Sex chromosomes of a woman - XX, men - XY. The X chromosome is medium submetacentric, the Y chromosome is small acrocentric.
In the area of secondary constrictions of chromosomes of groups D and G, there are copies of genes that carry information about the structure of rRNA, so the chromosomes of groups D and G are called nucleolus-forming.
Functions of chromosomes: 1) storage of hereditary information, 2) transfer of genetic material from the mother cell to the daughter cells.
Lecture number 9.
The structure of a prokaryotic cell. Viruses
Prokaryotes include archaebacteria, bacteria, and blue-green algae. prokaryotes- unicellular organisms that lack a structurally formed nucleus, membrane organelles and mitosis.
Biology 5,6,7,8,9,10,11 class, USE, GIA
Nucleus is an important structural eukaryotic cell component, which contains DNA molecules- genetic information. It has a round or oval shape. The nucleus stores, transmits and implements hereditary information, and also provides protein synthesis. More about cellular organization, composition and functions of the nucleus of an animal or plant cell, consider the table below.
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nuclear envelope. It has a porous two-membrane structure. |
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Chromosomes. Dense elongated or filamentous formations that can be seen only with cell division. |
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Nucleoli. They are spherical or irregular in shape. |
Participate in the synthesis process RNA, which is part of ribosomes. |
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nuclear juice (karyoplasm). A semi-liquid medium located inside the nucleus. |
A substance that contains nucleoli and chromosomes. |
Despite differences in structure and function, all cell parts constantly interact with each other, they are united by one main function - ensuring the vital activity of the cell, timely cell division and proper metabolism within it.
Only eukaryotic cells have a nucleus. At the same time, some of them lose it in the process of differentiation (mature segments of sieve tubes, erythrocytes). Ciliates have two nuclei: a macronucleus and a micronucleus. There are multinucleated cells that have arisen by combining several cells.
However, in most cases there is only one nucleus in each cell.
The cell nucleus is its largest organelle (except for the central vacuoles of plant cells). It is the very first of the cellular structures that was described by scientists. Cell nuclei are usually spherical or ovoid in shape.
The nucleus regulates all cell activity. It contains chromatids- filamentous complexes of DNA molecules with histone proteins (a feature of which is the content of a large amount of amino acids lysine and arginine in them).
The DNA of the nucleus stores information about almost all hereditary traits and properties of the cell and organism. During cell division, chromatids spiralize, in this state they are visible under a light microscope and are called chromosomes.
Chromatids in a non-dividing cell (during interphase) are not completely despiralized.
Tightly coiled parts of chromosomes are called heterochromatin. It is located closer to the shell of the nucleus. To the center of the nucleus is euchromatin- more despiralized part of chromosomes.
RNA synthesis takes place on it, i.e., genetic information is read out, genes are expressed.
DNA replication precedes nuclear division, which in turn precedes cell division. Thus, the daughter nuclei receive ready-made DNA, and the daughter cells receive ready-made nuclei.
The internal contents of the nucleus are separated from the cytoplasm nuclear envelope, consisting of two membranes (external and internal).
Thus, the cell nucleus refers to two-membrane organelles. The space between the membranes is called the perinuclear space.
The outer membrane in certain places passes into the endoplasmic reticulum (ER).
If ribosomes are located on the ER, then it is called rough. Ribosomes can also be located on the outer nuclear membrane.
In many places, the outer and inner membranes fuse with each other, forming nuclear pores.
Their number is not constant (they number in the thousands on average) and depends on the activity of biosynthesis in the cell. Through the pores, the nucleus and cytoplasm exchange various molecules and structures. Pores are not just holes, they are complex for selective transport. Their structure is determined by various nucleoporin proteins.
Molecules of mRNA, tRNA, subparticles of ribosomes come out of the nucleus.
Various proteins, nucleotides, ions, etc. enter the nucleus through pores.
Ribosome subunits are assembled from rRNA and ribosomal proteins into nucleolus(there may be several).
The central part of the nucleolus is formed by special sections of chromosomes (nucleolar organizers), which are located next to each other. The nucleolar organizers contain a large number of copies of the rRNA-coding genes. Before cell division, the nucleolus disappears and re-forms already during telophase.
The liquid (gel-like) content of the cell nucleus is called nuclear juice (karyoplasm, nucleoplasm).
Its viscosity is almost the same as that of hyaloplasm (the liquid content of the cytoplasm), but the acidity is higher (after all, DNA and RNA, which are abundant in the nucleus, are acids). Proteins, various RNAs, ribosomes float in the nuclear juice.
Structural elements of the nucleus are clearly expressed only in a certain period of the cell cycle in interphase. During cell division (during mitosis or meiosis), some structural elements disappear, others are significantly transformed.
Classification of structural elements of the interphase core:
Chromatin;
Nucleolus;
Karyoplasm;
Karyolemma.
Chromatin is a dye-receptive substance (chromos), hence its name.
Chromatin consists of chromatin fibrils, 20-25 nm thick, which can be loosely or compactly located in the nucleus. On this basis, two types of chromatin are distinguished:
Euchromatin - loose or decondensed chromatin, weakly stained with basic dyes;
Heterochromatin is compact or condensed chromatin that stains well with the same dyes.
During the preparation of the cell for division in the nucleus, chromatin fibrils spiralize and chromatin is converted into chromosomes.
After division in the nuclei of daughter cells, despiralization of chromatin fibrils occurs and the chromosomes are again converted into chromatin. Therefore, chromatin and chromosomes are different phases of the same substance.
According to the chemical structure, chromatin consists of:
Deoxyribonucleic acid (DNA) 40%;
Proteins about 60%;
Ribonucleic acid (RNA) 1%.
Nuclear proteins are represented by the forms:
Alkaline or histone proteins 80-85%;
Acid proteins 15-20%.
Histone proteins are associated with DNA and form polymer chains of deoxyribonucleoprotein (DNP), which are chromatin fibrils, clearly visible under electron microscopy.
In certain areas of chromatin fibrils, transcription from DNA of various RNAs is carried out, with the help of which the synthesis of protein molecules is then carried out. Transcription processes in the nucleus are carried out only on free chromosomal fibrils, that is, in euchromatin.
In condensed chromatin, these processes are not carried out and therefore heterochromatin is inactive chromatin. The ratio of euchromatin and heterochromatin in the nucleus is an indicator of the activity of synthetic processes in a given cell. On chromatin fibrils in the S-period of the interphase, the processes of DNA replication are also carried out. These processes occur both in euchromatin and in heterochromatin, but in heterochromatin they occur much later.
The nucleolus is a spherical formation (1-5 microns in diameter) that perceives basic dyes well and is located among the chromatin.
One nucleus can contain from 1 to 4 or even more nucleoli. In young and frequently dividing cells, the size of the nucleoli and their number are increased.
The nucleolus is not an independent structure. It is formed only in interphase in certain regions of some chromosomes - nucleolar organizers, which contain genes encoding a ribosomal RNA molecule. In the region of the nucleolar analyzer, transcription from DNA to ribosomal RNA is carried out.
In the nucleolus, ribosomal RNA combines with protein and the formation of ribosomal subunits.
Microscopically in the nucleolus distinguish:
Fibrillar component - localized in the central part of the nucleolus and is a thread of ribonucleoprotein (RNP);
The granular component is localized in the peripheral part of the nucleolus and represents an accumulation of ribosome subunits.
In the prophase of mitosis, when the spiralization of chromatin fibrils and the formation of chromosomes occur, the processes of RNA transcription and the synthesis of ribosome subunits cease and the nucleolus disappears.
At the end of mitosis, decondensation of chromosomes occurs in the nuclei of newly formed cells and a nucleolus appears.
Karyoplasm (nucleoplasm) or nuclear juice consists of water, proteins and protein complexes (nucleoproteins, glycoproteins), amino acids, nucleotides, sugars. Under a light microscope, the karyoplasm is structureless, but with electron microscopy, granules (15 nm) consisting of ribonucleoproteins are detected in it.
Karyoplasm proteins are mainly enzyme proteins, including glycolysis enzymes that break down carbohydrates and form ATP.
Non-histone (acidic) proteins form a structural network in the nucleus (nuclear protein matrix), which, together with the nuclear envelope, takes part in the creation of an internal order, primarily in a certain localization of chromatin.
With the participation of karyoplasm, the metabolism in the nucleus, the interaction of the nucleus and cytoplasm are carried out.
Karyolemma (nucleolemma) - the nuclear membrane separates the contents of the nucleus from the cytoplasm (barrier function), at the same time provides a regulated metabolism between the nucleus and the cytoplasm. The nuclear envelope is involved in the fixation of chromatin.
The karyolemma consists of two bilipid membranes - the outer and inner nuclear membranes, separated by a perinuclear space, 25 to 100 nm wide.
There are pores in the karyolemma with a diameter of 80-90 nm. In the pore region, the outer and inner nuclear membranes pass into each other, and the perinuclear space is closed.
The lumen of the pore is closed by a special structural formation - the pore complex, which consists of a fibrillar and a granular component. The granular component is represented by protein granules 25 nm in diameter, arranged along the edge of the pore in three rows.
Fibrils depart from each granule and unite in a central granule located in the center of the pore. The pore complex plays the role of a diaphragm that regulates its permeability. The pore sizes are stable for a given cell type, but the number of pores may change during cell differentiation. There are no nuclear pores in the nuclei of spermatozoa. Attached ribosomes can be localized on the outer nuclear membrane. In addition, the outer nuclear membrane may continue into the tubules of the endoplasmic reticulum.
Heterochromatin - sections of chromatin that are in a condensed (compact) state during the cell cycle. A feature of heterochromatin DNA is its extremely low transcribability. HETEROCHROMATIN
(from hetero... and chromatin), sections of chromatin that are in a condensed (densely packed) state throughout the entire cell cycle. They are intensely stained with nuclear dyes and are clearly visible under a light microscope even during interphase.
Heterochromatic regions of chromosomes, as a rule, are replicated later than euchromatic ones and are not transcribed, that is, they are genetically very inert. The nuclei of active tissues and embryonic cells are mostly poor in G. There are facultative and constitutive (structural) G. Facultative G. is present only in one of the homologous chromosomes. An example of G. of this type is the second X-chromosome in female mammals, which is inactivated during early embryogenesis due to its irreversible condensation.
Structural G. is contained in both homologous chromosomes, localized predominantly. in the exposed regions of the chromosome - in the centromere, telomere, nucleolar organizer (during interphase it is located near the nuclear envelope), is depleted in genes, enriched in satellite DNA and can inactivate neighboring genes (i.e.
n. position effect). This type of G. is very variable both within the same species and within closely related species. It can affect chromosome synapsis, the frequency of induced breaks, and recombination. Structural G.'s sites are characterized by adhesion (adhesion) of sister chromatids.
EUCHROMATIN
(from Greek eu - well, completely and chromatin), sections of chromosomes that retain a despiralized state in the resting nucleus (in interphase) and spiralize during cell division (in prophase); contain most of the genes and are potentially capable of transcription.
E. differs from heterochromatin in a lower content of methylated bases and blocks of repetitive DNA sequences, a large number of non-histone proteins and acetylated histone molecules, less dense packing of chromosomal material, which is believed to be especially important for the activity of E. and makes it potentially more accessible to enzymes, providing transcription.
E. can acquire the properties of facultative heterochromatin - inactivate, which is one of the ways to regulate gene activity.
Publication date: 2015-02-18; Read: 229 | Page copyright infringement
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The structure and functions of the cell nucleus.
The nucleus is an essential part of a eukaryotic cell. The main function of the nucleus is to store genetic material in the form of DNA and transfer it to daughter cells during cell division. In addition, the nucleus controls protein synthesis, controls all the life processes of the cell.
(in a plant cell, the nucleus was described by R. Brown in 1831, in an animal cell by T. Schwann in 1838)
Most cells have one nucleus, usually rounded, less often irregular.
The size of the nucleus ranges from 1 μm (in some protozoa) to 1 mm (in the eggs of fish, amphibians).
There are binuclear cells (liver cells, ciliates) and multinuclear cells (in the cells of striated muscle fibers, as well as in the cells of a number of species of fungi and algae).
Some cells (erythrocytes) are non-nuclear, this is a rare phenomenon, it is secondary.
The core includes:
1) nuclear envelope;
2) karyoplasm;
3) nucleolus;
4) chromatin or chromosomes.
Chromatin is in the nondividing nucleus, chromosomes are in the mitotic nucleus.
The shell of the nucleus consists of two membranes (outer and inner). The outer nuclear membrane connects to the membrane channels of the EPS. It contains ribosomes.
The core membranes have pores (3000-4000). Through the nuclear pores, various substances are exchanged between the nucleus and the cytoplasm.
Karyoplasm (nucleoplasm) is a jelly-like solution that fills the space between the structures of the nucleus (chromatin and nucleoli).
It contains ions, nucleotides, enzymes.
The nucleolus, usually spherical in shape (one or more), is not surrounded by a membrane, contains fibrillar protein filaments and RNA.
Nucleoli are not permanent formations; they disappear at the beginning of cell division and are restored after its completion. Nucleoli are found only in non-dividing cells.
In the nucleolus, the formation of ribosomes, the synthesis of nuclear proteins takes place. The nucleoli themselves are formed in areas of secondary chromosome constrictions (nucleolar organizers). In humans, nucleolar organizers are located on chromosomes 13,14,15,21 and 22.
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The cell nucleus in its structure belongs to the group of two-membrane organelles. However, the nucleus is so important for the life of the eukaryotic cell that it is usually considered separately. The cell nucleus contains chromatin (despiralized chromosomes), which is responsible for the storage and transmission of hereditary information.
In the structure of the cell nucleus, the following key structures are distinguished:
- the nuclear envelope, which consists of an outer and an inner membrane
- nuclear matrix - everything that is contained inside the cell nucleus,
- karyoplasm (nuclear juice) - liquid contents similar in composition to hyaloplasm,
- nucleolus,
- chromatin.
In addition to the above, the nucleus contains various substances, subunits of ribosomes, RNA.
The structure of the outer membrane of the cell nucleus is similar to the endoplasmic reticulum.
Often, the outer membrane simply passes into the ER (the latter, as it were, branches off from it, is its outgrowth).
Ribosomes are located on the outer side of the nucleus.
The inner membrane is more durable due to the lamina lining it.
In addition to supporting function, chromatin is attached to this nuclear lining.
The space between the two nuclear membranes is called the perinuclear space.
The membrane of the cell nucleus is permeated with many pores connecting the cytoplasm with the karyoplasm. However, in terms of their structure, the pores of the cell nucleus are not just holes in the membrane. They contain protein structures (pore complex of proteins) responsible for the selective transport of substances and structures. Only small molecules (sugars, ions) can pass passively through the pore.
The chromatin of the cell nucleus consists of chromatin filaments. Each chromatin thread corresponds to one chromosome, which is formed from it by spiralization.
The stronger the chromosome is untwisted (turned into a chromatin thread), the more it is involved in the synthesis processes on it.
The same chromosome can be spiralized in some areas, and despiralized in others.
Each chromatin thread of the cell nucleus is structurally a complex of DNA and various proteins, which, among other things, perform the function of twisting and unwinding chromatin.
Cell nuclei may contain one or more nucleoli. The nucleoli are composed of ribonucleoproteins, from which ribosome subunits are subsequently formed.
This is where rRNA (ribosomal RNA) is synthesized.
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 structure-multiple formation that 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 common 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 reserve products that are used at the early stages of embryogenesis and which can be synthesized in the cell 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 chemicals that cause the synthesis of rRNA to cease.
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 fission, 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 the necessary amount of 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.
Micrograph of the nucleolus
nucleolus- chromosomal regions that determine the synthesis of rRNA and the formation of cellular ribosomes. In growing oocytes, several hundreds of nucleoli - amplification of the nucleoli. Nucleoli are absent in the cells of crushing eggs and in diff. cl - blood cells
The number of nucleoli depends on the number of nucleolar organizers - the areas on which the nucleoli of the interphase nucleus are formed in the telophase form secondary constrictions x-m. In humans, yao has 13, 14, 15, 21, and 22 chromosomes in the short arms (10 per diploid set). 82). The cat has 2; in a pig - 2; mouse - 4; a cow has 8. A cold-blooded one. vertebrates and birds usually 1pair yao x-m
The localization of RAO is determined on mitotic x-maxes by staining with silver salts, associated with RAO proteins, more precisely, the determination of RAO by the FISH method. Nucleoli can fuse with each other.
Multiplicity of ribosomal genes
at rupture x-we at the site of the secondary constriction of the nucleolus can
occur on each of the fragments xm - many copies of ribosomal genes - polycistrons - moderate repeats. E. coli has 6-7 identical rRNA operons scattered throughout the genome - ~1% of the total DNA. The number of rRNA genes is constant in the cell
Amplified nucleoli - mb rRNA genes are excessively replicated. At the same time, additional replication of rRNA genes occurs in order to ensure the production of a large number of ribosomes. As a result of this oversynthesis of rRNA genes, their copies can become free, extrachromosomal. These extrachromosomal copies of rRNA genes can function independently, resulting in a mass of free additional nucleoli, but no longer structurally associated with nucleolus-forming chromosomes. This phenomenon is called rRNA gene amplification. studied in detail on growing amphibian oocytes.
In X. laevis, rDNA amplification occurs in prophase I. In this case, the amount of amplified rDNA (or rRNA genes) becomes 3000 times greater than
per haploid amount of rDNA, and corresponds to 1.5x106 rRNA genes. These supernumerary extrachromosomal copies form hundreds of additional nucleoli in growing oocytes. On average, one additional nucleolus accounts for several hundred or thousands of rRNA genes.
Amplified nucleoli are also found in insect oocytes. 3x106 extrachromosomal copies of rRNA genes were found in oocytes of the banded swimmer.
After the period of maturation of the oocyte, during its two successive divisions, the nucleoli are not included in the mitotic chromosomes, they are separated from the new nuclei and degrade.
Tetrachymena pyriformis has a single rRNA gene in the haploid micronucleus genome. There are ~200 copies in the macronucleus.
In yeast, extrachromosomal copies of rRNA genes are cyclic DNA l ~ 3 μm, so there is one rRNA gene.
STRUCTURE OF THE NUCLEOL
In the nucleolus, a granular component (GC) and a fibrillar component (FC) are distinguished.
Granular component represents
granules 15-20 nm, usually located on the periphery of the nucleolus, although HA and FA may be evenly distributed.
FK and GK are able to form filamentous structures - nucleolonemes- nucleolar filaments ~100-200 nm, which can form separate clumps.
fibrillar component- represents thin (3-5 nm) fibrils - a diffuse part of the nucleoli, in the center of the nucleolus - 1 or 3-5 separate zones: fibrillar centers - parts of the accumulation of fibrils with low density, surrounded by a zone of high density fibrils - dense fibrillar component
chromatin - adjacent to or surrounding the nucleolus. 30nm fibrils of chromatin along the periphery of the nucleolus can enter the gaps, m-y nucleolonemal areas.
protein mesh matrix -
nc regressive staining method - uranyl ions associated with DNA are easily washed out with EDTA chelaton than with RNA? stained structures of sod RNA: granules (strongly), pfc (weaker), chromatin (not stained)
Pulse labeling (3H-uridine), the first traces of labeling were found first (after 1-15 min) in PFA, and then (up to 30 min) HA turned out to be labeled. the FC label was not detected? 45S pre-rRNA is synthesized in the PFC region, and the granular component of the nucleolus corresponds to preribosomal particles (55S-, 40S RNP).
staining with osmium-amine, DNase labeled with gold, binding of labeled actinomycin, direct molecular hybridization with labeled rDNA - that the fibrillar centers contain DNA responsible for rRNA synthesis. The zones of fibrillar centers differ from the rest of the chromatin in that they consist of thin chromatin fibrils, significantly depleted in histone H1 (as shown using colloidal gold-labeled antibodies).
fts: inactive ribosomal genes, spacer regions.
Pre-rRNA transcription occurs at the fc periphery, where pfc is the 45S pre-rRNA located in the form of “herringbones” on decondensed rDNA regions. After completion
During transcription, 45S RNA loses its connection with the transcription unit on DNA in the zone of the dense fibrillar component, in some still incomprehensible way passes into the granular zone, where rRNA processing, formation and maturation of ribosomal subunits take place.
Fibrillar center and nucleolar organizer
The structure and chemical characteristics of PC turned out to be almost identical to those of the nucleolar organizers of mitotic chromosomes. Both of them are built from closely associated fibrils, 6-10 nm thick; both of them have a characteristic feature - they stain with silver salts, which depends on the presence of special nucleolar proteins, contain RNA polymerase I.
the number of FCs in interphase nucleoli does not correspond to the number of nucleolar organizers in mitosis. Thus, in SPEV cells, the number of FCs can be 2–4 times higher than the number of nucleolar organizers.
Moreover, the amount of PC increases as the ploidy of the cell (G2, 4n) and its transcriptional activity increase.
This reduces the size of each individual fibrillar center. However, the total volumes of FC, when recalculated for the haploid chromosome set, remain constant in the interphase, but exceed this number twice as compared with the metaphase. In other words, upon activation of rRNA synthesis, such a change in the number of PCs and their sizes is observed, which may indicate some kind of fragmentation of the original PCs in relatively inactive nucleoli.
The opposite picture is observed with the attenuation of synthetic processes in differentiating cells of the erythroid series of mice (Table 12). It can be seen that in proerythroblasts proliferating and actively synthesizing hemoglobin, the number of fibrillar centers depends on the ploidy of the cell (88 in the G1 phase, 118 in the G2 phase of the cell cycle), the size of individual FCs changes little. After the cessation of reproduction of these cells and the fall of their synthetic activity, the parameters of the nucleolus change dramatically. Their volume, already starting from the stage of basophilic erythroblast
decreases 4-5 times, and at the final stage of differentiation (normoblast) - a hundred times. At the same time, the number of PCs drops sharply (10-40 times) and the volume increases by almost 10 times the size of an individual fibrillar center.
Based on these observations, we can present the general scheme of activation and inactivation of the nucleolus (Fig. 90) in the following way using the example of one nucleolar organizer.
In an inactive form, the nucleolar organizer is presented as one large fibrillar center, which includes a compactly folded part of the chromosomal DNA chain carrying tandemly arranged ribosomal genes (transcriptional units). At the beginning of nucleolus activation, p-genes are decondensed on the periphery of such a fibrillar center, these p-genes begin to be transcribed, RNP transcripts are formed on them, which, upon maturation, give rise to the appearance of ribosomal precursor granules along the periphery of the activated nucleolus. As transcription intensifies, the single fibrillar center seems to disintegrate
into a number of smaller fibrillar centers connected to each other by completely decompacted rDNA regions. The higher the transcriptional activity of the nucleolus, the greater the number of small interconnected fibrillar centers surrounded by a dense fibrillar component (DFC) containing 45S rRNA. With full activation of the nucleolus, all small fibrillar centers decondense; in this case, the zones of the dense fibrillar component contain the entire rDNA in the active state. This structure is observed in the amplified nucleoli of growing oocytes. In the case of inactivation of the nucleolus, gradual condensation of rDNA occurs, fibrillar centers are formed again, they combine with each other, their size increases in parallel with a decrease in the proportion of PFC. With complete inactivation, as in the case of normoblasts, the nucleolus is represented by one large (4-5 μm) spherical FC, without concomitant transcription of PFC: it is surrounded by a zone of condensed chromatin. Such an inactivated nucleolus is similar in its structural features
with a nucleolar organizer as part of mitotic chromosomes.
Structural types of nucleoli
The above descriptions provide a basis for understanding the diversity of the nucleolus structure in cells with an appropriate level of rRNA synthesis. However, in addition to varying degrees of severity of the granular and fibrillar components, there are other variants of the structural organization of the nucleoli. Usually, several structural types of nucleoli are distinguished: reticular or nucleolonemic, compact, annular, residual (resting), segregated (Fig. 91).
The reticular type of the nucleolus is most characteristic of most cells; it is characterized by a nucleolonemic structure, an abundance of granules and dense fibrillar material. In many cases, fibrillar centers are poorly identified, probably due to high levels of transcription. This type of nucleolus is found in animal and plant cells. For example, the reticular type of the nucleolus, characteristic of the giant polytene chromosomes of dipterous insects, is very similar to that of the giant chromosomes.
barley antipodial cells.
The compact type of the nucleolus differs from the previous one in a less pronounced nucleolonema, a higher frequency of occurrence of fibrillar centers. Such nucleoli are characteristic of actively proliferating cells (plant meristem cells, tissue culture cells, etc.). It is likely that both of these types can pass into each other, in any case, they are most often found in cells with a high level of RNA and protein synthesis.
Ring-shaped nucleoli are found in animal cells. In a light microscope, they have the shape of a ring with an optically bright central zone - this is a fibrillar center surrounded by RNP fibrils and granules. These nucleoli are about 1 µm in size. Typical ring-shaped nucleoli are characteristic of lymphocytes, endotheliocytes, i.e. for cells with a relatively low level of transcription.
Residual nucleoli are characteristic of cells that have completely lost the ability to synthesize rRNA (normoblasts, differentiated enterocytes, cells of the prickly layer of the skin epithelium, etc.).
Often they are so small and so surrounded by condensed chromatin that they are difficult to detect under a light microscope. In some cases, they can be activated again and go into a compact or reticular form.
Segregated nucleoli are characteristic of cells treated with various antibiotics or chemicals that cause the cessation of rRNA synthesis (actinomycin D, amphotericin, etc.), as well as antibiotics that affect the synthesis of DNA and proteins (mitomycin, puromycin, many carcinogens, etc.) . The term "segregation" is used in this case due to the fact that there is a kind of separation, separation of different components of the nucleoli, accompanied by a progressive decrease in its volume. At the same time, large fibrillar centers and the granular-fibrillar component are separated from each other.
Nucleolar proteins
Up to 60% of the dry weight of the isolated nucleoli is accounted for by proteins, the number of which can be several hundred different types. In addition to the proteins of nucleolar-associated chromatin,
The nucleolus includes ribosome proteins and specific nucleolar proteins associated with the transcription of ribosomal genes, with the processing of 45S rRNA, such as RNA polymerase I, transcription factors, topoisomerases, methylases, nucleases, protein kinases, and phosphatases. Part of the nucleolar proteins has an affinity for silver - argentophilic proteins: RNA polymerase I, transcription factor UBF, nucleolin (C-23), nucleophosmin (newmatrin or B-23).
Argentophilia is characteristic of proteins enriched in sulfhydryl and disulfide bonds. As already mentioned, interphase nucleoli and zones of nucleolar organizers on mitotic chromosomes have clear argentophilia.
The nucleolar proteins proper are located at specific sites of their activity. Thus, RNA polymerase I and the rRNA transcription factor UBF are located in fibrillar centers (FC) and/or in the dense fibrillar component (PFC).
Ag-philic is also a protein with a pier. weighing 195 kDa, which is a large subunit of RNA polymerase I involved
in rRNA synthesis. This protein is localized in the zone of fibrillar centers, along their periphery. On planar preparations of nucleoli, areas above the axial part of the "herringbones", directly above the location of the granules of RNA polymerase I, have argentophilia. In addition, using immunomorphological methods, RNA polymerase I is detected in the zone of nucleolar organizers of mitotic chromosomes. This circumstance does not contradict the data that transcription completely stops during mitosis. It is likely that during mitosis, genes loaded with inactive RNA polymerase I are transferred along with it in the region of nucleolar organizers from one cell generation to another.
The nucleolar-specific protein fibrillarin (B-36, m.w. 34 kDa) is located in the PFC, where it processes pre-rRNA in a complex with other RNPs, which include U3 snRNA, which is necessary for the initial stage of 45S rRNA processing. Fibrillarin is also found in the residual nucleoli - in the "nucleolar matrix".
Protein C23 (110 kDa) or "nucleolin" is localized in the zone of the dense fibrillar component and in the fibrillar centers of the nucleoli, but also in the zones of the nucleolar organizers of mitotic chromosomes. Therefore, it is found on both transcribed and inactive regions of ribosomal genes. In preparations of spread nucleoli, it is found above transcription units ("herringbones"), it is found in fractions containing ribosome precursors. Its functions are not completely clear, although it has become known that the C23 protein can play an important structural role in the transcription process: it binds to nucleolar chromatin with its N-terminus, on which lysine groups are located, and its C-terminus with a transcribed spacer (tsi) on 45S rRNA.
It was found that this protein binds not to DNA of a transcription unit, but to DNA having a nucleosomal structure (probably with spacer regions).
The B-23 protein (nucleophosin, m.v. 37 kDa) is localized in the PFC region using immunocytochemical methods and mainly in
zone of the granular component. It is believed that B-23 is involved in the intermediate and terminal stages of ribosome biogenesis, and in the transport of pre-ribosomes.
General scheme of the nucleolus as a special locus of ribosome synthesis
With the formation of rRNA synthesis in the nucleoli on the surface of the FC, transcription units are activated, binding to transcription factors and RNA_polymerase I, which begins to read the primary rRNA transcript. As the first RNA polymerase I passes through, the next RNA polymerase sits on the released site of the transcription unit and the synthesis of new rRNA begins. Simultaneously and sequentially, one p-gene can contain up to hundreds of RNA polymerases I, from which transcripts of varying degrees of completeness depart. The end product is pre-rRNA or 45S rRNA. During synthesis, the growing rRNA chains are dressed with ribosomal proteins that enter the nucleus from the cytoplasm, so that RNP precursor chains are immediately formed. The set of transcription products of several transcription
units forms a PFC zone around the FC. The final product of this synthesis is a ribonucleoprotein strand, or a globule with a sedimentation constant of about 80S, containing one 45S rRNA molecule. After separation of 45S rRNA at the terminal point of the transcription unit, cleavage occurs - processing of 45S rRNA, at the end of which 40S and 60S ribosomal subunits are formed. Synthesis of small subunits in the nucleolus takes approximately 30 min, and large subunits - about 1 hour. In the nucleolus, the immature 60S ribosomal subunit, together with two rRNA fragments (28S and 5.8S), binds to the third (5S), which was synthesized independently of chromosomes with nucleolar organizers on other chromosomes. Such newly formed ribosomal subunits exit the nucleus into the cytoplasm in a special way through the nuclear pores. In the cytoplasm, such immature ribosomes can bind to additional proteins. The 40S subunit first binds to the mRNA, and only then to the large 60S subunit, forming a complete 80S functioning ribosome (Fig. 92).
New, non-canonical functions of the nucleoli
Recent evidence indicates that, in addition to rRNA synthesis, the nucleolus is involved in many other aspects of gene expression.
The first hints (1965) on signs of polyfunctionality of the nucleoli were obtained in the study of heterokaryons. Thus, when human HeLa cells were fused with chicken erythrocytes, heterokaryons were obtained with initially completely different nuclei. The nuclei of HeLa cells were functionally active; various RNAs were synthesized in them. The initial nuclei of chicken erythrocytes contained supercondensed chromatin, did not contain nucleoli, and were not transcribed. In the heterokaryon, after fusion with HeLa cells in the nuclei of chicken erythrocytes, chromatin began to decondense, transcription was activated, and nucleoli appeared. Immunocytochemical methods were used to study the appearance in heterokaryons of proteins characteristic of chicken cells. Despite the fact that HeLa cells had a ready system for the functioning of ribosomes and nucleoli were formed, the appearance of chicken proteins was delayed until then.
until nucleoli appear in the nuclei of erythrocytes. This meant that the nucleolus of a chicken erythrocyte must somehow be involved in the formation of chicken mRNAs; the nucleolus must play some role in chicken mRNA production.
More recently, evidence has been accumulated to support this possibility. The maturation (splicing, see below) of c-myc mRNA in mammalian cells has been found to occur in the nucleolus. Spliceosomal small RNAs (sn RNA) and pre-mRNA splicing factors were found in the nucleoli.
Further, in the nucleoli, RNAs are found that are part of the SRP particles involved in the synthesis of proteins in the endoplasmic reticulum. Telomerase RNA, ribonucleoprotein (reverse transcriptase), was associated with the nucleolus. There are many data on the localization in the nucleoli of the processing of small nuclear RNAs that make up spliceosomes, and even on the processing of tRNAs.
Nucleolus during mitosis: peripheral chromosomal material
Under a light microscope, the nucleolus is revealed during interphase,
in mitotic cells it disappears. When using time-lapse microfilming, one can observe in living cells how, as chromosomes condense in interphase, the nucleolus disappears. At first, it is slightly compacted, but then, by the time of the rupture of the nuclear membrane, it begins to quickly lose density, becomes loose and quickly disappears before our eyes, as if melting. It can be seen that part of the nucleolar material spreads between the chromosomes. In metaphase and anaphase, there are no nucleoli as such. The first signs of new nucleoli appear after the middle telophase, when the chromosomes of the daughter nuclei, which have a new nuclear membrane, have already loosened enough. At this time, dense bodies, prenucleoli, appear near the decondensing chromosomes. Usually their number is higher than the number of nucleoli in interphase. Later, already in the G1 period of the cell cycle, the prenucleoli grow, begin to unite with each other, their total number decreases, but the total volume increases. The total volume of the nucleolus doubles in S-G2 phases. In some cases, prophase
(human cell cultures) during the condensation of chromosomes, large nucleoli break up into smaller ones, which disappear in mitosis.
In fact, there is no complete disappearance or “dissolution” of the nucleolus: there is a change in its structure, the reduction of one part of its components while maintaining the other. Thus, it was shown that argentophilic granules in interphase nucleoli, detected in a light microscope, begin to merge with each other in prophase, simultaneously decreasing in volume, they occupy the minimum size in metaphase, being localized in the zones of nucleolar organizers of chromosomes. In this form, they exist until the middle telophase, when they are detected as separate multiple "prenucleoli" scattered among the decondensed chromosomes. Already at the end of telophase, such argentophilic prenucleoli begin to grow. Thus, it can be seen that during mitosis, only a part of the nucleolar component undergoes disappearance, while the argentophilic component is preserved, constantly existing during mitosis.
and is transferred on chromosomes to daughter nuclei.
Radioautographic studies have shown that the disappearance of the nucleoli coincides with the cessation of the synthesis of cellular (mainly ribosomal) RNA, which resumes in the late telophase, coinciding in time with the appearance of new nucleoli.
In addition, it was found that the activity of RNA polymerase I also disappears in the middle stages of mitosis. This gave reason to believe that the new formation of nucleoli is associated with the restoration of rRNA synthesis in daughter cells.
But on the other hand, there are facts pointing to the permanent presence of nucleolar components throughout the entire cell cycle. This applies primarily to the Ag-filic material of the nucleoli.
During mitosis in animals and plants, the chromosomes are surrounded by a matrix, which is an accumulation of loosely located fibrils and granules of ribonucleoproteins, similar in composition to the components that make up the interphase nucleoli.
During chromosome condensation, some of the nucleoli dissociate and go into the cytoplasm (most of the RNP particles), while others are closely associated with the chromosome surface, forming the basis of the "matrix", or peripheral chromosomal material (PCM).
This fibrillar-granular material, synthesized before mitosis, is transferred by chromosomes to daughter cells. In the early telophase, even in the absence of RNA synthesis, as chromosomes decondense, a structural redistribution of PCM components occurs. Its fibrillar components begin to assemble into small associates—prenucleoli, which can merge with each other, and assemble in the zone of the nucleolar chromosome organizer in the late telophase, where rRNA transcription resumes.
The nucleolar proteins involved in rRNA transcription (RNA polymerase I, topoisomerase I, transcription initiation factor UBF, etc.) accumulate in the nucleolar organizer zone, while proteins associated with pre-rRNA processing (fibrillarin,
nucleolin, B-23), as well as some of the pre-rRNA and small nucleolar RNPs, are carried by the surface of chromosomes as part of the peripheral chromosomal material.
In addition, PCM may include some non-histone proteins from the nuclear interphase core.
