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Morphological structure of viruses. Morphology of viruses

Morphological structure of viruses. Morphology of viruses

The morphology and structure of viruses are studied using an electron microscope, since their size is small and comparable to the thickness of the bacterial shell. The shape of the virions can be different: rod-shaped (tobacco mosaic virus), bullet-shaped (rabies virus), spherical (polio viruses, HIV), in the form of a spermatozoon (many bacteriophages).

The size of viruses is determined using electron microscopy, ultrafiltration through filters with a known pore diameter, ultracentrifugation. One of the smallest viruses is the poliomyelitis virus (about 20 nm), the largest is smallpox (about 350 nm).

There are simply arranged (for example, the polio virus) and complexly arranged (for example, influenza viruses, measles) viruses. In simply arranged viruses, the nucleic acid is associated with a protein shell called a capsid (from Latin capsa - case). The capsid consists of repeating morphological subunits - capsomeres. Nucleic acid and capsid, interacting with each other, form the nucleocapsid. In complex viruses, the capsid is surrounded by an additional lipoprotein shell - the supercapsid (a derivative of the membrane structures of the host cell), which has "spikes". Virions are characterized by a helical, cubic and complex type of capsid symmetry. The spiral type of symmetry is due to the helical structure of the nucleocapsid, the cubic type of symmetry is due to the formation of an isometric hollow body from the capsid containing the viral nucleic acid.

Capsid and supercapsid protect virions from environmental influences, determine selective interaction (adsorption) with cells, and determine the antigenic and immunogenic properties of virions. The internal structures of viruses are called the core. In virology, the following taxonomic categories are used: family (name ends with viridae), subfamily (name ends with virinae), genus (name ends with virus).

However, the names of genera and especially subfamilies are not formulated for all viruses. The type of virus has not received a binomial name, like bacteria.

The classification of viruses is based on the following categories:

§ type of nucleic acid (DNA or RNA), its structure, number of strands (one or two),

§ features of the reproduction of the viral genome;

§ the size and morphology of virions, the number of capsomeres and the type of symmetry;

§ the presence of a supercapsid;

§ sensitivity to ether and deoxycholate;

§ place of reproduction in the cell;

§ antigenic properties, etc.

Viruses infect vertebrates and invertebrates, as well as plants and bacteria. Being the main causative agents of human infectious diseases, viruses also participate in the processes of carcinogenesis, can be transmitted in various ways, including through the placenta (rubella virus, cytomegalovirus, etc.), affecting the human fetus. They can lead to post-infectious complications - the development of myocarditis, pancreatitis, immunodeficiency, etc.

In addition to ordinary viruses, there are also so-called non-canonical viruses - prions - protein infectious particles, which are agents of a protein nature, having the form of fibrils with a size of 10.20x100.200 nm. Prions, apparently, are both inducers and products of an autonomous human or animal gene and cause encephalopathy in them under conditions of a slow viral infection (Creutzfeldt.Jakob disease, kuru, etc.). Other unusual agents that are close to viruses are viroids, small, protein-free circular, supercoiled RNA molecules that cause disease in plants.

Chapter 3

PHYSIOLOGY OF MICROORGANISMS

The physiology of microorganisms studies the vital activity of microbial cells, the processes of their nutrition, respiration, growth, reproduction, patterns of interaction with the environment.

Subject of study medical microbiology is the physiology of pathogenic and opportunistic microorganisms capable of causing human diseases. Elucidation of the physiology of these microorganisms is important for making a microbiological diagnosis, understanding the pathogenesis, treating and preventing infectious diseases, regulating the relationship between a person and the environment, etc.

The chemical composition of bacteria

The composition of microorganisms includes water, proteins, nucleic acids, carbohydrates, lipids, minerals.

Water is the main component of a bacterial cell, accounting for about 80% of its mass. It is in a free or bound state with the structural elements of the cell. In spores, the amount of water decreases to 18.20%. Water is a solvent for many substances, and also plays a mechanical role in providing turgor. During plasmolysis - the loss of water by the cell in a hypertonic solution - the exfoliation of protoplasm from the cell membrane occurs. Removal of water from the cell, drying suspend the processes of metabolism. Most microorganisms tolerate drying well. With a lack of water, microorganisms do not multiply. Drying in a vacuum from a frozen state (lyophilization) stops reproduction and promotes long-term preservation of microbial species.

Proteins (40.80% dry weight) determine the most important biological properties of bacteria and usually consist of combinations of 20 amino acids. Bacteria contain diaminopimelic acid (DAP), which is absent in human and animal cells. Bacteria contain more than 2000 different proteins that are in structural components and are involved in metabolic processes. Most proteins have enzymatic activity. Proteins of a bacterial cell determine the antigenicity and immunogenicity, virulence, and species of bacteria.

The nucleic acids of bacteria perform functions similar to the nucleic acids of eukaryotic cells: the DNA molecule in the form of a chromosome is responsible for heredity, ribonucleic acids(informational, or matrix, transport and ribosomal) are involved in protein biosynthesis.

Bacteria can be characterized (taxonomically) by the content of the sum of guanine and cytosine (GC) in mole percent (M%) of the total number of DNA bases. A more accurate characterization of microorganisms is the hybridization of their DNA. The basis of the hybridization method

DNA - the ability of denatured (single-stranded) DNA to renature, i.e. combine with a complementary strand of DNA to form a double-stranded DNA molecule.

Bacterial carbohydrates are represented simple substances(mono- and disaccharides) and complex compounds. Polysaccharides are often found in capsules. Some intracellular polysaccharides (starch, glycogen, etc.) are reserve nutrients.

Lipids are mainly part of the cytoplasmic membrane and its derivatives, as well as the bacterial cell wall, for example, the outer membrane, where, in addition to the biomolecular layer of lipids, there is LPS. Lipids can act as reserve nutrients in the cytoplasm. Bacterial lipids are represented by phospholipids, fatty acids and glycerides. Nai large quantity lipids (up to 40%) contain Mycobacterium tuberculosis.

Bacterial minerals are found in the ash after the cells are burned. Phosphorus, potassium, sodium, sulfur, iron, calcium, magnesium, as well as trace elements (zinc, copper, cobalt, barium, manganese, etc.) are detected in large quantities. They are involved in the regulation of osmotic pressure, pH, redox potential , activate enzymes, are part of enzymes, vitamins and structural components of microbial cells.

Bacteria nutrition

Features of the nutrition of a bacterial cell consist in the intake of nutrient substrates inside through its entire surface, as well as in the high rate of metabolic processes and adaptation to changing environmental conditions.

Food types. The wide distribution of bacteria is facilitated by a variety of types of food. Microorganisms need carbohydrate, nitrogen, sulfur, phosphorus, potassium and other elements. Depending on the carbon sources for nutrition, bacteria are divided into autotrophs (from the Greek autos - itself, trophe - food), which use carbon dioxide CO 2 and other inorganic compounds to build their cells, and heterotrophs (from the Greek heteros - another, trophe - food), feeding on ready-made organic compounds. Autotrophic bacteria are nitrifying bacteria found in the soil; sulfur bacteria living in water with hydrogen sulfide; iron bacteria living in water with ferrous iron, etc.

Depending on the oxidizable substrate, called an electron or hydrogen donor, microorganisms are divided into two groups. Microorganisms that use inorganic compounds as hydrogen donors are called lithotrophic (from the Greek lithos - stone), and microorganisms that use organic compounds as hydrogen donors are called organotrophs.

Considering the source of energy, phototrophs are distinguished among bacteria, i.e. photosynthetic (for example, blue-green algae that use the energy of light), and chemotrophs that need chemical energy sources.

growth factors. Microorganisms for growth on nutrient media require certain additional components, which are called growth factors. Growth factors are compounds necessary for microorganisms that they themselves cannot synthesize, so they must be added to nutrient media. Among the growth factors, there are: amino acids necessary for building proteins; purines and pyrimidines, which are required for the formation of nucleic acids; vitamins that are part of some enzymes. To denote the relationship of microorganisms to growth factors, the terms "auxotrophs" and "prototrophs" are used. Auxotrophs need one or more growth factors; prototrophs can synthesize the compounds necessary for growth themselves. They are able to synthesize components from glucose and ammonium salts.

Power mechanisms. Admission various substances into a bacterial cell depends on the size and solubility of their molecules in lipids or water, the pH of the medium, the concentration of substances, various factors of membrane permeability, etc. The cell wall allows small molecules and ions to pass through, retaining macromolecules weighing more than 600 D. The main regulator of the entry of substances into the cell is cytoplasmic membrane. It is conditionally possible to distinguish four mechanisms for the penetration of nutrients into a bacterial cell: these are simple diffusion, facilitated diffusion, active transport, and group translocation. The simplest mechanism for the entry of substances into the cell is simple diffusion, in which the movement of substances occurs due to the difference in their concentration on both sides of the cytoplasmic membrane. Substances pass through the lipid part of the cytoplasmic membrane (organic molecules, drugs) and less often through water-filled channels in the cytoplasmic membrane. Passive diffusion is carried out without energy consumption.

Facilitated diffusion also occurs as a result of the difference in the concentration of substances on both sides of the cytoplasmic membrane. However, this process is carried out with the help of carrier molecules localized in the cytoplasmic membrane and possessing specificity. Each carrier transports the corresponding substance across the membrane or transfers it to another component of the cytoplasmic membrane - the carrier itself.

Carrier proteins can be permeases, the site of synthesis of which is the cytoplasmic membrane. Facilitated diffusion proceeds without energy expenditure, substances move from a higher concentration to a lower one.

Active transport occurs with the help of permeases and is aimed at the transfer of substances from a lower concentration to a higher one, i.e. as if against the current, therefore, this process is accompanied by the expenditure of metabolic energy (ATP), which is formed as a result of redox reactions in the cell.

Transfer (translocation) of groups is similar to active transport, differing in that the transferred molecule is modified in the process of transfer, for example, it is phosphorylated. The exit of substances from the cell is carried out due to diffusion and with the participation of transport systems-enzymes of bacteria. Enzymes recognize their respective metabolites (substrates), interact with them, and accelerate chemical reactions. Enzymes are proteins involved in the processes of anabolism (synthesis) and catabolism (decay), i.e. metabolism. Many enzymes are interconnected with the structures of the microbial cell. For example, in the cytoplasmic membrane there are redox enzymes involved in respiration and cell division; enzymes that provide cell nutrition, etc. Redox enzymes of the cytoplasmic membrane and its derivatives provide energy for intensive processes of biosynthesis of various structures, including the cell wall. Enzymes associated with cell division and autolysis are found in the cell wall. The so-called endoenzymes catalyze the metabolism that takes place inside the cell.

Exoenzymes are secreted by the cell into environment, splitting the macromolecules of nutrient substrates to simple connections absorbed by the cell as sources of energy, carbon, etc. Some exoenzymes (penicillinase, etc.) inactivate antibiotics, performing a protective function.

There are constitutive and inducible enzymes. Constitutive enzymes include enzymes that are synthesized by the cell continuously, regardless of the presence of substrates in the nutrient medium. Inducible (adaptive) enzymes are synthesized by a bacterial cell only if there is a substrate for this enzyme in the medium. For example, p-galactosidase is practically not formed by Escherichia coli on a medium with glucose, but its synthesis increases sharply when grown on a medium with lactose or another p-galactosidosis.

Some enzymes (the so-called aggression enzymes) destroy tissue and cells, causing a wide distribution of microorganisms and their toxins in the infected tissue. These enzymes include hyaluronidase, collagenase, deoxyribonuclease, neuraminidase, lecitovitellase, etc. Thus, streptococcal hyaluronidase, splitting hyaluronic acid connective tissue, promotes the spread of streptococci and their toxins.

More than 2000 enzymes are known. They are combined into six classes: oxidoreductases - redox enzymes (they include dehydrogenases, oxidases, etc.); transferases that transfer individual radicals and atoms from one compound to another; hydrolases that accelerate hydrolysis reactions, i.e. splitting substances into simpler ones with the addition of water molecules (esterases, phosphatase, glycosidase, etc.); lyases that cleave off chemical groups from substrates in a non-hydrolytic way (carboxylases, etc.); isomerases that convert organic compounds into their isomers (phosphohexoisomerase, etc.); ligases, or synthetases, accelerating the synthesis of complex compounds from simpler ones (asparagine synthetase, glutamine synthetase, etc.).

Differences in the enzymatic composition are used to identify microorganisms, as they determine their various biochemical properties: saccharolytic (breakdown of sugars), proteolytic (decomposition of proteins) and others, identified by the final degradation products (formation of alkalis, acids, hydrogen sulfide, ammonia, etc.) .

Enzymes of microorganisms are used in genetic engineering (restriction enzymes, ligases, etc.) to obtain biologically active compounds, acetic, lactic, citric and other acids, lactic acid products, in winemaking and other industries. Enzymes are used as bioadditives in washing powders (Oka, etc.) to destroy protein pollution.

Breath bacteria

Respiration, or biological oxidation, is based on redox reactions that take place with the formation of ATP, the universal accumulator of chemical energy. Energy is necessary for a microbial cell for its vital activity. When breathing, the processes of oxidation and reduction occur: oxidation - the return of hydrogen or electrons by donors (molecules or atoms); reduction - the addition of hydrogen or electrons to an acceptor. The acceptor of hydrogen or electrons can be molecular oxygen (such respiration is called aerobic) or nitrate, sulfate, fumarate (such respiration is called anaerobic - nitrate, sulfate, fumarate). Anaerobiosis (from the Greek aeg - air + bios - life) - vital activity that occurs in the absence of free oxygen. If the donors and acceptors of hydrogen are organic compounds, then this process is called fermentation. During fermentation, the enzymatic breakdown of organic compounds, mainly carbohydrates, occurs under anaerobic conditions. Taking into account the final product of the breakdown of carbohydrates, alcohol, lactic acid, acetic acid and other types of fermentation are distinguished.

In relation to molecular oxygen, bacteria can be divided into three main groups: obligate, i.e. obligatory, aerobes, obligate anaerobes and facultative anaerobes.

Obligate aerobes can only grow in the presence of oxygen. Obligate anaerobes (clostridia of botulism, gas gangrene, tetanus, bacteroids, etc.) grow only in a medium without oxygen, which is toxic to them. In the presence of oxygen, bacteria form oxygen peroxide radicals, including hydrogen peroxide and superoxide oxygen anion, which are toxic to obligate anabolic bacteria because they do not form the corresponding inactivating enzymes. Aerobic bacteria inactivate hydrogen peroxide and superoxide anion with the corresponding enzymes (catalase, peroxidase and superoxide dismutase). Facultative anaerobes can grow in both the presence and absence of oxygen, since they are able to switch from respiration in the presence of molecular oxygen to fermentation in its absence. Facultative anaerobes are able to carry out anaerobic respiration, called nitrate: nitrate, which is an acceptor of hydrogen, is reduced to molecular nitrogen and ammonia. Among obligate anaerobes, aerotolerant bacteria are distinguished that persist in the presence of molecular oxygen, but do not use it.

For the cultivation of anaerobes in bacteriological laboratories, anaerostats are used - special containers in which air is replaced by a mixture of gases that do not contain oxygen. Air can be removed from nutrient media by boiling, using chemical oxygen adsorbents placed in anaerobic balloons or other containers with crops.

Growth and reproduction of bacteria

The vital activity of bacteria is characterized by growth - the formation of structural and functional components of the cell and the increase in the bacterial cell itself, as well as reproduction - self-reproduction, leading to an increase in the number of bacterial cells in the population.

Bacteria reproduce by binary fission in half, less often by budding.

Actinomycetes, like fungi, can reproduce by spores. Actinomycetes, being branching bacteriums, multiply by fragmentation of filamentous cells. Gram-positive bacteria divide by ingrowth of the synthesized division partitions into the cell, and gram-negative bacteria divide by constriction, as a result of the formation of dumbbell-shaped figures, from which two identical cells are formed.

Cell division is preceded by the replication of the bacterial chromosome according to a semi-conservative type (the double-stranded DNA chain opens and each strand is completed by a complementary strand), leading to the doubling of the DNA molecules of the bacterial nucleus - the nucleoid. Replication of chromosomal DNA is carried out from the starting point ori (from English, origin - the beginning).

The chromosome of a bacterial cell is connected in the region of ori with the cytoplasmic membrane. DNA replication is catalyzed by DNA polymerases. First, unwinding (despiralization) of the DNA double strand occurs, resulting in the formation of a replication fork (branched chains); one of the chains, being completed, binds nucleotides from the 5 "- to the 3"-end, the other is completed segment by segment.

DNA replication occurs in three stages: initiation, elongation, or chain growth, and termination. The two chromosomes formed as a result of replication diverge, which is facilitated by an increase in the size of the growing cell: the chromosomes attached to the cytoplasmic membrane or its derivatives (for example, mesosomes) move away from each other as the cell volume increases. Their final isolation ends with the formation of a constriction or division septum. Cells with a division septum diverge as a result of the action of autolytic enzymes that destroy the core of the division septum. In this case, autolysis can take place unevenly: dividing cells in one area remain connected by a part of the cell wall in the region of the division septum. Such cells are located at an angle to each other, which is typical for diphtheria corynebacteria.

Reproduction of bacteria in a liquid nutrient medium. Bacteria seeded in a certain, unchanging volume of the nutrient medium, multiplying, consume nutrients, which subsequently leads to the depletion of the nutrient medium and the cessation of bacterial growth. The cultivation of bacteria in such a system is called periodic cultivation, and the culture is called periodic. If the cultivation conditions are maintained by the continuous supply of fresh nutrient medium and the outflow of the same volume of culture fluid, then such cultivation is called continuous, and the culture is called continuous.

When growing bacteria on a liquid nutrient medium, near-bottom, diffuse, or surface (in the form of a film) culture growth is observed. The growth of a periodic culture of bacteria grown on a liquid nutrient medium is divided into several phases, or periods:

§ lag phase;

§ phase of logarithmic growth;

§ phase of stationary growth, or maximum concentration

§ bacteria;

§ phase of bacterial death.

These phases can be depicted graphically as segments of the bacterial reproduction curve, which reflects the dependence of the logarithm of the number of living cells on the time of their cultivation. Lag phase (from English, lag - delay) - the period between sowing bacteria and the start of reproduction. The average duration of the lag phase is 4.5 hours. Bacteria increase in size and prepare for division; the amount of nucleic acids, protein and other components increases. The phase of logarithmic (exponential) growth is a period of intensive division of bacteria.

Its duration is about 5.6 hours. Under optimal growth conditions, bacteria can divide every 20-40 minutes. During this phase, bacteria are the most vulnerable, which is explained by the high sensitivity of the metabolic components of a rapidly growing cell to inhibitors of protein synthesis, nucleic acids, etc. Then, the stationary growth phase begins, in which the number of viable cells remains unchanged, constituting the maximum level (M-concentration) . Its duration is expressed in hours and varies depending on the type of bacteria, their characteristics and cultivation. The process of bacterial growth is completed by the death phase, which is characterized by the death of bacteria under conditions of depletion of nutrient sources and the accumulation of bacterial metabolic products in it. Its duration varies from 10 hours to several weeks. The intensity of growth and reproduction of bacteria depends on many factors, including the optimal composition of the nutrient medium, redox potential, pH, temperature, etc.

Reproduction of bacteria on a dense nutrient medium. Bacteria growing on solid nutrient media form isolated round-shaped colonies with smooth or irregular edges (S- and R-shapes; see Chapter 5), of varying consistency and color, depending on the pigment of the bacteria.

Water-soluble pigments diffuse into the nutrient medium and stain it, for example, Pseudomonas aeruginosa (Pseudomonas aeruginosa) stains the medium blue. Another group of pigments is insoluble in water but soluble in organic solvents. Thus, the colonies of the "wonderful stick" have a blood-red pigment that is soluble in alcohol. And, finally, there are pigments that are insoluble neither in water nor in organic compounds.

The most common pigments among microorganisms are carotenes, xanthophylls, and melanins. Melanins are insoluble black, brown or red pigments synthesized from phenolic compounds. Melanins, along with catalase, superoxide cismutase, and peroxidases, protect microorganisms from the effects of toxic oxygen peroxide radicals. Many pigments have antimicrobial, antibiotic-like effects.

The appearance, shape, color, and other features of colonies on a dense nutrient medium can be taken into account when identifying bacteria, as well as selecting colonies to obtain pure cultures.

Under industrial conditions, when obtaining the biomass of microorganisms for the preparation of antibiotics, vaccines, diagnostics, eubiotics, the cultivation of bacteria and fungi is carried out in fermenters with strict observance of the optimal parameters for the growth and reproduction of cultures (see Chapter 6).

LECTURE No. 5.

VIROLOGY.

All viruses exist in two qualitatively different forms. Extracellular form - virion - includes all the constituent elements of a virus particle. Intracellular form - virus - can be represented by only one nucleic acid molecule, tk. Once in the cell, the virion breaks down into its constituent elements. At the same time, an intracellular virus is a self-replicating form that is incapable of division. On this basis, the definition of a virus implies a fundamental difference between cellular forms of existence (bacteria, fungi, protozoa) that reproduce by division and a replicating form that reproduces from a viral nucleic acid. But this is not limited to the distinguishing features of viruses from pro- and eukaryotes. The fundamental differences include:

1. the presence of one type of nucleic acid (DNA or RNA);

2. lack of cellular structure and protein-synthesizing systems;

3. the possibility of integration into the cellular genome and synchronous replication.

The shape of the virion can be very different (rod-shaped, ellipsoid, spherical, filamentous, in the form of a spermatozoon), which is one of the signs of the taxonomic affiliation of this virus.

The dimensions of viruses are so small that they are comparable to the thickness of the cell membrane. The smallest (parvoviruses) are 18 nm in size, and the largest (variola virus) are about 400 nm.

The classification of viruses is based on the type of nucleic acid that forms the genome, which made it possible to distinguish two sub-kingdoms:

riboviruses- RNA-containing or RNA viruses;

deoxyriboviruses- DNA-containing or DNA viruses.

Subkingdoms are divided into Families, Subfamilies, Genera and Species.

When systematizing viruses, the following main criteria were identified: the similarity of nucleic acids, size, the presence or absence of a supercapsid, the type of symmetry of the nucleocapsid, the characteristics of nucleic acids, polarity, the number of strands in the molecule, the presence of segments, the presence of enzymes, intranuclear or cytoplasmic localization, antigenic structure and immunogenicity, tropism for tissues and cells, the ability to form inclusion bodies. An additional criterion is the symptomatology of the lesions, i.e. the ability to cause generalized or organ-specific infections.

According to the structural organization, they distinguish simply organized ("naked") and complexly organized ("dressed") viruses.

The structure of a simple virion is arranged in such a way that viral Nucleic Acid, those. the genetic material of the virus is reliably protected by a symmetrical protein shell - capsid, the functional and morphological combination of which forms nucleocapsid.

The capsid has a strictly ordered structure based on the principles of helical or cubic symmetry. It is formed by subunits of the same structure - capsomeres organized in one or two layers. The number of capsomeres is strictly specific for each species and depends on the size and morphology of the virions. Capsomeres, in turn, are formed by protein molecules - protomers. They can be monomeric - composed of a single polypeptide or polymeric - composed of several polypeptides. The symmetry of the capsid is explained by the fact that a large number of capsomeres is required for the genome packing, and their compact connection is possible only with a symmetrical arrangement of subunits. The formation of a capsid resembles the process of crystallization and proceeds according to the principle of self-assembly. The main functions of the capsid are determined by the protection of the viral genome from external influences, ensuring the adsorption of the virion on the cell, the penetration of the genome into the cell as a result of the interaction of the capsid with cell receptors, and determine the antigenic and immunogenic properties of virions.

The nucleocapsid follows the symmetry of the capsid. At spiral symmetry the interaction of nucleic acid and protein in the nucleocapsid is carried out along one axis of rotation. Each virus with helical symmetry has a characteristic length, width and periodicity. Most human pathogenic viruses, including the influenza virus, have helical symmetry. The organization according to the principle of helical symmetry gives the viruses a rod-like or filamentous shape. This arrangement of subunits forms a hollow channel, inside which the viral nucleic acid molecule is compactly packed. Its length can be many times greater than the length of the virion. The tobacco mosaic virus, for example, has a virion length of 300 nm, and its RNA reaches 4000 nm. With such an organization, the protein sheath better protects hereditary information, but requires more protein, because. the coating consists of relatively large blocks. At cubic symmetry the nucleic acid is surrounded by capsomeres, forming an icosahedron - a polyhedron with 12 vertices, 20 triangular faces and 30 corners. The organization of the virion according to this principle gives the viruses a spherical shape. The principle of cubic symmetry is the most economical for the formation of a closed capsid, because for its organization, small protein blocks are used, forming a large internal space in which the nucleic acid freely fits.

Some bacteriophages have double symmetry, when the head is organized according to the principle of cubic, and the process - according to the principle of spiral symmetry.

For large viruses, no permanent symmetry.

An integral structural and functional component of the nucleocapsid are internal proteins, providing the correct supercoiled packaging of the genome, performing structural and enzymatic functions.

The functional specificity of viral enzymes is determined by the place of their localization and the mechanism of formation. Based on this, viral enzymes are divided into virus-induced and virion. The former are encoded in the viral genome, the latter are part of the virions. Virion enzymes are also divided into two functional groups: enzymes of the first group ensure the penetration of viral nucleic acids into the cell and the exit of daughter populations; enzymes of the second group are involved in the processes of replication and transcription of the viral genome. Along with their own, viruses actively use cellular enzymes that are not virus-specific. But their activity can be modified during the reproduction of the virus.

There is a group of so-called. complex or "dressed" viruses, which, unlike "naked", have a special lipoprotein shell on top of the capsid - supercapsid or peplos, organized by a double layer of lipids and specific viral glycoproteins penetrating the lipid bilayer and forming outgrowths-thorns(ash meters or supercapsid proteins ). Surface supercapsid proteins are an important component that facilitates the penetration of viruses into sensitive cells. It is these special proteins, called F-proteins ( fusio - fusion), the fusion of viral supercapsids and cell membranes is ensured. The supercapsid is formed at the later stages of the reproductive cycle during the budding of daughter populations and is a derivative structure from the membranes of a virus-infected cell. Thus, the composition of lipids depends on the nature of the "budding" of the viral particle. For example, in the influenza virus, the composition of the lipid bilayer is similar to that of cell membranes. Because herpesviruses bud through the nuclear membrane, the set of lipids in their supercapsid reflects the composition of the nuclear membrane. The sugars that make up glycoproteins also come from the host cell.

On the inner surface of the supercapsid, the so-called. matrix proteins (M proteins) a structural layer is formed that promotes the interaction of the supercapsid with the nucleocapsid, which is extremely important at the final stages of virion self-assembly.

Nevertheless, the main structural and functional component of the virus is its gene, which determines all the properties of the viral particle, both inside and outside the target cell. The genome encodes information about the morphological, biochemical, pathogenic and antigenic properties of its carrier. The genome of the viral particle is haploid. Nucleic acids are represented by single-stranded RNA molecules or double-stranded DNA molecules. The exceptions are reoviruses, the genome of which is formed by two strands of RNA, and parvoviruses, in which the genome is represented as a single strand of DNA. Viruses contain only one type of nucleic acid.

Viral DNA are organized as circular covalently linked supercoiled or linear structures with molecular weights from 1 x 10 6 to 1 x 10 8 , which is 10 to 100 times less than the molecular weight of bacterial DNA. The genome contains up to several hundred genes. Transcription of viral DNA takes place in the nucleus of the infected cell . Nucleotide sequences occur once, but at the ends of the molecule there are direct and inverted (180 o expanded) repeating nucleotide sequences. This ensures the ability of the DNA molecule to close into a ring. In addition, they are a kind of markers of viral DNA.

Viral RNA are represented by single- and double-stranded molecules and in their own way chemical composition are indistinguishable from cellular RNA. Single-stranded molecules can be segmented, which leads to an increase in the coding capacity of the genome. In addition, they have helical regions such as the double helix of DNA, formed by pairing of complementary nitrogenous bases. Double-stranded RNA can be linear or circular.

Depending on the specifics of intracellular behavior and the functions performed, viral RNAs are divided into groups:

1. Plus-strand RNA, which have the ability to translate the information encoded in it to the ribosomes of the target cell, i.e. function as mRNA. The RNAs of plus-strand viruses have characteristic modified cap-shaped ends necessary for specific recognition of ribosomes. They are called plus strands or positive genome.

2. Negative strands of RNA are unable to translate genetic information directly to ribosomes and cannot function as mRNA. However, they are a template for mRNA synthesis. They are called minus threads or negative gene.

3. double strands, one of which functions as -RNA, the other, complementary to it, as +RNA.

Many viral nucleic acids + RNA and DNA-containing viruses are infectious in themselves, because contain all the genetic information necessary for the synthesis of new viral particles. This information is realized after the penetration of the virion into the sensitive cell. Double-stranded RNAs and most -RNAs cannot show infectious properties.

The interaction of a virus with a target cell is a complex and multistage process of coexistence of two forms of living matter - precellular and cellular. Here, the whole complex of the effects of the viral genome on the genetically encoded biosynthetic processes of the host cell is manifested.

The implementation of the reproductive cycle largely depends on the type of infection of the cell and the nature of the interaction of the virus with a sensitive (possible to be infected) cell.

In a virus-infected cell, viruses can be in various states:

1. reproduction of numerous new virions;

2. presence of the nucleic acid of the virus in an integrated state with the chromosome of the cell in the form of a provirus;

3. existence in the cytoplasm of the cell in the form of circular nucleic acids resembling bacterial plasmids.

It is these conditions that determine a wide range of disorders caused by the virus: from a pronounced productive infection, ending in cell death, to prolonged interaction of the virus with the cell in the form of a latent (latent) infection or malignant transformation of the cell.

Four types of virus interaction with a sensitive cell have been identified:

1. productive type - ends with the formation of a new generation of virions and their release as a result of lysis of infected cells ( cytolytic form), or exit from the cell without its destruction ( non-cytolytic form). According to the non-cytolytic type of interaction, most often occur persistent chronic infections characterized by the formation of daughter populations of the pathogen after the completion of the acute phase of the disease. Cell death is caused by early suppression of cellular protein synthesis, accumulation of toxic and specifically damaging viral components, damage to lysosomes and release of their enzymes into the cytoplasm;

2. Integrative type , or virogeny - characterized by the incorporation (integration) of viral DNA in the form of a provirus into the cell chromosome and subsequent functioning as its integral part with co-replication. This type of interaction occurs latent infection, bacterial lysogeny and viral cell transformation;

3. abortive type - does not end with the formation of new virions, since the infectious process in the cell is interrupted at one of the stages. Occurs when a virus interacts with a resting cell, or when a cell is infected with a defective virus.

Both viruses and virions can be defective.

Defective viruses exist as independent species and are functionally inferior, tk. their replication requires a "helper virus", i.e. the defect is determined by the inferiority of the genome. They are divided into 3 groups:

1. Defective interfering particles, which are virions that contain only part of the genetic information of the original virus and replicate only with the participation of a related "helper virus";

2. Companion viruses differ from the previous ones in that for their reproduction they require the participation of any "helper virus", not necessarily related;

3. Integrated genomes are proviruses, i.e. viral genomes built into the chromosome of the cell, but have lost the ability to turn into a full-fledged virus;

Defective virionsmake up a group that is formed during the formation of large daughter populations, and their defectiveness is determined mainly by morphological inferiority (empty capsids, unenveloped nucleocapsids, etc.). A special form of defective virions - pseudovirions, having a normal capsid containing part of its own nucleic acid and fragments of the nucleic acid of the host, or part of the chromosome of the host cell and part of the nucleic acid of another virus.

The significance of defective viruses lies in their ability to transfer genetic material from a donor cell to a recipient cell.

4. Virus interference - occurs when a cell is infected with two viruses and does not occur with any combination of pathogens. Interference is realized either due to the induction by one virus of cellular inhibitors that suppress the reproduction of another, or due to damage to the receptor apparatus or cell metabolism by the first virus, which excludes the possibility of reproduction of the second. Distinguish homologous(related viruses) and heterologous(unrelated viruses) interference.

According to the nature of the interaction of the virus genome with the cell genome, autonomous and integration infection. During autonomous infection, the virus genome is not integrated into the cell genome, while during integration, the integration of the viral genome into the cell occurs.

Productive type of interaction between a virus and a cell , i.e. Virus reproduction is a unique form of expression of foreign (viral) genetic information in human, animal, plant and bacterial cells, which consists in subordinating the cellular matrix-genetic mechanisms of viral information. This is the most complex process of interaction between two genomes occurring in 6 stages:

1. adsorption of virions;

2. penetration of the virus into the cell;

3. stripping and release of the viral genome;

4. synthesis of viral components;

5. formation of virions;

6. release of virions from the cell.

First reproduction stage - adsorption, i.e. attachment of the virion to the cell surface. It proceeds in two phases. First phase - non-specific due to ionic attraction and other mechanisms of interaction between the virus and the cell. Second phase - highly specific, due to the homology and complementarity of the receptors of sensitive cells and the protein ligands of viruses that recognize them. Recognizing and interacting viral proteins are called attachment and are represented by glycoproteins, as part of the lipoprotein shell of the capsid or supercapsid of the virus.

Specific cell receptors have a different nature, being proteins, lipids, carbohydrate components of proteins and lipids. One cell can carry from ten to one hundred thousand specific receptors, which allows tens and hundreds of virions to gain a foothold on it. The number of infectious viral particles adsorbed on a cell defines the term "multiplicity of infection". However, a virus-infected cell is in most cases tolerant of re-infection with a homologous virus.

The presence of specific receptors underlies tropism viruses to certain cells, tissues and organs.

Second stage - entry of the virus into the cell can happen in several ways.

1. Receptor-dependent endocytosis occurs as a result of the capture and absorption of the virion by a sensitive cell. In this case, the cell membrane with the attached virion invaginates with the formation of an intracellular vacuole (endosome) containing the virus. Next, the lipoprotein envelope of the virus fuses with the endosome membrane and the virus enters the cytoplasm of the cell. Endosomes combine with lysosomes, which break down the remaining viral components.

2. Viropexis - consists in the fusion of the viral supercapsid with the cell or nuclear membrane and occurs with the help of a special fusion protein – F-squirrel, which is part of the supercapsid. As a result of viropexis, the capsid is inside the cell, and the supercapsid, together with the protein, integrates (embeds) into the plasma or nuclear membrane. Inherent only in complex viruses.

3. Phagocytosis - by means of which viruses penetrate into phagocytic cells, which leads to incomplete phagocytosis.

Third stage - stripping and releasing the viral genome occurs as a result of deproteinization, modification of the nucleocapsid, removal of surface viral structures and the release of an internal component that can cause an infectious process. The first stages of "undressing" begin even in the process of penetration into the cell by fusion of viral and cellular membranes or when the virus exits the endosome into the cytoplasm. The subsequent stages are closely related to their intracellular transport to the sites of deproteinization. Different viruses have their own specialized stripping sites. Transport to them is carried out using intracellular membrane vesicles, in which the virus is transferred to the ribosomes, the endoplasmic reticulum, or to the nucleus.

Fourth stage - synthesis of viral components starts at the moment shady or eclipse phases, which is characterized by the disappearance of the virion. The shadow phase ends after the formation of the component components of the virus necessary for the assembly of daughter populations. The virus uses the genetic apparatus of the cell for this, suppressing the synthetic reactions necessary for it itself. Synthesis of proteins and nucleic acids of the virus, i.e. its reproduction, separated in time and space, is carried out in different parts cells and is called disjunctive.

In an infected cell, the viral genome encodes the synthesis of two groups of proteins:

- non-structural proteins, serving the intracellular reproduction of the virus at its various stages, which include RNA or DNA polymerases that provide transcription and replication of the viral genome, regulatory proteins, precursors of viral proteins, enzymes that modify viral proteins;

- structural proteins, which are part of the virion (genomic, capsid and supercapsid).

The synthesis of proteins in the cell is carried out in accordance with the processes transcriptions by "rewriting" the genetic information from the nucleic acid into the nucleotide sequence of messenger RNA (mRNA) and broadcasts(reading) mRNA on ribosomes to form proteins. The term "translation" refers to the mechanisms by which the sequence of nucleic bases of mRNA is translated into a specific amino acid sequence in the synthesized polypeptide. In this case, discrimination of cellular mRNAs occurs and synthetic processes on ribosomes pass under viral control. The mechanisms for transmitting information regarding the synthesis of mRNA in different groups of viruses are not the same.

Double-stranded DNA containing viruses implement genetic information in the same way as the cellular genome, according to the scheme: virus genomic DNA→ mRNA transcription→viral protein translation. At the same time, DNA-containing viruses, the genomes of which are transcribed in the nucleus, use a cellular polymerase for this process, and the genomes of which are transcribed in the cytoplasm, their own virus-specific RNA polymerase.

Genome –RNA-containing viruses serves as a template from which mRNA is transcribed, with the participation of virus-specific RNA polymerase. Their protein synthesis occurs according to the scheme: virus genomic RNA→mRNA transcription→virus protein translation.

The group of RNA-containing retroviruses, which includes human immunodeficiency viruses and oncogenic retroviruses, stands apart. They have a unique way of transferring genetic information. The genome of these viruses consists of two identical RNA molecules, i.e. is diploid. Retroviruses contain a special virus-specific enzyme - reverse transcriptase, or reversetase which carries out the process of reverse transcription. It consists in the following: complementary single-stranded DNA (cDNA) is synthesized on the genomic RNA template. It is copied with the formation of double-stranded complementary DNA, which integrates into the cellular genome and is transcribed into mRNA using cellular DNA-dependent RNA polymerase. The synthesis of proteins of these viruses is carried out according to the scheme: virus genomic RNA→complementary DNA→mRNA transcription→virus protein translation.

Transcription is regulated by cellular and virus-specific mechanisms. It consists in sequential reading of information from the so-called. "early" and "late" genes. In the former, information is encoded for the synthesis of virus-specific transcription and replication enzymes, and in the latter, for the synthesis of capsid proteins.

Synthesis of viral nucleic acids, i.e. replication of viral genomes, leads to the accumulation in the cell of copies of the original viral genomes, which are used in the assembly of virions. The method of replication depends on the type of nucleic acid of the virus, the presence of virus-specific and cellular polymerases, and the ability of viruses to induce the formation of polymerases in the cell.

Double-stranded DNA viruses replicate in the usual semi-conservative way: after the DNA strands are untwisted, new strands are completed complementary to them. Each newly synthesized DNA molecule consists of one parent and one synthesized strand.

Single-stranded DNA viruses in the process of replication, cellular DNA polymerases are used to create a double-stranded viral genome, the so-called. replicative form. At the same time, a –DNA strand is complementarily synthesized on the initial +DNA strand, which serves as a template for the +DNA strand of the new virion.

Single-stranded +RNA viruses induce the synthesis of RNA-dependent RNA polymerase in the cell. With its help, on the basis of the genomic +RNA strand, the -RNA strand is synthesized, a temporary double RNA is formed, called replication intermediate. It consists of a complete +RNA strand and numerous partially completed -RNA strands. When all -RNA strands are formed, they are used as templates for the synthesis of new +RNA strands.

Single stranded RNA viruses contain RNA-dependent RNA polymerase. The genomic –RNA strand is transformed by viral polymerase into incomplete and complete +RNA strands. Incomplete copies act as mRNA for the synthesis of viral proteins, and complete copies are a template for the synthesis of the genomic RNA strand of the offspring.

Double-stranded RNA viruses replicate similarly to single-stranded RNA viruses. The difference is that +RNA strands formed during transcription function not only as mRNA, but also participate in replication. They are a matrix for the synthesis of RNA strands. Together, they form genomic double-stranded RNA virions.

Diploid +RNA viruses or retroviruses replicate with the help of viral reverse transcriptase, which synthesizes a DNA strand on the template of the RNA virus, from which the +DNA strand is copied to form a double strand of DNA closed in a ring. Next, the double strand of DNA integrates with the chromosome of the cell, forming a provirus. Numerous virion RNAs are formed as a result of transcription of one of the strands of integrated DNA with the participation of cellular DNA-dependent RNA polymerase.

Fifth stage - virion assembly takes place in an orderly manner. self-assembly when the constituent parts of the virion are transported to the assembly sites of the virus. These are specific areas of the nucleus and cytoplasm, called replication complexes. The connection of the components of the virion is due to the presence of hydrophobic, ionic, hydrogen bonds and stereochemical correspondence.

The formation of viruses is a multi-stage, strictly sequential process, with the formation of intermediate forms that differ from mature virions in the composition of polypeptides. The assembly of simply arranged viruses occurs on replication complexes and consists in the interaction of viral nucleic acids with capsid proteins and the formation of nucleocapsids. In complex viruses, nucleocapsids are first formed on replication complexes, which then interact with modified cell membranes, which are the future lipoprotein shell of the virion. In this case, the assembly of viruses that replicate in the nucleus occurs with the participation of the nuclear membrane, and the assembly of viruses that replicate in the cytoplasm is carried out with the participation of the membranes of the endoplasmic reticulum or the cytoplasmic membrane, where glycoproteins and other proteins of the virion envelope are embedded. In some complex RNA viruses, a matrix protein is involved in the assembly - M protein- which is located under the cell membrane modified by this protein. Possessing hydrophobic properties, it acts as an intermediary between the nucleocapsid and the supercapsid. Complex viruses in the process of formation include components of the host cell in their composition. If the self-assembly process is violated, "defective" virions are formed.

sixth stage - release of viral particles from the cell completes the process of virus reproduction and occurs in two ways.

explosive way when viruses lacking a supercapsid cause cell destruction and enter the extracellular space. A large number of virions simultaneously emerge from a dead cell.

budding or exocytosis , characteristic of complex viruses, the supercapsid of which is derived from cell membranes. First, the nucleocapsid is transported to cell membranes, which are already embedded with virus-specific proteins. In the area of contact, the protrusion of these areas begins with the formation of a kidney. The formed kidney is separated from the cell in the form of a complex virion. The process is not lethal for the cell, and the cell is able to remain viable for a long time, producing viral offspring.

Budding of viruses that form in the cytoplasm can occur either through the plasma membrane or through the membranes of the endoplasmic reticulum and the Golgi apparatus, followed by exit to the cell surface.

Viruses that form in the nucleus bud into the perinuclear space through the modified nuclear envelope and are transported to the cell surface as part of cytoplasmic vesicles.

Integrative type of virus-cell interaction (virogeny) is the coexistence of a virus and a cell as a result of the integration of the nucleic acid of the virus into the chromosome of the host cell, in which the viral genome replicates and functions as a major part of the cell's genome.

This type of interaction is characteristic of moderate DNA-containing bacteriophages, oncogenic viruses, and some infectious DNA- and RNA-containing viruses.

Integration requires the presence of a circular form of double-stranded DNA of the virus. Such DNA is attached to cellular DNA at the site of homology and is integrated into a specific region of the chromosome. In RNA viruses, the integration process is more complex and begins with a reverse transcription mechanism. Integration occurs after the formation of a double-stranded DNA transcript and its closure into a ring.

Additional genetic information during virogeny imparts new properties to the cell, which can cause oncogenic transformation of cells, autoimmune and chronic diseases.

Abortive type of interaction of the virus with the cell does not end with the formation of viral progeny and can occur under the following conditions:

1. infection of a sensitive cell occurs with a defective virus or a defective virion;

2. infection with a virulent virus of cells genetically resistant to it;

3. infection of a sensitive cell with a virulent virus in non-permissive (nonpermissive) conditions.

More often, an abortive type of interaction is observed when an insensitive cell is infected with a standard virus. However, the mechanism of genetic resistance is not the same. It may be associated with the absence of specific receptors on the plasma membrane, the inability of this type of cell to initiate the translation of viral mRNA, and the absence of specific proteases or nucleases necessary for the synthesis of viral macromolecules.

Changes in the conditions under which virus reproduction occurs can also lead to abortive interaction: an increase in body temperature, a change in pH in the focus of inflammation, the introduction of antiviral drugs, etc. However, when non-permissive conditions are eliminated, the abortive type of interaction turns into a productive one with all the ensuing consequences.

Interfering interaction is determined by the state of immunity to secondary infection of a cell already infected with a virus.

heterologous interference occurs when infection with one virus completely blocks the possibility of replication of the second virus within the same cell. One of the mechanisms is associated with the inhibition of the adsorption of another virus by blocking or destroying specific receptors. Another mechanism is related to the inhibition of mRNA translation of any heterologous mRNA in the infected cell.

Homologous interference typical of many defective viruses, especially repassable ones in vitro and high multiplicity of infection. Their reproduction is possible only when the cell is infected with a normal virus. Sometimes a defective virus can interfere with the reproductive cycle of a normal virus and form defective interfering virus particles (DI). DI particles contain only part of the genome of a normal virus. By the nature of the defect, DI particles are deletion particles and they can be considered as lethal mutants. The main property of DI particles is the ability to interfere with a normal homologous virus and even play the role of helpers in replication. The ability to adsorb and penetrate into the cell is associated with the normal structure of the capsid. The release and expression of a defective nucleic acid leads to various biological effects: it inhibits synthetic processes in the cell, inhibits the synthesis and transformation of proteins of normal viruses due to homologous interference. Circulation of DI particles and co-infection with a normal homologous virus causes the appearance of indolent, long-term forms of diseases, which is associated with the ability of DI particles to replicate much faster due to the simplicity of the genome, while the defective population has a noticeable decrease in the severity of the cytopathic effect characteristic of a normal virus.

The process of interaction of the virus with the body in most cases is cytospecific and is determined by the ability of the pathogen to multiply in certain tissues. However, some viruses have a wider range of tropism and reproduce in a wide variety of cells and organs.

The specificity factors of the virus responsible for its tropism and the variety of affected cells include the number of specific receptors (both in the virion and in the cell) that ensure the full interaction of the virus with the cell. The number of such receptors is usually limited.

In some cases, the very physiological specificity of cells, and hence their bimolecular organization, contributes to the manifestation of the virulence of the pathogen. For example, the G-protein of the rabies virus envelope has a high affinity for neuronal acetylcholine receptors, which ensures its ability to penetrate the cells of the nervous tissue. It should be noted that neurotropic viruses cause especially severe diseases, because nerve cells do not regenerate. Moreover, reproduction of the pathogen makes them targets for cytotoxic immune responses.

Quite often, the virulence of viruses increases due to mutations. Of particular importance in this case is the ability of viruses to reverse mutation of genes (reversion). Genes encoding protein structure can restore their structure and transform previously avirulent virus strains into virulent ones.

Not less than importance have and features of a susceptible macroorganism.

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- These are the smallest particles of life, they are 50 times smaller than bacteria. Usually viruses cannot be seen in a light microscope, since their individuals are more than half the wavelength of light. Resting individuals of a virus are called virion. Viruses exist in two forms: resting, or extracellular (viral particles, or virions), and reproducing, or intracellular (complex "virus - host cell").

The forms of viruses are different, they can be filiform, spherical, bullet-shaped, rod-shaped, polygonal, brick-shaped, cubic, while some have a cubic head and process. Each virion consists of nucleic acid and proteins.

In the virions of viruses, only one type of nucleic acid is always present - either RNA or DNA. Moreover, both one and the other can be single-stranded and double-stranded, and DNA can be linear or circular. RNA in viruses is always only linear, but it can be represented by a set of RNA fragments, each of which carries a certain part of the genetic information necessary for reproduction. By the presence of a particular nucleic acid, viruses are called DNA-containing and RNA-containing. It should be especially noted that in the kingdom of viruses, the function of the custodian of the genetic code is performed not only by DNA, but also by RNA (it can also be double-stranded).

Viruses have a very simple structure. Each virus consists of only two parts - core and capsid. The core of the virus, which contains DNA or RNA, is surrounded by a protein coat - capsid (lat. capsa- "receptacle", "box", "case"). Proteins protect the nucleic acid, and also cause enzymatic processes and minor changes in proteins in the capsid. The capsid consists of a certain way stacked of the same type of protein molecules - capsomeres. Usually this is either a spiral type of laying (Fig. 22), or a type symmetrical polyhedron(isometric type) (Fig. 23).

All viruses are conditionally divided into simple and complex. Simple viruses consist only of a core with nucleic acid and a capsid. Complex viruses on the surface of the protein capsid they also have an outer shell, or supercapsid, containing a bilayer lipoprotein membrane, carbohydrates and proteins (enzymes). This outer shell (supercapsid) is usually built from the membrane of the host cell. material from the site

On the surface of the capsid there are various outgrowths - spikes, or "carnations" (they are called fibers), and shoots. With them, the virion attaches to the surface of the cell, into which it then penetrates. It should be noted that on the surface of the virus there are also special attachment proteins, binding the virion with specific groups of molecules - receptors(lat. recipio-“I receive”, “I accept”), located on the surface of the cell into which the virus penetrates. Some viruses attach to protein receptors, others to lipids, and others recognize carbohydrate chains in proteins and lipids. In the process of evolution, viruses "learned" to recognize cells that are sensitive to them by the presence of special receptors on the cell surface of their hosts.

The morphology and structure of viruses are studied using an electron microscope. One of the smallest is the polio virus (about 20 nm), the largest is smallpox (about 350 nm).

Viruses are composed of the following main components:

1. Core - genetic material (DNA or RNA) that carries information about several types of proteins necessary for the formation of a new virus.

2. A protein shell, which is called a capsid (from the Latin capsa - a box). It is often built from identical repeating subunits - capsomeres. Capsomeres form structures with a high degree symmetry.

3. Additional lipoprotein membrane (supercapsid). It is formed from the plasma membrane of the host cell and is found only in relatively large viruses (influenza, herpes).

Schematically, the structure of an RNA-containing virus with a helical symmetry type and an additional lipoprotein envelope is shown on the left in the figure, its enlarged cross section is shown on the right.

The capsid and the additional shell have protective functions, as if protecting the nucleic acid. In addition, they contribute to the penetration of the virus into the cell. A fully formed virus is called virion.

The shape of the virions depends on the way the protein subunits are folded into the capsid. This stacking can have helical or cubic symmetry. Bacteriophages have a mixed or combined type of symmetry.

Tobacco mosaic virus has both RNA and protein subunits arranged in a spiral and is filamentous or rod-shaped. With this symmetry, the protein sheath protects the nucleic acid better, but it requires more protein than with cubic symmetry. The true number of subunits in different virions is 60 or a multiple of this value (420 subunits for the polyoma virus, 540 for the reovirus, 960 for the herpes virus, 1500 for the adenovirus).

Most closed-case viruses have cubic symmetry. It is based on various combinations of equilateral triangles (capsomeres) formed by spherical protein subunits. In this case, tetrahedra, octahedrons and icosahedrons can be formed. Icosahedrons have 20 triangular faces and 12 vertices. This is the most efficient and economical symmetry. Therefore, spherical animal viruses most often have the shape of an icosahedron.

In the influenza virus, the nucleocapsid has a rod-shaped helical structure, and the supercapsid lipoprotein envelope gives the virion a spherical shape.

The number of capsomeres for viruses of this type is constant and has diagnostic value.

Simply arranged viruses have only a capsid (polio virus), complex viruses also have a supercapsid (measles, influenza viruses).

The classification of viruses is based on the following categories.

Table of contents of the subject "Types of Microorganisms. Viruses. Virion.":
1. Microorganisms. Types of microorganisms. Classification of microorganisms. Prions.
2. Viruses. Virion. Morphology of viruses. Virus sizes. nucleic acids of viruses.
3. Capsid of the virus. Functions of the capsid of viruses. Capsomeres. Virus nucleocapsid. Helical symmetry of the nucleocapsid. Cubic symmetry of the capsid.
4. Virus supercapsid. Dressed up viruses. Naked viruses. Matrix proteins (M-proteins) of viruses. reproduction of viruses.
5. Interaction of a virus with a cell. The nature of the virus-cell interaction. Productive interaction. Virogeny. Virus interference.
6. Types of cell infection by viruses. The reproductive cycle of viruses. The main stages of the reproduction of viruses. Adsorption of the virion to the cell.
7. Penetration of the virus into the cell. Viropexis. Undressing the virus. Shadow phase (eclipse phase) of virus reproduction. The formation of viral particles.
8. Transcription of the virus in the cell. Translation of viruses.
9. Replication of the virus in the cell. Collection of viruses. Release of progeny virions from the cell. Viruses. Virion. Morphology of viruses. Virus sizes. nucleic acids of viruses.

Extracellular form - virion- includes all constituent elements (capsid, nucleic acid, structural proteins, enzymes, etc.). Intracellular form - virus- can be represented by only one nucleic acid molecule, since, when it enters the cell, the virion breaks down into its constituent elements.

Morphology of viruses. Virus sizes.

Nucleic acids of viruses

Viruses contain only one type of nucleic acid, DIC or RNA, but not both types at the same time. For example, smallpox, herpes simplex, Epstein-Barr viruses are DNA-containing, and togaviruses, picornaviruses are RNA-containing. The genome of the viral particle is haploid. The simplest viral genome encodes 3-4 proteins, the most complex - more than 50 polypeptides. Nucleic acids are represented by single-stranded RNA molecules (excluding reoviruses, in which the genome is formed by two strands of RNA) or double-stranded DNA molecules (excluding parvoviruses, in which the genome is formed by one strand of DNA). In the hepatitis B virus, the strands of the double-stranded DNA molecule are unequal in length.

Viral DNA form circular, covalently linked supercoiled (for example, in papovaviruses) or linear double-stranded structures (for example, in herpes and adenoviruses). Their molecular weight is 10-100 times less than the mass of bacterial DNA. Transcription of viral DNA (mRNA synthesis) is carried out in the nucleus of a virus-infected cell. In viral DNA, at the ends of the molecule, there are direct or inverted (180" unfolded) repeating nucleotide sequences. Their presence ensures the ability of the DNA molecule to close into a ring. These sequences, present in single- and double-stranded DNA molecules, are a kind of viral DNA markers.

Rice. 2-1. Sizes and morphology of the main pathogens viral infections human.

Viral RNA represented by single- or double-stranded molecules. Single-stranded molecules can be segmented - from 2 segments in arenaviruses to 11 segments in rotaviruses. The presence of segments leads to an increase in the coding capacity of the genome. Viral RNA subdivided into the following groups: plus strands of RNA (+RNA), minus strands of RNA (-RNA). In various viruses, the genome can form +RNA or -RNA strands, as well as double strands, one of which is -RNA, the other (complementary to it) - +RNA.

Plus-strand RNA are represented by single chains with characteristic endings (“caps”) for ribosome recognition. This group includes RNAs that can directly translate genetic information on the ribosomes of a virus-infected cell, that is, perform the functions of mRNA. Plus strands perform the following functions: they serve as mRNA for the synthesis of structural proteins, as a template for RNA replication, and they are packaged into a capsid to form a daughter population. RNA minus strands are unable to translate genetic information directly on ribosomes, meaning they cannot function as mRNA. However, such RNAs serve as templates for mRNA synthesis.

Infectivity of nucleic acids of viruses

Many viral nucleic acids are infectious in themselves, as they contain all the genetic information necessary for the synthesis of new viral particles. This information is realized after the penetration of the virion into the sensitive cell. Nucleic acids of most +RNA- and DNA-containing viruses exhibit infectious properties. Double-stranded RNAs and most RNAs are not infectious.