Supporting the shape and some features of the morphology of the nucleus. The nuclear matrix includes the nuclear lamina, the residual nucleolus and the so-called diffuse matrix - a network of filaments and granules connecting the nuclear lamina with the residual nucleolus.

The components of the nuclear matrix were first isolated and described in the early 1960s. The term “nuclear matrix” was introduced in the mid-1970s in connection with the accumulation of information about non-chromatin proteins of the nuclear skeleton and its role in the functioning of the cell nucleus. The term was coined to refer to residual nuclear structures that can be obtained from successive nuclear extractions.

Description [ | ]

The nuclear matrix can be obtained by treating isolated nuclei with nucleases and subsequent extraction of histones with a 2 M NaCl solution. As such, the nuclear matrix is ​​not a clear morphological structure. The composition of the nuclear matrix remaining after extraction of chromatin from the nucleus and removal of the nuclear envelope using nonionic detergents, as well as removal of DNA and RNA residues using nucleases, is similar in different objects. It is composed of 98% non-histone proteins, and also contains 0.1% DNA, 1.2% RNA and 1.1% phospholipids. Protein composition nuclear matrix is ​​approximately the same in different types of cells. It is characterized by the presence of lamins, as well as many minor proteins with masses from 11-13 to 200 kDa.

Morphologically, the nuclear matrix consists of the nuclear lamina, the diffuse matrix (also known as the internal or interchromatic network), and the residual nucleolus. The lamina is a proteinaceous reticulate layer lining the inner membrane of the nuclear envelope. The diffuse matrix is ​​revealed only after chromatin is isolated from the nucleus. It is a loose fibrous network located between sections of chromatin. Sometimes it contains ribonucleoprotein granules. The residual nucleolus is a dense structure, repeating the shape of the nucleolus and consisting of densely packed fibrils.

DNA loops that are associated with the nuclear matrix are discrete topological domains. It has been shown that in nuclei there are from 60,000 to 125,000 DNA sections protected from nucleases and located on all three components of the nuclear matrix. For the formation of sites for attachment of DNA loops to the nuclear matrix, MAR elements (SAR, S/MAR) are important - genome elements that specifically bind to the isolated nuclear matrix under conditions in vitro. These elements contain DNA about 200 base pairs long and are located at a distance of 5 to 112,000 bp. from each other. The fruit fly has at least 10,000 MARs in its nucleus.

The locations of MAR elements are very similar to DNA binding sites. , involved in the formation of chromatin loops. It has been shown that the nuclear matrix is ​​associated with DNA replication: more than 70% of newly synthesized DNA is localized in the zone of the internal nuclear matrix. The fraction of DNA associated with the nuclear matrix is ​​enriched in replication forks. In addition, it was found in the nuclear matrix

NUCLEAR MATRIX on the left - diagram of the structure of nuclei before extraction; on the right - after extraction; 1 - near-membrane protein layer (lamina) and pore complexes; 2 - interchromatin matrix protein network; 3 - protein matrix of the nucleolus

Nuclear Matrix: Definition Euchromatin and heterochromatin are associated within the nucleus with a network of nonchromatic fibrillar and granular structures. Even 50 years ago, the existence of a fraction of nuclear proteins was shown that forms a fibrillar network of nucleoproteins in the nucleus. The term nuclear matrix was proposed for this structure by Berezney and Coffey (1974). Due to the fact that the concept of the nuclear matrix is ​​operationally defined, different authors include different structures in its composition. Thus, in most cases it is believed that the nuclear matrix (scaffold) is an intranuclear network of fibrillar and granular components, a peripheral lamina with pore complexes and a residual nucleolus involved in the processes of genome functioning (initiation of DNA synthesis and replication, as well as in the synthesis, RNA processing and transport), its maintenance and the location of chromosomes in the nucleus.

Nuclear matrix: structure Proteins DNA RNA Phospholipids Some studies suggest that the structural unity of the nuclear matrix is ​​due to metal-protein interactions, similar to those that occur during matrix release using methods based on the inclusion of calcium or copper ions, as well as magnesium.

The protein composition of the nuclear matrix very much depends on the methods and conditions of its isolation. Only a few of the many matrix proteins have been isolated and characterized: Structural proteins - lamin A, lamin B 1, B 2 and lamin C, nucleoprotein B-23 and residual hn proteins. RNP particles, matrine; Regulatory proteins - non-histone chromosomal proteins, nuclear acidic proteins, high mobility group (HMG) nuclear proteins, various transcription factors and nucleic acid metabolic enzymes. Of these, topoisomerase II should be especially noted, which is also one of the components of the matrix (and metaphase chromosomes) and is present there in fairly large quantities, determining the topological status of chromosomal DNA. The sequence of identically oriented lamins A, B and C (molecular weight 65 -70 kD) form the nuclear lamina (a rigid structure underlying the nuclear membrane, involved in the organization of chromatin). The nuclear lamina contacts chromatin and nuclear RNAs. As a result of the association of the three main polypeptides, through dimeric interaction, they are folded into 10 nm structures that attach to specific proteins of the nuclear membrane through C-lamin. Vlamin is apparently associated with certain regions of chromosomes. Lamin A mediates the connection between lamins C and B. An important function of nuclear matrix polypeptides is the disintegration of the nuclear envelope during mitosis. Matrins play the role of the main structural proteins of the matrix in the narrow sense. These are matrine 3 (12 k. D, has slightly acidic properties), matrine 4 (105 k. D, basic), matrine D-G (60 -75 k. D, basic) and matrine 12 and 13 (42 -48 k. D , have acidic properties).

Nuclear matrix: interaction with DNA Regions of DNA that specifically bind to the nuclear matrix apparently take an important part in the processes of regulation of gene activity, as well as in the processes of RNA replication, splicing and its transfer from the nucleus to the cytoplasm. Lamins, topoisomerases II, special AT-rich sequence binding protein 1 (SATB 1) and matrix binding factor-B 1 (SAFB 1) are key players in fundamental nuclear processes. In eukaryotic organisms, chromatin is attached to the nuclear matrix by short DNA sequences of about 100-2000 bp. , these are the so-called matrix binding regions (MARs). The strong interaction between MARs and insoluble nuclear matrix proteins protects these sequences from ion buffer and nucleases. As a rule, MAR/SAR sequences flank genes, but in some cases they are found inside genes, but as part of introns, as well as near enhancers. Interactions of DNA with the nuclear matrix are divided into: permanent (that is, present in the inactive nucleus) function-dependent (temporary, dynamic) Higher structures chromatin of interphase and metaphase chromosomes is likely to be supported by permanent MARs. Dynamic temporal associations of MARs will be implicated in genomic functions as they relate to transcription or replication of the genetic locus to which they are associated.

MARs and transcriptional regulation Let us describe transcriptional regulation using the example of T-cell differentiation. Following antigen stimulation, the naïve helper CD 4 T lymphocyte differentiates into effector Th 1 and Th 2 cells. In mice, IFNG (interferon-γ cytokine gene) will be silent in naïve T cells but transcribed in activated Th 1 cells. In naive T cells, IFNG exists in a linear conformation, but in Th 1 cells it is present in the form of loops connected to the nuclear matrix by MARs 7 kb on one side and 14 kb on the other side of the locus. The lack of selective DNA attachment to the nuclear matrix in naïve T cells indicates that dynamic DNA-matrix connections form loops that promote expression of the IFNG locus. The molecular mechanisms of permanent communication can be illustrated by the example of a locus that contains a cluster of coordinately regulated genes IL 4, IL 13 and IL 5. These genes are expressed in Th 2 cells, but are silent in naive T lymph cells. Following Th 2 activation, SATB 1 (special AT-rich sequence-binding protein-1) gene expression is rapidly induced and MARs form small loops to promote gene expression. Down-regulation of SATB 1 expression by RNA interference prevents both the formation of this loop structure and the transcriptional activation of the locus. In SATB 1 -null thymocytes, the expression of many genes is impaired and T cell development in SATB 1 -deficient mice is prematurely blocked. These results indicate that SATB 1 binding on MARs regulates the expression of T cell differentiation genes by higher order chromatin reorganization.

Transcription in Eukaryotic Cells In eukaryotic cells, mRNA synthesis is concentrated in foci within the nucleus that contain RNA polymerases, RNA transcriptases, mRNA transcription factors, and processing factors. The persistence of RNA polymerase II and general transcription factors in nuclei after extraction of soluble proteins and nucleases suggests that the transcription factors are assembled on the nuclear matrix. It is proposed that dynamic interactions between MARs and the matrix integrate proximal and distal regulatory sequences and assemble them close to transcription factors, thereby promoting efficient regulation of gene expression. The association of MARs and the nuclear matrix topologically confines DNA into loop structures, protecting DNA intermediates from the influence of cis-regulatory elements. Thus, we can say that MARs perform functions such as a platform for a wide range of matrix proteins. Such interactions form complex nucleoprotein structures that: isolate chromatin domains regulate gene expression

A simplified model depicting the function of MARs in gene regulation. Transcriptional activation is accompanied by the anchoring of MARs into the nuclear matrix. This leads to the formation of loops. The transcription complex assembles at the site of MARs. The interaction of MARs with the nuclear matrix integrates coding sequences, DNA regulatory elements, and transcription factors. At the end of S phase, the transcription complex breaks down.

MARs and Replication To ensure that the genome is copied accurately and only once per cell cycle, eukaryotes have evolved complex mechanisms to regulate DNA replication. At the site of replication, the nuclear matrix contains factors necessary for DNA replication: DNA polymerase proliferating cell nuclear antigen (PCNA) single-stranded binding protein (RPA) The choice and size of the replicon is thought to be determined in early G 1 phase. MCM 2 (DNA replication licensing factor), ORC 1, 2 (origin recognition complex) are gradually loaded into the replication complex, but are quickly excluded in the S phase. This is consistent with a model in which MARs stably anchor the ends of the replicon, and during G 1, Oris attaches to the nuclear matrix, where the matrix accumulates factors to form a prereplication complex. Subsequently, as the number of Oris increases in S phase, certain protein factors detach from chromatin and undergo proteolysis - as part of a control mechanism to prevent re-replication - thus freeing Oris from the nuclear matrix. At the ends of a replicon, MARs can act as barriers between adjacent replicons, preventing the accumulation of supercoiled DNA structure while providing binding sites for topoisomerase II, which can allow replication of intermediates.

Diagram of DNA replication on a nuclear matrix (a) Replicons are defined in the early G 1 phase of the cell cycle by the attachment of MARs to the nuclear matrix. (b) At the end of G 1 - the beginning of replication (Oris) - replication factors are assembled at these sites (c) Once the necessary mitogenic stimuli have been received, the cells enter S phase, in which Oris are activated. After initiation of replication at a specific locus, initiation factors dissociate from the nuclear matrix. Two DNA replication loops gradually appear (shown in blue) (d) At the end of S phase, the replication complexes break down.

Nuclear matrix lipids Phospholipids (sphingomyelin - usually predominates, PC, PE, cardiolipin (in rats)); Neutral lipids (free cholesterol, many triglycerides and free fatty acids, few cholesteryl esters, and no diglycerides at all (in rats)). Two types of contacts of DNA loops with the nuclear matrix are suggested: dynamic - functional, due to phospholipids, possibly cardiolipin and sphingomyelin, through its sphingosine group (participation of sphingomyelin at the initiation points of DNA replication on the matrix, especially since sphingomyelin has a strong destabilizing effect on the secondary DNA structure); Østable - durable due to neutral lipids (regulation of nucleic acid synthesis both at the level of modification of protein kinase C activity and as a result of interaction with the DNA matrix (fatty acids, cholesterol)).

We have already become acquainted with the fact that in the interphase nucleus, unfolded chromosomes are not located chaotically, but are strictly ordered. Such organization of the chromosome in the three-dimensional space of the nucleus is necessary not only for chromosome segregation and separation from neighbors to occur during mitosis, but is also necessary for ordering the processes of chromatin replication and transcription. It can be assumed that in order to carry out these tasks, there must be some kind of framework intranuclear system, which can serve as a unifying basis for all nuclear components - chromatin, nucleolus, nuclear envelope. Such a structure is protein nuclear core or matrix. It is necessary to immediately make a reservation that the nuclear matrix does not represent a clear morphological structure: it is revealed as a separate morphological heterogeneous component upon extraction from the nuclei of almost all areas of chromatin, the bulk of RNA and lipoproteins of the nuclear envelope. From the nucleus, which does not lose its general morphology, remaining a spherical structure, there remains a kind of frame, a skeleton, which is sometimes also called the “nuclear skeleton”.

Components of the nuclear matrix (residual nuclear proteins) were first isolated and characterized in the early 60s. It was found that with sequential treatment of isolated rat liver nuclei with a 2 M NaCI solution and then with DNase, complete dissolution of chromatin occurs, and the main structural elements of the nucleus remain: the nuclear envelope, associated components - nucleomes (nuclear filaments) containing protein and RNA , and nucleoli. It has been hypothesized that chromatin fibrils in native nuclei are attached to these axial protein filaments like a “bottle brush” (see Fig. 67).

Much later (mid-70s) these works were developed and led to the emergence of a mass of new information about non-chromatin proteins of the nuclear core and its role in the physiology of the cell nucleus. At the same time, the term “nuclear matrix” was proposed to denote the residual structures of the nucleus that can be obtained as a result of successive extractions of nuclei with various solutions. What was new in these techniques was the use of nonionic detergents, such as Triton X-100, which dissolve nuclear lipoprotein membranes.

The sequence of processing of isolated nuclei, leading to the production of nuclear matrix preparations enriched with protein, is as follows (see Table 6).

Table 6. Extraction (in %) of nuclear components in the process of obtaining nuclear protein matrix

Isolated nuclei obtained in solutions of 0.25 M sucrose, 0.05 M Tris-HCI buffer and 5 mM MgCI 2 were placed in a solution of low ionic strength (LS), where the bulk of the DNA was degraded due to endonuclease cleavage. In 2 M NaCI (HS), chromatin was subsequently dissociated into histones and DNA, and further extraction of DNA fragments and various proteins took place. Subsequent treatment of nuclei in a 1% Triton X-100 solution led to almost complete loss of nuclear envelope phospholipids and the formation of a nuclear matrix (NM) containing DNA and RNA residues, which were further dissolved by treatment with nucleases, resulting in the final nuclear protein matrix fraction ( NPM). It consists of 98% non-histone proteins; it also contains 0.1% DNA, 1.2% RNA, and 1.1% phospholipids.

The chemical composition of the nuclear matrix obtained in this way is similar in different objects (see Table 7).

Table 7. Composition of the nuclear protein matrix

According to its morphological composition, the nuclear matrix consists of at least three components: a peripheral protein mesh (fibrous) layer - lamina (nuclear lamina, fibrous lamina), an internal or interchromatic network (skeleton) and a “residual” nucleolus (Fig. 68) .

The lamina is a thin fibrous layer underlying the inner membrane of the nuclear envelope. It also includes complexes of nuclear pores, which are, as it were, embedded in the fibrous layer. This part of the nuclear matrix is ​​often called the “pore complex – lamina” fraction (PCL – “pore complex – lamina”). In intact cells and nuclei, lamin is mostly not detected morphologically, because a layer of peripheral chromatin is closely adjacent to it. Only sometimes can it be observed in the form of a relatively thin (10-20 nm) fibrous layer located between the inner membrane of the nuclear envelope and the peripheral layer of chromatin.

The structural role of the lamina is very important: it forms a continuous fibrous protein layer along the periphery of the nucleus, sufficient to maintain the morphological integrity of the nucleus. Thus, the removal of both membranes of the nuclear membrane using Triton X-100 does not cause disintegration or dissolution of the nuclei. They retain their round shape and do not spread out even if they are transferred to low ionic strength when chromatin swelling occurs.

The intranuclear framework or network is morphologically revealed only after chromatin extraction. It is represented by a loose fibrous network located between sections of chromatin; often this spongy network includes various granules of RNP nature.

Finally, the third component of the nuclear matrix is ​​the residual nucleolus - a dense structure that repeats the shape of the nucleolus and also consists of densely packed fibrils.

The morphological expression of these three components of the nuclear matrix, as well as the amount in the fractions, depends on a number of conditions for processing the nuclei. Matrix elements are best identified after isolation of nuclei in relatively high (5 mM) concentrations of divalent cations.

It was found that the formation of disulfide bonds is of great importance for identifying the protein component of the nuclear matrix. So, if the nuclei are pre-incubated with iodoacetamide, which prevents the formation of S-S bonds, and then stepwise extraction is carried out, then the nuclear matrix is ​​represented only by the PCL complex. If we use sodium tetrathionate, which causes the closure of S-S bonds, then the nuclear matrix is ​​represented by all three components. In nuclei pre-treated with hypotonic solutions, only the lamina and residual nucleoli are detected.

All these observations led to the conclusion that the components of the nuclear matrix are not frozen rigid structures, but components with dynamic mobility, which can change not only depending on the conditions of their isolation, but also on the functional characteristics of native nuclei. For example, in mature erythrocytes of chickens, the entire genome is repressed and chromatin is localized mainly at the periphery of the nucleus; in this case, the internal matrix is ​​not detected, but only a lamina with pores. In the erythrocytes of 5-day-old chick embryos, the nuclei of which retain transcriptional activity, elements of the internal matrix are clearly expressed.

As can be seen from the table. 7, the main component of the residual structures of the nucleus is protein, the content of which can range from 98 to 88%. The protein composition of the nuclear matrix from different cells is quite similar. It is characterized by three proteins of the fibrous layer, called lamins. In addition to these major polypeptides, the matrix contains a large number of minor components with molecular weights from 11-13 to 200 kDa.

Lamins are represented by three proteins (lamins A, B, C). Two of them, lamins A and C, are close to each other immunologically and in peptide composition. Lamin B differs from them in that it is a lipoprotein and therefore binds more tightly to the nuclear membrane. Lamin B remains associated with membranes even during mitosis, while lamins A and C are released upon destruction of the fibrous layer and diffusely distributed throughout the cell.

As it turned out, lamins are similar in their amino acid composition to intermediate microfilaments (vimentin and cytokeratin) that are part of the cytoskeleton. Often, the fraction of isolated nuclei, as well as preparations of the nuclear matrix, contain significant amounts of intermediate filaments, which remain associated with the nuclear periphery even after removal of the nuclear membranes.

Unlike intermediate filaments, lamins do not form filamentous structures during polymerization, but are organized into networks with an orthogonal type of molecular packing. Such continuous lattice areas that underlie the inner membrane of the nuclear envelope can be disassembled during phosphorylation of lamins, and polymerize again when they are dephosphorylated, which ensures the dynamism of both this layer and the entire nuclear envelope.

The molecular characterization of intranuclear core proteins has not yet been developed in detail. It has been shown that it includes a number of proteins that take part in the domain organization of DNA in the interphase nucleus in the creation of a rosette-shaped, chromomeric form of chromatin packaging. The assumption that the elements of the internal matrix represent the cores of the rosette structures of chromomeres is confirmed by the fact that the polypeptide composition of the matrix of interphase nuclei (with the exception of lamina proteins) and the residual structures of metaphase chromosomes (axial structures or “scaffolds”) are almost the same. In both cases, these proteins are responsible for maintaining the loop organization of DNA.

The cell nucleus is the central organelle, one of the most important. Its presence in the cell is a sign high organization body. A cell that has a formed nucleus is called eukaryotic. Prokaryotes are organisms consisting of a cell that does not have a formed nucleus. If we consider all its components in detail, we can understand what function the cell nucleus performs.

Core structure

1. Nuclear envelope.
2. Chromatin.
3. Nucleoli.
4. Nuclear matrix and nuclear juice.

The structure and function of the cell nucleus depends on the type of cell and its purpose.

Nuclear envelope

The nuclear envelope has two membranes - outer and inner. They are separated from each other by the perinuclear space. The shell has pores. Nuclear pores are necessary so that various large particles and molecules can move from the cytoplasm to the nucleus and back.

Nuclear pores are formed by the fusion of the inner and outer membranes. Pores are round openings with complexes that include:

1. A thin diaphragm that closes the hole. It is penetrated by cylindrical channels.
2. Protein granules. They are located on both sides of the diaphragm.
3. Central protein granule. It is associated with peripheral granules by fibrils.

The number of pores in the nuclear membrane depends on how intensively synthetic processes take place in the cell.

The nuclear envelope consists of outer and inner membranes. The outer one passes into the rough ER (endoplasmic reticulum).

Chromatin

Chromatin is the most important substance included in the cell nucleus. Its functions are the storage of genetic information. It is represented by euchromatin and heterochromatin. All chromatin is a collection of chromosomes.

Euchromatin is parts of chromosomes that actively participate in transcription. Such chromosomes are in a diffuse state.

Inactive sections and entire chromosomes are condensed clumps. This is heterochromatin. When the state of the cell changes, heterochromatin can transform into euchromatin, and vice versa. The more heterochromatin in the nucleus, the lower the rate of ribonucleic acid (RNA) synthesis and the lower the functional activity of the nucleus.

Chromosomes

Chromosomes are special structures that appear in the nucleus only during division. A chromosome consists of two arms and a centromere. According to their form they are divided into:

• Rod-shaped. Such chromosomes have one large arm and the other small.
• Equal-armed. They have relatively identical shoulders.
• Mixed shoulders. The arms of the chromosome are visually different from each other.
• With secondary constrictions. Such a chromosome has a non-centromeric constriction that separates the satellite element from the main part.

In each species, the number of chromosomes is always the same, but it is worth noting that the level of organization of the organism does not depend on their number. So, a person has 46 chromosomes, a chicken has 78, a hedgehog has 96, and a birch tree has 84. Largest number The fern Ophioglossum reticulatum has chromosomes. It has 1260 chromosomes per cell. The male ant of the species Myrmecia pilosula has the smallest number of chromosomes. He only has 1 chromosome.

It was by studying chromosomes that scientists understood the functions of the cell nucleus.

Chromosomes contain genes.

Gene

Genes are sections of deoxyribonucleic acid (DNA) molecules that encode specific compositions of protein molecules. As a result, the body exhibits one or another symptom. The gene is inherited. Thus, the nucleus in the cell performs the function of transmitting genetic material next generations of cells.

Nucleoli

The nucleolus is the densest part that enters the cell nucleus. The functions it performs are very important for the entire cell. Usually has a round shape. The number of nucleoli varies in different cells - there may be two, three, or none at all. Thus, there is no nucleolus in the cells of crushed eggs.

Structure of the nucleolus:

1. Granular component. These are granules that are located on the periphery of the nucleolus. Their size varies from 15 nm to 20 nm. In some cells, HA may be evenly distributed throughout the nucleolus.
2. Fibrillar component (FC). These are thin fibrils, ranging in size from 3 nm to 5 nm. Fk is the diffuse part of the nucleolus.

Fibrillar centers (FCs) are areas of fibrils that have a low density, which, in turn, are surrounded by fibrils with a high density. Chemical composition and the structure of the PCs is almost the same as that of the nucleolar organizers of mitotic chromosomes. They consist of fibrils up to 10 nm thick, which contain RNA polymerase I. This is confirmed by the fact that the fibrils are stained with silver salts.

Structural types of nucleoli

1. Nucleolonemal or reticular type. Characterized by a large number granules and dense fibrillar material. This type of nucleolar structure is characteristic of most cells. It can be observed both in animal cells and in plant cells.
2. Compact type. It is characterized by a low severity of nucleonoma and a large number of fibrillar centers. It is found in plant and animal cells, in which the process of protein and RNA synthesis actively occurs. This type of nucleoli is characteristic of cells that are actively reproducing (tissue culture cells, plant meristem cells, etc.).
3. Ring type. In a light microscope, this type is visible as a ring with a light center - a fibrillar center. The size of such nucleoli is on average 1 micron. This type is characteristic only of animal cells (endotheliocytes, lymphocytes, etc.). In cells with this type of nucleoli there are quite low level transcriptions.
4. Residual type. In cells of this type of nucleoli, RNA synthesis does not occur. Under certain conditions, this type can become reticular or compact, i.e., activated. Such nucleoli are characteristic of cells of the spinous layer of the skin epithelium, normoblast, etc.
5. Segregated type. In cells with this type of nucleolus, rRNA (ribosomal ribonucleic acid) synthesis does not occur. This occurs if the cell is treated with any antibiotic or chemical. The word “segregation” in this case means “separation” or “separation”, since all components of the nucleoli are separated, which leads to its reduction.

Almost 60% of the dry weight of the nucleoli is protein. Their number is very large and can reach several hundred.

The main function of the nucleoli is the synthesis of rRNA. Ribosome embryos enter the karyoplasm, then leak through the pores of the nucleus into the cytoplasm and onto the ER.

Nuclear matrix and nuclear sap

The nuclear matrix occupies almost the entire cell nucleus. Its functions are specific. It dissolves and distributes everything evenly nucleic acids in a state of interphase.

The nuclear matrix, or karyoplasm, is a solution that contains carbohydrates, salts, proteins and other inorganic and organic substances. It contains nucleic acids: DNA, tRNA, rRNA, mRNA.

During cell division, the nuclear membrane dissolves, chromosomes are formed, and the karyoplasm mixes with the cytoplasm.

The main functions of the nucleus in a cell

1. Informative function. It is in the nucleus that all the information about the heredity of the organism is located.
2. Inheritance function. Thanks to genes located on chromosomes, an organism can pass on its characteristics from generation to generation.
3. Merge function. All cell organelles are united into one whole in the nucleus.
4. Regulation function. All biochemical reactions in the cell and physiological processes are regulated and coordinated by the nucleus.

One of the most important organelles is the cell nucleus. Its functions are important for the normal functioning of the entire organism.

Nucleoli – dense, intensely colored round formations in the core measuring 1-2 microns. There may be several of them. Nucleoli are formed in the nucleus in the region of nucleolar organizers, which are usually located in the region of secondary constrictions of some chromosomes. There are genes encoding ribosomal RNA. The nucleoli consist of granular and fibrillar components. Nucleolar granules are ribosomal subunits, and filaments are molecules of the resulting ribosomal RNA. The latter bind to proteins coming from the cytoplasm to form ribosomal subunits. These subunits enter the cytoplasm through nuclear pores, where they combine into ribosomes and bind to messenger RNA for protein synthesis. The higher the functional, synthetic activity of a cell, the more numerous and larger its nucleoli.

Transcription of non-ribosomal genes.

Nuclear protein matrix.

Karyoplasm(nuclear juice) is a liquid component of the nucleus, a true solution of biopolymers in which chromosomes and the nucleolus are located in suspension. In terms of its physicochemical properties, karyoplasm is close to hyaloplasm.

Nuclear envelope.

Nuclear envelope separates the nucleus from the cytoplasm, delimits its contents and ensures the exchange of substances between the nucleus and the cytoplasm. The nuclear envelope consists from two biological membranes, between which is located perinuclear space 15-40 nm wide. The outer membrane of the nucleus is covered with ribosomes and passes into the membranes of the granular endoplasmic reticulum. Adjacent to the inner membrane is a layer of protein filaments ( lamina) karyoskeleton, through which chromosomes are attached to the nuclear membrane (Fig. 2-9).

There are holes in the nuclear membrane - nuclear pores with a diameter of 90 nm (Fig. 2-10). They are not just holes, but very complexly organized pore complexes They contain proteins that form three rows of 8 granules along the edge of the pore, and in the center of the pore there is 1 granule, connected by protein threads to peripheral granules.

This creates partition, diaphragm 5 nm thick. These pore complexes are selectively permeable: small ions cannot pass through them, but long strands of messenger RNA and ribosomal subunits are transported.

The core has several thousand pores, occupying from 3 to 35% of its surface. Their number is much greater in cells with intense synthetic and metabolic processes. No pores are found in the nuclear membranes of mature spermatozoa, where protein biosynthesis does not occur. It is also noted that the higher the functional activity of the cell, the more convoluted the karyolemma is (to increase the area of ​​​​metabolism between the nucleus and the cytoplasm).