The cells have a developed cytoskeleton. Cytoskeleton

The cytoskeleton is formed by three components: microtubules, microfilaments, and intermediate filaments.

Microtubules penetrate the entire cytoplasm of the cell. Each of them is a hollow cylinder with a diameter of 20–30 nm. The wall of microtubules is formed by 13 threads (protofilaments), twisted in a spiral one above the other. Each thread, in turn, is composed of tubulin protein dimers. Tubulin synthesis occurs on the membranes of granular ER, and assembly into a spiral occurs in the cell center.

Accordingly, many microtubules have a radial direction relative to the centrioles. From here they spread throughout the cytoplasm.

Most microtubules have fixed (“-”) and free (“+”) ends. The free end ensures the lengthening and shortening of the tubes. The formation of microtubules by self-assembly involves small spherical bodies - satellites (microtubule organizing centers) contained in the cell center and basal bodies of cilia, as well as centromeres of chromosomes. If the microtubules of the cytoplasm are completely destroyed, they grow from the cell center at a speed of 1 µm/min. The destruction of microtubules leads to a change in cell shape(an animal cell usually takes on a spherical shape). In this case, the structure of the cell and the distribution of organelles are disrupted.

In a cage microtubules can be located:

Ø in the form of separate elements;

Ø in bundles in which they are connected to each other by cross bridges (neuron processes);

Ø as part of pairs or doublets (axial filament of cilia and flagella);

Ø as part of triplets (centrioles and basal bodies).

In the last two variants, the microtubules partially merge with each other.

Functions of microtubules:

a) maintaining the shape and polarity of the cell;

b) ensuring the orderliness of the arrangement of cell components;

c) participation in the formation of other, more complex organelles (centrioles, cilia, etc.);

d) participation in intracellular transport;

e) ensuring the movement of chromosomes during mitotic cell division;

e) ensuring the movement of cilia.

Microfilaments. Microfilaments are thin protein filaments with a diameter5 – 7 nm, found in almost all cell types. They can be located in the cytoplasm in bundles, network-like layers, or singly.

The main protein of microfilaments is actin, which accounts for up to 5% of the total number of proteins. In addition to it, microfilaments may include myosin, tropomyosin, as well as several dozen actin-binding proteins. The actin molecule usually looks like two helically twisted filaments. Directly below the plasma membrane is a cortical network in which microfilaments are intertwined and connected to each other using special proteins, such as filamin. The cortical network determines the smooth change in cell shape, gradually rearranging itself with the participation of actin-splitting enzymes. Thus, it prevents sharp and sudden deformation of the cell under mechanical stress. Individual microfilaments of the cortical network are attached to the integral and transmembrane proteins of the plasmalemma, as well as to the so-called adhesive junctions (focal contacts), which connect the cell with the components of the intercellular substance or with other cells. Microfilaments are more resistant to physical and chemical influences than microtubules.

Main functions of microfilaments:

1) ensuring a certain rigidity and elasticity of the cell due to the cortical network of microfilaments;

2) a change in the consistency of the cytosol, including the transition from sol to gel;

3) participation in endocytosis and exocytosis;

4) ensuring the motility of non-muscle cells (for example, neutrophils and macrophages), which is based on a change in the shape of the cell surface due to regulated actin polymerization;

5) participation in the contraction of muscle cells and fibers;

6) stabilization of local protrusions of the plasma membrane provided by bundles of cross-linked actin filaments (microvilli, stereocilia);

7) participation in the formation of intercellular connections (girdling desmosomes, etc.).

Intermediate filaments are ropes woven by protein threadsabout 10 nm thick. This indicator determined their allocation to an intermediate place between microtubules and microfilaments. Intermediate filaments form three-dimensional networks in the cells of various tissues of the animal body. They surround the nucleus and can be located in various parts of the cytoplasm, form intercellular connections (desmosomes and hemidesmosomes), and are located inside the processes of nerve cells.

The main functions of intermediate filaments:

1) structural;

2) supporting;

3) the function of distribution of organelles in certain areas of the cell.

The cytoskeleton is a set of thread-like protein structures - microtubules and microfilaments that make up the musculoskeletal system of the cell. Only eukaryotic cells have a cytoskeleton; prokaryotic (bacterial) cells do not have it, which is an important difference between these two types of cells. The cytoskeleton gives the cell a certain shape even in the absence of a rigid cell wall. It organizes the movement of organelles in the cytoplasm (the so-called flow of protoplasm), which underlies amoeboid movement. The cytoskeleton is easily rebuilt, providing, if necessary, a change in cell shape. The ability of cells to change shape determines the movement of cell layers in the early stages embryonic development. During cell division ( mitosis) the cytoskeleton “disassembles” (dissociates), and its self-assembly occurs again in the daughter cells.

The cytoskeleton performs three main functions.

1. Serves as a mechanical framework for the cell, which gives the cell its typical shape and provides communication between the membrane and organelles. The framework is a dynamic structure that is constantly updated as external conditions and the state of the cell change.

2. Acts as a “motor” for cellular movement. Motor (contractile) proteins are found not only in muscle cells, but also in other tissues. The components of the cytoskeleton determine the direction and coordinate movement, division, change in cell shape during growth, movement of organelles, and movement of the cytoplasm.

3. Serves as “rails” for the transport of organelles and other large complexes within the cell.

24. The role of the immunocytochemistry method in the study of the cytoskeleton. Features of the organization of the cytoskeleton in muscle cells.

Immunocytochemical analysis is a method that allows for immunological analysis of cytological material while maintaining cell morphology. ICC is one of many types of immunochemical method: enzyme immunoassay, immunofluorescence, radioimmune, etc. The basis of the ICC method is the immunological reaction of antigen and antibody.

The cytoplasm of eukaryotic cells is permeated by a three-dimensional network of protein threads (filaments) called the cytoskeleton. Depending on their diameter, filaments are divided into three groups: microfilaments (6-8 nm), intermediate fibers (about 10 nm) and microtubules (about 25 nm). All these fibers are polymers consisting of subunits of special globular proteins.

Microfilaments (actin filaments) are composed of actin, a protein most abundant in eukaryotic cells. Actin can exist as a monomer (G-actin, “globular actin”) or a polymer (F-actin, “fibrillar actin”). G-actin is an asymmetric globular protein (42 kDa), consisting of two domains. As ionic strength increases, G-actin reversibly aggregates to form a linear, coiled-coil polymer, F-actin. The G-actin molecule carries a tightly bound ATP molecule, which, when converted to F-actin, is slowly hydrolyzed to ADP, i.e. F-actin exhibits the properties of ATPase.

B. Intermediate fiber proteins

The structural elements of intermediate fibers are proteins belonging to five related families and exhibiting a high degree of cellular specificity. Typical representatives of these proteins are cytokeratins, desmin, vimentin, glia fibrillary acidic protein [GFAP] and neurofilament. All of these proteins have a basic core structure in the central part, which is called a supercoiled α-helix. Such dimers associate antiparallel to form a tetramer. The aggregation of tetramers in a head-to-head manner produces a protofilament. Eight protofilaments form the intermediate fiber.

In contrast to microfilaments and microtubules, free intermediate fiber monomers are hardly found in the cytoplasm. Their polymerization leads to the formation of stable non-polar polymer molecules.

V. Tubulin

The cytoskeleton performs three main functions.

1. Serves as a mechanical framework for the cell, which gives the cell a typical shape and provides communication between the membrane and organelles. The framework is a dynamic structure that is constantly updated as external conditions and the state of the cell change.

3. Serves as “rails” for the transport of organelles and other large complexes within the cell.

Microfilaments and intermediate fibers.

Microfilaments built from F-actin penetrate the microvilli, forming nodes. These microfibers are held together by actin-binding proteins, the most important of which are fimbrin and villin. Calmodulin and myosin-like ATPase connect the outermost microfibers to the plasma membrane. .

A cell can change the set of synthesized cytoskeletal proteins depending on conditions, but this process is slow. The design of the cytoskeleton can quickly change even without the synthesis of new molecules, due to the polymerization and depolymerization of threads. In the cell, there is an ongoing exchange between the filaments and a solution of protein monomers in the cytoplasm. In many cells, approximately half of the actin and tubulin molecules are found as monomers in the cytoplasm and half are part of the microfilament filaments. The cell regulates the stability of the cytoskeletal filaments by attaching special proteins to them that change the rate of polymerization. The general principle of the functioning of the cytoskeleton is dynamic instability. For example, the shape of an erythrocyte in the form of a biconcave disk is supported by a near-membrane cytoskeleton made of fibers formed by the protein spectrin. Spectrin is associated with the protein ankerin (anchor), which connects to the cytoplasmic membrane protein responsible for the transport of anions (Cl - , HCO - 3). Defects in the proteins spectrin and ankyrin cause unusual red blood cell shapes. Such red blood cells are destroyed very quickly in the spleen. Diseases caused by such disorders are called hereditary spherocytosis or hereditary elliptocytosis.

Rice. Cytoskeleton of eukaryotes. Actin microfilaments are colored red, microtubules are colored green, cell nuclei are colored blue.

Keratin intermediate filaments in the cell.

Thus, eukaryotic cells have a kind of framework that, on the one hand, gives them a certain shape, and on the other, allows for the possibility of changing it, allowing cells to move and move their organelles from one part of the cell to another. In addition to the main components of the cytoskeleton, auxiliary proteins play an important role in its organization and functional integration. These proteins are responsible for attaching organelles to the cytoskeleton, ensuring directed movement of organelles, and coordinating the functions of the cytoskeleton.

Cytoskeletal disorders. The cytoskeleton is not a passive cellular structure that provides only cellular morphology. The role of the cytoskeleton in the motor function of cells, in the structure of the plasma membrane and, very importantly, in the receptor function of cells has been proven. It has been noted that changes in the cytoskeleton disrupt the process of release of the active substance (hormone, mediator, etc.), and also change the receptor function of target cells. As a result, the reception of various stimulating substances by cells (in particular, nerve cells) is disrupted. In addition, there is a disturbance in the motor activity of cells (for example, beta cells of the pancreas), resulting in insulin deficiency. Therefore, the manifestations of diabetes are quite constant in chromosomal syndromes (Turner, Klinefelter, Down, etc.). Other examples of diseases with cytoskeletal disorders are Duchenne muscular dystrophy and Becker muscular dystrophy. Both forms result from mutations in the gene encoding the dystrophin protein. Dystrophin, in turn, is part of the cytoskeleton. As a result, muscle biopsy reveals characteristic changes - muscle degeneration and fiber necrosis.

Organelles containing microtubule triplets

Centrioles. The centriole is cylindrical, 150 nm in diameter and 500 nm in length; the wall is formed by 9 triplets (triplet - consisting of three) microtubules. The centriole, the organizing center of the mitotic spindle, is involved in cell division. During the S phase of the cell cycle, the centrioles double. The new centriole formed is located at right angles to the original centriole. During mitosis, pairs of centrioles, each of which consists of an original and a newly formed one, diverge to the poles of the cell and participate in the formation of the mitotic spindle.

Basal body consists of 9 triplets of microtubules located at the base of the cilium or flagellum; serves as a matrix in organizing the axoneme.

Axoneme consists of 9 peripheral pairs of microtubules and two centrally located single microtubules. In each peripheral pair of microtubules, subfibril A and subfibril B are distinguished. So-called external and internal handles are associated with subfibril A. They contain the protein dynein, which has the ability to break down ATP. The axoneme is the main structural element of the cilium and flagellum.

Eyelash– cell outgrowth 5-10 µm long and 0.2 µm thick, containing an axoneme. Cilia are present in the epithelial cells of the airway and reproductive tract; move mucus with foreign particles and remnants of dead cells and create a fluid flow near the cell surface. Under the influence of tobacco smoke, the cilia of the airways are destroyed, which contributes to the retention of secretions in the bronchi.

Rice. Diagram of the cross section of a cilium. (From the book by B. Alberts et al. “Molecular Biology of the Cell”, volume 3.)

Scheme of the structure of a eukaryotic epithelial cell

Drawing by V.P. Andreeva

Intracellular space inside a cell - This is a zone of cytosol of intracellular contents unstructured by membranes. Cytosol is the liquid part of the cytoplasm and makes up about half the volume of the cell. Here proteins are synthesized, some of which are assembled on polysomes and remain in the cytosol. The cytosol communicates directly with the contents of the nucleus through large nuclear pores. In the nucleus, the processes of transcription of RNA from DNA take place, and both normal cellular and viral ones are synthesized during viral infections of cells. RNA from the nucleus is transported to the cytosol on polyribosomes for protein synthesis. Synthesized proteins under control chaperones(“catalysts” for the polypeptide chain to adopt a biologically significant conformation) are directed to special areas of the endoplasmic reticulum. Excess, damaged, and viral proteins are broken down in the cytosol by the so-called proteasomes. "Proteasomes" are multiprotease complexes consisting of 28 subunits. Proteasomes break down viral proteins into peptide antigens. The resulting antigen peptides bind to molecules of the major histocompatibility complex (MHC-I) and are sent for expression to the cell membrane. Antigen-MHC-I complexes located on the cell membrane are recognized by CD8 + T lymphocytes, which are activated and provide antiviral protection, as well as protection against cytosolic intracellular infections.

Extracellular space inside the cell - this is a space (zone, compartment) associated with the external extracellular environment and limited by membranes of structures and vesicles, including the Golgi apparatus, endoplasmic reticulum, lysosomes, endosomes, phagosomes and phagolysosomes. This zone is of particular importance in the structure of antigen-presenting cells, which include macrophages and dendritic cells (a variant of lymphocytes). Chains of molecules of the major histocompatibility complex (MHC-III) are synthesized on the ribosomes of the endoplasmic reticulum of these cells. The conformation of these molecules will only occur if they combine with peptides, formed as a result of proteolysis (cleavage) of proteins - antigens captured by the cell through endocytosis or phagocytosis. This occurs when phagolysosomes fuse with vesicles containing unconformed MHC-II molecules. With the participation of the peptide, the MHC-II molecule takes on the correct conformation, moves towards the membrane and is expressed on it. Complexes of antigen-peptides with MHC-II molecules are recognized by CD4 + T lymphocytes, which play a major role in protective reactions against extracellular infections.

Modern Cytology Concepts

Different cell types in different organisms are characterized by universal processes. These are signaling within the cell, regulation of the cell cycle, apoptosis, heat shock, degradation of intracellular proteins.

Apoptosis – a biological mechanism of cell death due to one or another signal from the outside or inside, which activates certain enzyme systems inside the cell that cause mitochondrial damage, DNA fragmentation and then fragmentation of the nucleus and cytoplasm of the cell. As a result, the cell disintegrates into membrane-enclosed apoptotic bodies, which can be phagocytosed by neighboring epithelial cells and macrophages. The contents of the dying cell do not enter the extracellular environment. Inflammation does not develop in the tissue. The life of multicellular organisms is impossible without programmed cell death, which regulates development, tissue homeostasis, cellular response to DNA damage and aging.

Heat shock

Heat shock can be caused not only by too high, but also by too low a temperature, poisons and many other influences, for example, a disruption in the circadian activity cycle. Under the influence of these factors, proteins with an “incorrect” tertiary structure appear in the cell. Many heat shock proteins help bring denatured or misfolded proteins into solution and refold them.

The heat shock reaction is accompanied by the cessation of the synthesis of proteins common to the cell and the accelerated synthesis of various protective proteins. These proteins protect DNA, messenger RNA, ribosome precursors, and other structures important for the cell from damage. The heat shock response is unusually ancient and conservative. Some heat shock proteins show homology in bacteria and humans.

Ubiquitin protein molecules are attached to the N-terminus of damaged, worn-out, unfinished and functionally inactive proteins, making them a target for enzymes of the protease class. The ubiquitin-associated protein is degraded in special multicomponent complexes called proteasomes. Ubiquitin is an example of a heat shock protein that functions in cells under normal conditions. In some cells, up to 30% of abnormal proteins are synthesized. The Nobel Prize in Chemistry was awarded in 2004 for the discovery of the role of ubiquitin in protein degradation.

Chaperones(from English letters - an elderly lady accompanying a young girl at balls) - a family of specialized intracellular proteins that ensure rapid and correct folding of newly synthesized protein molecules.

In addition, other chaperone proteins are known. For example, the chaperone HSP 70. Its synthesis is activated under many stresses, in particular during heat shock (hence the name Heart shook protein 70 - heat shock protein). The number 70 means molecular weight in kilodaltons. The main function of this protein is to prevent denaturation of other proteins when temperature increases. Chaperones are one of the most vital proteins of all living things. They arose at the earliest stages of evolution, perhaps even before the division of organisms into prokaryotes and eukaryotes

Transmission of an external signal into the cell

Cells cannot decide for themselves what the body needs. They must receive a signal from the outside and only after that intracellular regulation will be involved in maintaining the necessary processes. Famous biochemists William Elliot and Daphne Elliot give an analogy with sailing. “Each ship is an organizational unit “cage”, where order and discipline are maintained, all mechanisms work in an orderly manner, etc. At the same time, the goals and sailing routes for ships are determined by external signals (hormones) of the top management (endocrine glands and brain).

The cell usually receives a signal about the “state of affairs” around it using receptors. N.N. Mushkambarov and S.L. Kuznetsov identifies several mechanisms of action of signaling substances.

1) The substance interacts with the plasma membrane receptor, which induces signal transmission into the cell and in this case occurs chemical modification(phosphorylation, dephosphorylation) of certain proteins. (The phosphoryl group carries a strong negative charge, which contributes to a change in the conformation of the protein molecule.)

2) The substance interacts with the plasma membrane receptor, which is also an ion channel that opens when the regulator binds.

3) The extracellular regulator penetrates into the target cell, binds to a cytoplasmic or nuclear receptor protein and then acts as transcriptional factor that affects the expression of certain genes. This is how hormones of a steroid nature work (for example, male and female sex hormones).

Prostaglandins and NO (nitric oxide) sometimes act as signaling molecules. They penetrate the target cell and affect the activity of regulatory enzymes. The end result is modification of certain proteins.

The most commonly used mechanism is the first type. At the same time, the specific ways of its implementation are very diverse.

Signaling within the cell

Water-soluble signaling molecules, including known neurotransmitters, peptide hormones and growth factors, attach to specific protein receptors on the surface of target cells. Surface receptors bind a signaling molecule (ligand) with great affinity, and this extracellular event generates an intracellular signal that changes cell behavior.

Receptors are integral membrane proteins.

There are many signaling pathways starting from the membrane receptor.

(Changes in membrane receptors are accompanied by the occurrence of various diseases. For example, a defect in the receptor for the male sex hormone testosterone leads to the fact that individuals with the male genotype (2A + XY) look like females; all mammals that were not exposed to testosterone during the embryonic period develop along the female pathway. Mutant males have normal testes that produce testosterone, but the tissues of these males do not respond to the hormone due to defective receptors. As a result, such males develop all the secondary sexual characteristics of females and their testes do not descend into the scrotum, but remain in abdominal cavity. This syndrome (testicular feminization or Morris syndrome) occurs in mice, rats, cattle, and also in humans. Although only the gene that codes for the testosterone receptor is changed, all the different types of cells that normally respond to the hormone are affected. Thus, a single external signal can turn on different sets of genes in different types of cells.

The vast majority of surface receptors for hydrophilic signaling molecules, having bound a ligand on the outer side of the membrane, undergo a conformational change. This change creates intracellular signal, changing the behavior of the target cell. Intracellular signaling molecules are often called second intermediaries(messengers, English messenger - messenger), considering the extracellular ligand the “first messenger”. Secondary (intracellular) messengers include cyclic adenosine monophosphate (cAMP), cyclic guanosine 3΄,5΄ - monophosphate (cGMP), calcium cations, inositol 1,4,5-triphosphate, diacylglycerol. In addition, signaling pathways mediated by proteins, lipids, including free fatty acids, nitric oxide (NO), as well as pathways that do not contain a second messenger are known. An example of the latter option is the effect of γ-interferon on the transcription of certain genes, with an antiviral focus. Intracellular signaling pathways regulating cellular activity are very complex, not fully understood, and many discoveries are still ahead. Suffice it to say that the intracellular signaling pathway involving insulin, despite many years of research, has not yet been deciphered.

-A set of thread-like protein structures - microtubules and microfilaments that make up the musculoskeletal system of the cell.

The cytoskeleton is a highly dynamic cytoplasmic system. Many cytoskeletal structures can easily be destroyed and reappear, changing their location or morphology. These cytoskeletal features are based on polymerization-depolymerization reactions of the main structural cytoskeletal proteins and their interaction with other proteins, both structural and regulatory.

Only eukaryotic cells have a cytoskeleton; prokaryotic (bacterial) cells do not have it, which is an important difference between these two types of cells. The cytoskeleton gives the cell a certain shape even in the absence of a rigid cell wall. It organizes the movement of organelles in the cytoplasm (the so-called flow of protoplasm), which underlies amoeboid movement. The cytoskeleton is easily rebuilt, providing, if necessary, a change in cell shape. The ability of cells to change shape determines the movement of cell layers in the early stages of embryonic development. During cell division (mitosis), the cytoskeleton “disassembles” (dissociates), and in daughter cells its self-assembly occurs again.

The functions of the cytoskeleton are diverse. It helps maintain cell shape and carries out all types of cellular movements. In addition, the cytoskeleton can take part in the regulation of the metabolic activity of the cell.

The cytoskeleton is formed by proteins. In the cytoskeleton, several main systems are distinguished, named either by the main structural elements visible during electron microscopic studies (Microfilaments, intermediate filaments, microtubules), or by the main proteins included in their composition (actin-myosin system, keratins, tubulin-dynein system ).

Intermediate filaments are the least understood structure among the major components of the cytoskeleton with respect to their assembly, dynamics, and function. Their properties and dynamics are very different from those of both microtubules and actin filaments. The functions of intermediate filaments still remain in the realm of hypotheses.

Cytoplasmic intermediate filaments are found in the vast majority of ukaryotic cells, both in vertebrates and invertebrates, and in higher plants. Rare examples of animal cells in which intermediate filaments are not found cannot be considered definitive, since intermediate filament proteins can form unusual structures.

Morphological microtubules are hollow cylinders with a diameter of about 25 nm with a wall thickness of about 5 nm. The cylinder wall consists of protofilaments - linear tubulin polymers with longitudinally oriented heterodimers. As part of microtubules, protofilaments run along their long axis with a slight shift relative to each other, so that the tubulin subunits form a three-start helix. The microtubules of most animals contain 13 protofilaments.

Actin filaments play a key role in the contractile apparatus of muscle and non-muscle cells, and also take part in many other cellular processes, such as motility, maintaining cell shape, cytokinesis

Actin filaments or fibrillar actin (F-actin) are thin fibrils with a diameter of 6-8 nm. They are the result of polymerization of globular actin - G-actin. In a cell, actin filaments, with the help of other proteins, can form many different structures.

Components of the musculoskeletal system of the cell. Only eukaryotic cells have a cytoskeleton; prokaryotic (bacterial) cells do not have it, which is an important difference between these two types of cells. The cytoskeleton gives the cell a certain shape even in the absence of a rigid cell wall. It organizes the movement of organelles in the cytoplasm (the so-called flow of protoplasm), which underlies amoeboid movement. The cytoskeleton is easily rebuilt, providing, if necessary, a change in cell shape. The ability of cells to change shape determines the movement of cell layers in the early stages embryonic development . During cell division (mitosis) the cytoskeleton “disassembles” (dissociates), and its self-assembly occurs again in the daughter cells.

The cytoskeleton performs three main functions.

3. Serves as “rails” for the transport of organelles and other large complexes within the cell.
24. The role of the immunocytochemistry method in the study of the cytoskeleton. Features of the organization of the cytoskeleton in muscle cells.

The cytoplasm of eukaryotic cells is permeated by a three-dimensional network of protein threads (filaments), called the cytoskeleton. Depending on their diameter, filaments are divided into three groups: microfilaments (6-8 nm), intermediate fibers (about 10 nm) and microtubules (about 25 nm). All these fibers are polymers consisting of subunits of special globular proteins.

B. Intermediate fiber proteins

The structural elements of intermediate fibers are proteins belonging to five related families and exhibiting a high degree of cellular specificity. Typical representatives of these proteins are cytokeratins, desmin, vimentin, glia fibrillary acidic protein [GFAP] and neurofilament. All of these proteins have a basic rod structure in the central part, which is called a supercoiled α-helix. Such dimers associate antiparallel to form a tetramer. The aggregation of tetramers in a head-to-head manner produces a protofilament. Eight protofilaments form the intermediate fiber.

In contrast to microfilaments and microtubules, free intermediate fiber monomers are hardly found in the cytoplasm. Their polymerization leads to the formation of stable non-polar polymer molecules.

V. Tubulin

Microtubules are built from the globular protein tubulin, which is a dimer of α- and β-subunits. Tubulin monomers bind GTP, which is slowly hydrolyzed by GDP and GTP. Two types of proteins are associated with microtubules: structural translocator proteins.
25. The nucleus in plant and animal cells, structure, functions, relationship between the nucleus and the cytoplasm.

The nucleus was discovered by R. Brown in 1831. The significance of the nucleus is determined, first of all, by the presence of DNA in it.

Usually there is one nucleus in a cell. However, there are also multinucleated cells. The core diameter ranges from 5 to 20 µm; Due to its relatively large size, this cellular structure is clearly visible under a light microscope. The shape of the nucleus can be different: spherical, elongated, disc-shaped. The location of the nucleus in the cell is not constant. In a young plant cell, most often the nucleus is located closer to its center. In adult cells, the nucleus shifts to the periphery, which is associated with the appearance of a large central vacuole. The chemical composition of the nucleus is represented mainly by nucleic acids and proteins. Thus, isolated pea cell nuclei contain DNA - 14%, RNA - 12%, basic proteins - 22.6%, other proteins - 51.3%. The nuclear envelope consists of two membranes, each about 8 nm thick, separated by a perinuclear space 20-30 nm wide, which is filled with fluid.

The outer membrane on the surface has a complex folded structure, in places connected to the endoplasmic reticulum. The outer membrane contains a large number of ribosomes. The inner membrane may develop invaginations. The nuclear envelope has pores. There are from 10 to 100 pores with a diameter of about 20 nm per 1 μm2 of the nuclear membrane. Pores are complex formations; they have the shape of a watch glass, which is surrounded, as it were, by a rim. The rim consists of individual protein granules. In the center of the pore there is a central granule connected by threads to the rim granules. The pores of the nucleus are dynamic formations; they can open and close. In this way, regulation of the exchange between the nucleus and the cytoplasm can be carried out. The internal structure of the nucleus changes depending on its state. There are two periods of nuclear life: metabolic (between divisions) and the fission period. During the metabolic period, the nucleus also contains one or several spherical granules-nucleoli. The substance of the nucleolus consists of tightly intertwined threads - nucleonemes and contains about 80% protein, 10-15% RNA and some DNA. The nucleolus contains ribosomes. The nucleolus is formed on certain parts of the chromosome, called the nucleolar organizer, thus being a derivative of the chromosome. The main function of the nucleolus is that it synthesizes ribosomal RNA and assembles ribosomal subunits. Self-assembly of ribosomes subsequently occurs in the cytoplasm.

Kernel functions

The nucleus carries out two groups of general functions: one associated with the storage of genetic information itself, the other with its implementation, ensuring protein synthesis.
The first group includes processes associated with maintaining hereditary information in the form of an unchanged DNA structure. These processes are associated with the presence of so-called repair enzymes that eliminate spontaneous damage to the DNA molecule (break of one of the DNA chains, part of the radiation damage), which preserves the structure of DNA molecules practically unchanged over generations of cells or organisms. Further, reproduction or reduplication of DNA molecules occurs in the nucleus, which makes it possible for two cells to receive exactly the same volumes of genetic information, both qualitatively and quantitatively. Processes of change and recombination of genetic material occur in the nuclei, which is observed during meiosis (crossing over). Finally, nuclei are directly involved in the distribution of DNA molecules during cell division.
Another group of cellular processes ensured by the activity of the nucleus is the creation of the protein synthesis apparatus itself. This is not only the synthesis, transcription on DNA molecules of various messenger RNAs and ribosomal RNAs. In the nucleus of eukaryotes, the formation of ribosomal subunits also occurs by complexing ribosomal RNA synthesized in the nucleolus with ribosomal proteins, which are synthesized in the cytoplasm and transferred to the nucleus.

Interaction of nucleus and cytoplasm in development

Cytoplasm plays an important role in the implementation of hereditary information and the formation of certain characteristics of the body. The main part of the cytoplasm enters the zygote with the egg. Certain areas of the cytoplasm of the egg may contain factors that determine the fate of certain differentiating cells. Gene activity depends on the cytoplasm. In the cytoplasm of the egg there is an activator of DNA synthesis and a repressor of RNA synthesis, which act independently of each other. If nuclei from brain cells of an adult frog are transplanted into a mature oocyte, then RNA is synthesized in them and DNA is not synthesized. Some cytoplasmic organelles, which have their own protein synthesis system (mitochondria), can influence the development of certain traits. Inheritance of traits through the cytoplasm - cytoplasmic or extranuclear heredity. During development, a complex interaction between the nucleus and cytoplasm takes place. In plants and especially animals, the main role in the formation of the characteristics of the organism belongs to the nucleus.

In experiments on interspecific androgenesis with silkworms, B.L. Astaurov convincingly demonstrated the dominant role of the nucleus in the process of individual development. He obtained interspecific hybrids by inseminating wild silkworm eggs with domestic silkworm sperm and vice versa. The female nuclei were inactivated by heat shock (by heating). In this case, the nuclei of two sperm participated in the fertilization of the egg. Nuclear-cytoplasmic hybrids received cytoplasm from one species and nuclei from another. The developed individuals were always male and, according to all the studied characteristics, were similar to the species from which they received the nuclei.

However, the cytoplasm plays a very important role in the implementation of hereditary information and the formation of certain characteristics of the organism. It is known that the main part of the cytoplasm enters the zygote with the egg. The cytoplasm of the egg differs from the cytoplasm of somatic cells in the large variety of proteins, RNA and other types of molecules synthesized during oogenesis. Boveri, Conklin, Driesch and others have long pointed out that certain areas of the cytoplasm of the egg may contain factors that determine the fate of certain differentiating cells.
26. Spatial organization of intraphase chromosomes inside the nucleus, euchromatin, heterochromatin.

And the interphase nucleus as a whole, the spatial organization of chromosomes

As a result of the development of methods for obtaining preparations of metaphase chromosomes, it became possible to analyze the number of chromosomes and describe their morphology and size. True, the physical dimensions and morphology of the chromosome in cytological preparations are very

depended on the stage of mitosis and the conditions for preparing the corresponding cytological preparation. Many years passed before it was shown that the size and morphology of chromosomes in the G2 stage of the cell cycle differed little from actual mitotic chromosomes.

The development of cellular and molecular biology has made it possible to visualize individual chromosomes in the interphase nucleus, their

three-dimensional microscopy and even identification of individual areas. Research in this direction was carried out both on fixed and living cells. It turned out that long prophase and prometaphase chromosomes, well known to biologists from cytological preparations, are simply the result of stretching of chromosomes in the process of spreading them on glass. In later stages of mitosis, chromosomes resist stretching more effectively and maintain their natural size. Experiments on living cells use a variety of fluorescent labeling methods and 4D microscopy. Thus, for lifetime observations of individual chromosomes, a fluorescent label was first introduced into the DNA of all chromosomes cultured in cells, and then the nutrient medium was replaced with

Free from fluorochromes, the cells were allowed to go through several cell cycles. As a result, cells appeared in the culture.

This term refers to a complex of nuclear DNA with proteins (histones, non-histone proteins).

There are hetero- and euchromatin.

Heterochromatin - transcriptionally inactive, condensed chromatin of the intarphase nucleus. It is located mainly along the periphery of the nucleus and around the nucleoli. A typical example of heterochromatin is the Barr body.

Although historically less well understood than euchromatin, new discoveries suggest that heterochromatin plays a critical role in the organization and proper functioning of genomes from yeast to humans. Its potential importance is highlighted by the fact that 96% of the mammalian genome consists of noncoding and repetitive sequences. New discoveries regarding the mechanisms of heterochromatin formation have revealed unexpected things

Euchromatin – a transcriptionally active and less condensed part of chromatin, localized in lighter areas of the nucleus between heterochromatin, rich in genes. A region of the chromosome that is poorly stained or not stained at all. Diffuse in interphase. Actively transcribed. Euchromatin is characterized by less compaction of DNA compared to heterochromatin, and, as already mentioned, actively expressed genes are mainly localized in it.

Euchromatin, or “active” chromatin, consists primarily of coding sequences that make up only a small fraction (less than 4%) of the mammalian genome.

Thus, the collective term “euchromatin” most likely refers to the complex state(s) of chromatin, encompassing a dynamic and complex mixture of mechanisms that interact closely with each other and with the chromatin fibril to effect the transcription of functional RNAs.
27. Chemical composition of chromosomes: DNA and proteins.

Chemical and structural organization of chromosomes
Chromosomes, in interaction with extrachromosomal mechanisms, provide:
1) storage of hereditary information;
2) use of this information to create and maintain cellular organization;
3) regulation of reading hereditary information;
4) self-duplication of genetic material;
5) transfer it from the mother cell to the daughter cells.
The main chemical components of chromosomes are represented by DNA, basic (histone) and acidic (non-histone) proteins, which account for 40% and about 20%, respectively. Chromosomes contain RNA, lipids, polysaccharides, and metal ions.
DNA molecules encode hereditary information, which makes them the leading functional component of chromosomes.
The DNA of eukaryotic cells is represented by the following fractions:
1) unique nucleotide sequences;
2) repetitions of a certain sequence;
3) repetitions.
Chromosome elements - centromeres and chromatids

Histones are represented by five main fractions and play structural and regulatory roles. The number of non-histone protein fractions exceeds 100. Among them are enzymes for RNA synthesis and processing, DNA reduplication and repair. Acidic proteins of chromosomes also perform structural and regulatory roles. Chromosome RNA is represented partly by transcription products that have not yet left the site of synthesis. Some fractions have a regulatory function. The regulatory function of chromosome components is to “prohibit” or “permit” the reading of information from a DNA molecule.

The elementary structure of a chromosome, distinguishable using an electron microscope, is a thread with a diameter of 10-13 nm, which is a complex of DNA and histone proteins (nucleohistone). The thickness of the thread depends on the bodies located along its length - nucleosomes. The diameter of the internucleosomal regions is less than 1.5 nm, which coincides with the thickness of the DNA biohelix. The cores of the bodies are formed by 8 histone molecules of 4 different types - H2a, H2b, H3 and H4. They serve as the basis on which DNA fragments approximately 200 nucleotide pairs long are “twisted.” Histone H1 “stitches” DNA turns together. The functional significance of nucleosomes is unclear. There is evidence that transcribed DNA fragments encoding rRNA do not have a nucleosomal structure. For other genes, there are indications that the nucleosomal structure is lost during transcription. The twisting of DNA molecules onto histone bodies reduces the length of the DNA biohelix by 7 times, i.e., it serves the purpose of packaging hereditary material.
Data from microscopic and electron microscopic studies of chromatin and mitotic chromosomes provide the following diagram of the structural organization of the chromosome. A DNA double helix with a diameter of 1.5 nm is converted into a nucleohistone complex with a nucleosomal structure as a result of twisting and protein attachment. It looks like a thread with a diameter of 10-13 nm. With further twisting and attachment of proteins, a thread with a diameter of 20-25 nm appears. It is detected using an electron microscope in both interphase and mitotic chromosomes. As a result of further twisting of this thread, which occurs repeatedly and is supplemented by folding, mitotic chromosomes are formed. This scheme is preliminary; it combines the areas of interest of a cytogeneticist in medical genetic consultation (micromorphology of mitotic chromosomes) and a specialist in the functional organization of chromosomes at the ultrastructural and molecular levels.
Reorganization of the nucleohistone strand with the formation of a more compact structure is called helicalization (condensation), the opposite process to that described is despiralization (decondensation). Thanks to spiralization, dense packing of hereditary material is achieved, which is important when moving chromosomes during mitosis. The following figures indicate the packing density. The nucleus of a somatic diploid human cell contains about 6 pg of DNA, which corresponds to a nucleohistone thread almost 2 m long. The total length of all chromosomes of a human cell in metaphase of mitosis is 150 μm. A biospiral of 100 g of human DNA, if stretched into one strand, will cover a distance of 2.5 X 1010 km, which is more than 100 times the distance from the Earth to the Sun.
The presented information about the laying of the nucleohistone thread is consistent with genetic concepts about the continuity and linearity of the arrangement of genes along the length of chromosomes. They correspond to the assumption that each chromosome contains one DNA double helix. In special, so-called polytene chromosomes of insect cells, several double helices of DNA are simultaneously present. Since they are stacked side by side, this design is compatible with the principle of linear and continuous arrangement of genes.
For the study of karyotype, mitotic metaphase chromosomes are of particular importance. They are formed by two chromatids. The latter are daughter chromosomes, which will separate into daughter cells during mitosis. The chromatids are connected in the region of the primary constriction (centromere, kinetochore), to which the filaments of the spindle are attached. The fragments into which the primary constriction molds the chromosome are called arms, and the ends of the chromosome are called telomeres. Depending on the position of the primary constriction, metacentric (equal-armed), submetacentric (moderately unequal-armed), acrocentric and subacrocentric (unequal-armed expression) chromosomes are distinguished. In humans, chromosomes 1 and 3 pairs, the X chromosome are metacentric, pairs 2.6-12, 16-20 are submetacentric, pairs 4-5, 13-15, 21-22 are acrocentric and subacrocentric, and the Y chromosome. When using some methods of preparing drugs, hemi-chromatids are visible in chromosomes, but the question of their presence in the cell cannot be considered resolved. Perhaps they are the result of exposure of the chromosome substance to the material used to prepare the drug. Some chromosomes have secondary constrictions. They arise in areas of incomplete chromatin condensation, for example, in the pericentromeric regions of the long arm of human chromosomes 1, 9 and 16. Secondary constrictions separate the terminal sections of the short arms of 13-15, 21-22 human chromosomes in the form of satellites. In the area of secondary constrictions of some chromosomes, nucleolar organizers are located. They contain genes encoding rRNA and serve as the site for the formation of the nucleolus. The described structural features are used to identify chromosomes.
Although interphase chromosomes are generally characterized by a despiralized state, the degree of spiralization of individual fragments varies. There are euchromatin, structural heterochromatin and facultative heterochromatin. Euchromatin is formed by sections of chromosomes that despiral at the end of mitosis. In interphase nuclei, these are weakly stained filamentous structures. Structural genes are located in the euchromatin region. Structural heterochromatin is characterized by a highly coiled state, which persists throughout the entire mitotic cycle. It occupies constant regions similar in homologous chromosomes. Usually these are fragments adjacent to the centromere region, as well as located at the free ends (telomeres) of chromosomes. This type of heterochromatin apparently does not contain structural genes and its function is not clear. Each chromosome has its own order of arrangement of eu- and heterochromatic regions. This is used to identify individual chromosomes in human cytogenetic studies. Facultative heterochromatin is formed by the spiralization of one of two homologous chromosomes. A typical example is the genetically inactive X chromosome of somatic cells of female mammals and humans (sex chromatin bodies). The functional role of facultative heterochromatization is to compensate (reduce) the dosage of certain genes.