Viruses form an independent kingdom (Vira) and have the following features:

The genome is represented by one nucleic acid - DNA or RNA (respectively, 2 sub-kingdoms are distinguished - riboviruses and deoxyriboviruses).

Non-cellular structure. Nucleic acid is coated with a protein coat - capsid, which consists of separate subunits - capsomers(usually consists of 5-6 polypeptides). The capsid together with nucleic acid forms nucleocapsid. Simple viruses have such a structure (poliomyelitis viruses, adenoviruses, etc.). Complex viruses have an outer envelope - supercapsid, which contains lipids, glycolipids. The supercapsid is partly formed by the host cell.

Lack of protein-synthesizing systems (in the presence of enzymes of adsorption, proliferation, DNA - and RNA - dependent polymerases).

A special (disjunctive) method of reproduction: the proteins of the virus are synthesized on the ribosomes of the affected cell, in other areas - the nucleic acid of the virus, then the viral particles are assembled.

Small size; small viruses (podiovirus, etc.) - 25-30 nm (nanometers); medium (influenza virus, etc.) - 50-125 nm; large (variola virus) - 150-200 nm.

7. Filterability (pass through bacterial filters).

8. Crystallization (extracellular viruses purified from ballast substances, virions, able to form crystals).

9. The form of virioi (distinguish rod-shaped - at rabies virus, etc., in the form of a polyhedron, icosahedron - in adenoviruses, cuboidal forms - in variola virus, spherical - in influenza viruses, capitate(sperm-like) - bacteriophages).

The cultivation of viruses also has its own peculiarities. They are cultivated on actively multiplying cells with increased metabolic activity. I use the following living systems. In the body of laboratory animals: usually infect mice (adults and suckers), rabbits, monkeys (intramuscularly, intranasally, intraperitoneally, intracerebrally, on the cornea). On 9-12-day-old chicken embryos: the dish is cultured on the embryo-allantoic membrane, less often - in the allantoic or amniotic subnetwork. On cell culture: more often single-layer tissue culture from actively multiplying cells. Cells are grown on natural nutrient media (embryonic extracts, horse, human serum), enzymatic protein hydrolysates (tryptic hydrolyzate of lactalbumin), on synthetic media (for example, on medium 199, consisting of 63 components, including amino acids, vitamins, glucose, salts , human serum, an indicator of phenol red). The following types of cell cultures are used: primary trypsinized (usually chicken embryo fibroblasts; they are not intertwined and must always be prepared ex tempore; the disadvantage is that they are non-standard); transplanted (the same in all laboratories, since they are a specific clone of cells, for example, cells from portal tissues- human amnion, pig embryo kidney; cells from tumor tissues - HeLa (cervical cancer cells), HEp-2 and others; the disadvantage of this group is that cells often spontaneously degenerate, become atypical, polyploid, and are also spontaneously infected with latent viruses and mycoplasmas); semi-grafted diploid (for example, diploid cells of human lungs; they are stable, do not regenerate spontaneously, are not contaminated with viruses and mycoplasmas).

There are the following forms of viral infections. Abortive infection (occurs in an unresponsive immune organism): the virus either does not enter the cell, or after penetration dies and is pushed out of the cell. Productive infection: the virus is adsorbed on sensitive cells and enters the cell by immersing its membrane with the virus inside, into the cytoplasm of the cell ( viro-rexis); in the formed phagosome, the nucleic acid of the virus is freed from the protein envelopes ("stripping the virus"); after the final undressing, the viral nucleic acid that has penetrated into the cell switches the functioning of the cellular genome and the corresponding metabolic systems of the cell for reproduction virus; the formed viral particles leave the cell and invade neighboring cells. Often this interaction ends in cell death, this process is referred to as cytopathicaction(CPD). An early sign of CPP is cessation of mitosis; the cell temporarily swells, then deforms, shrinks, becomes more intensely stained, exfoliates from glass (in cultures) and dies. Sometimes, before death, cells form symplasts(merged multinucleated cells). Virogeny: the viral nucleic acid that has penetrated into the cell integrates (integrates) into the host cell's DNA (as in the case of a temperate phage) and in the form provirus exists in the cell and is passed on to its offspring. The phenomenon of virogeny is characteristic of both DNA and RNA viruses, since the latter have an enzyme reverse transcriptase(for example, retroviruses).

The modern classification of viruses is based on a number of features, including: the type of nucleic acid, the number of capsomeres, the presence of a supercapsid, sensitivity to ether, the range of susceptible hosts, pathogenicity, geographical distribution, etc.

Features of antiviral immunity. Immunity to viral infections may be due to the following factors. Factors of natural resistance: cellular unresponsiveness (as a result of phylogenesis, a person is immune to many viral diseases of animals and plants); inhibitors - substances of mucoprotein or lipoprotein nature, structurally identical to the receptors of sensitive cells (they circulate freely in the blood, other fluids and block the interaction of the virus with the cell); complement is involved in the formation of a specific (immune) antiviral response (lysozyme and other humoral factors do not play a protective role); phagocytosis is incomplete, but leukocytes, into which the virus has penetrated, produce interferon; interferon is synthesized by the cell after the penetration of the virus, it nonspecifically inhibits the reproduction of any viruses, disrupting the synthesis of viral proteins on ribosomes (only human interferon is active in the human body, which is produced by human leukocytes, or genetically engineered interferon - reaferon, produced by Escherichia coli, into the genome of which the human interferon gene has been introduced; interferon is widely used for the treatment and emergency prevention of viral infections); fever (high temperature interferes with the reproduction of viruses); the age factor (it matters, for example, with rotavirus infection, which children are more likely to get sick with); endocrine factors (hypofunction of many endocrine glands aggravates the course of viral infections); factors of the excretory system (contribute to the release of the body from viruses); the formation of intracellular inclusions may have a protective effect (Guarnieri's little body in case of smallpox, Babesha-Negri's little body in case of rabies).

The features of the acquired antiviral immunity in some cases determine persistent immunity (for example, after measles), in others - short-term (after rhinovirus infection). Antibodies act only on extracellularly located viruses (therefore, treatment with antiviral immunoglobulins is carried out early, until the bulk of the viruses have penetrated into the cells). Cells into which viruses have entered synthesize virus-dependent antigens and become foreign to the body, which leads to their destruction by killer T cells. The local resistance of cells is also important in defense reactions (for example, in a person refractory to poliomyelitis, cells of the nervous tissue and the gastrointestinal tract, to which the poliovirus has tropism, become resistant to the virus). Secretory immunoglobulins (slgA) are the main link of local immunity on the mucous membranes. Vaccination (with viral vaccines) creates not only specific immunity against an individual virus, but also forms resistance to other viruses (not only the production of antibodies and the formation of T-killers is stimulated, but also the production of interferon).

The morphology and structure of viruses is studied using an electron microscope, since their size is small and comparable to the thickness of the bacterial membrane.

The form of virions can be different: rod-shaped (tobacco mosaic virus), bullet-shaped (rabies virus), spherical (poliomyelitis viruses, HIV), in the form of a spermatozoon (many bacteriophages) (Fig. 8).

Rice. 8. Forms of virions:

1 smallpox virus; 2 herpes virus; 3 adenovirus; 4 papovavirus; 5 hepadnavirus; 6 paramyxovirus; 7 influenza virus; 8 coronavirus; 9 arenavirus; 10 retrovirus;

The sizes of viruses are determined using electron microscopy, by ultrafiltration through filters with a known pore diameter, by ultracentrifugation. Some of the smallest viruses are polio and foot and mouth disease (about 20 nm), circoviruses (16 nm), the largest variola virus (about 350 nm). Viruses have a unique genome as they contain either DNA or RNA. Therefore, a distinction is made between DNA-containing and RNA-containing viruses. They are usually haploid, meaning they have one set of genes. The genome of viruses is represented by various types of nucleic acids: double-stranded, single-stranded, linear, circular, fragmented.

Distinguish between simple (for example, polio virus) and complex (for example, influenza, measles) viruses. In viruses simply arranged, the nucleic acid is bound to a protein membrane called the capsid (from the Latin capsa sheath). The capsid consists of repeating morphological subunits of capsomeres. Nucleic acid and capsid interact with each other to form a nucleocapsid. In complex viruses, the capsid is surrounded by an additional lipoprotein envelope, a supercapsid (a derivative of the membrane structures of the host cell), which has spikes. Capsid and supercapsid protect virions from environmental influences, cause selective interaction (adsorption) with cells, and determine the antigenic and immunogenic properties of virions. The internal structures of viruses are called the core.

Virions are characterized by spiral, cubic and complex types of capsid symmetry. The spiral type of symmetry is due to the helical structure of the nucleocapsid, cubic formation of an isometric hollow body from a capsid containing viral nucleic acid.

In addition to common viruses, so called non-canonical viruses, prions, are protein infectious particles in the form of fibrils 10–20 x 100–200 nm in size. Prions, apparently, are both inducers and products of the autonomous gene of humans or animals and cause encephalopathy in them under conditions of a slow viral infection (Creutzfeldt Jakob disease, kuru, etc.). Other unusual agents close to viruses are viroids, small molecules of circular, supercoiled RNA that do not contain protein, which cause disease in plants.

LECTURE No. 5.

VIROLOGY.

All viruses exist in two qualitatively different forms. Extracellular form - virion - includes all the constituent elements of a viral particle. Intracellular form - virus - can be represented by only one nucleic acid molecule, since getting into the cell, the virion breaks down into its constituent elements. At the same time, the intracellular virus is a self-replicating form that is not capable of dividing. On this basis, the definition of a virus implies a fundamental difference between the cellular forms of existence (bacteria, fungi, protozoa), multiplying by division, and the replicating form, reproduced from the viral nucleic acid. But this is not limited to the distinctive 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 replication synchronous with it.

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

The size of the viruses is 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 subkingdoms:

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: similarity of nucleic acids, size, presence or absence of a supercapsid, type of nucleocapsid symmetry, characteristics of nucleic acids, polarity, number of strands in a molecule, presence of segments, presence of enzymes, intranuclear or cytoplasmic localization, antigenic structure and immunogenicity, tropism to 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.

Structural organization distinguishes simply organized ("naked") and complex ("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 envelope - capsid, the functional and morphological set of which forms nucleocapsid.

The capsid has a strictly ordered structure, which is based on the principles of spiral or cubic symmetry. It is formed by subunits of the same structure - capsomers organized in one or two layers. The number of capsomeres is strictly specific for each species and depends on the size and morphology of 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 packing the genome requires a large number of capsomeres, and their compact connection is possible only with a symmetric arrangement of subunits. The formation of the capsid resembles the crystallization process 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 cellular receptors, and determine the antigenic and immunogenic properties of virions.

The nucleocapsid repeats the symmetry of the capsid. At spiral symmetry the interaction of nucleic acid and protein in the nucleocapsid is carried out along the same axis of rotation. Each helically symmetric virus has a characteristic length, width, and frequency. Most viruses pathogenic to humans, including the influenza virus, have spiral symmetry. Helical symmetry organization gives viruses a rod-like or thread-like 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 the length of the virion. In the tobacco mosaic virus, for example, the virion is 300 nm long, 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 angles. 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, which form a large internal space in which nucleic acid can freely fit.

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

Large viruses are characterized by lack of permanent symmetry.

An integral structural and functional component of the nucleocapsid is internal proteins ensuring 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. Virionic 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 membrane on top of the capsid - supercapsid or peplos organized by a double layer of lipids and specific viral glycoproteins that penetrate the lipid bilayer and form 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 late stages of the reproductive cycle during the budding of daughter populations and is a derived structure from the membranes of a virus-infected cell. So, 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 herpes viruses bud through the nuclear membrane, then the set of lipids of their supercapsid reflects the composition of the nuclear membrane. The sugars that make up the glycoproteins also come from the host cell.

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

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 presented 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 a molecular weight from 1 · 10 6 to 1 · 10 8, which is 10 to 100 times less than the molecular weight of bacterial DNA. The genome contains up to several hundred genes. Viral DNA is transcribed in the nucleus of the infected cell . Nucleotide sequences occur only once, but at the ends of the molecule there are straight and inverted (180 ° rotated) repeating nucleotide sequences. This ensures the ability of the DNA molecule to close into a ring. In addition, they are a kind of viral DNA markers.

Viral RNA are represented by single- and double-stranded molecules and in their chemical composition do not differ from RNA of cellular origin. 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 a double helix of DNA, formed due to the pairing of complementary nitrogenous bases. Double-stranded RNA can be linear or circular.

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

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

2. RNA minus strands 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 strands or negative genome.

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 by themselves, because contain all the genetic information needed to synthesize new viral particles. This information is realized after the penetration of the virion into the sensitive cell. Double-stranded RNAs and most -RNAs cannot exhibit 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 cell infection and the nature of the interaction of the virus with a sensitive (potentially infected) cell.

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

1. reproduction of numerous new virions;

2. the stay 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 a 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 that ends with cell death, to a 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 interaction of the virus 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 destroying it ( noncytolytic form). The noncytolytic type of interaction most often occurs persistent chronic infections characterized by the formation of daughter populations of the pathogen after the end of the acute phase of the disease. Cell death is caused by early suppression of the synthesis of cellular proteins, the accumulation of toxic and specifically damaging viral components, damage to lysosomes and the release of their enzymes into the cytoplasm;

2. Integrative type , or virogeny - characterized by the embedding (integration) of viral DNA in the form of a provirus into the cell chromosome and subsequent functioning as its component with joint replication. According to this type of interaction, latent infection, lysogeny of bacteria 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. It occurs when a virus interacts with a resting cell, or when a cell is infected with a defective virus.

Viruses as well as virions can be defective.

Defective viruses exist as independent species and are functionally defective, because a helper virus is required for their replication; the defect is determined by the inferiority of the genome. Divided into 3 groups:

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

2. Satellite viruses they 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 a cell, but having lost the ability to transform into a full-fledged virus;

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

The value 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 every combination of pathogens. Interference is realized either due to the induction of cell inhibitors by one virus 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.

By the nature of the interaction of the genome of the virus with the genome, cells are distinguished autonomous and integration infection... During autonomous infection, the viral genome is not integrated into the cell genome, while during integration, the viral genome is integrated into the cellular one.

Productive type of interaction of the virus with the 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 the subordination of the cellular matrix-genetic mechanisms of viral information. This is a complex process of interaction between two genomes, which takes place 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. the formation of virions;

6. exit of virions from the cell.

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

Specific cell receptors are of 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 the cell defines the term "multiplicity of infection"... However, a virus-infected cell is generally tolerant of reinfection with a homologous virus.

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

The second stage - penetration 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 invades to form an intracellular vacuole (endosome) containing the virus. Further, the lipoprotein envelope of the virus merges with the endosome membrane and the virus enters the cytoplasm of the cell. Endosomes combine with lysosomes, which destroy 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, is integrated (incorporated) into the plasma or nuclear membrane. It is inherent only in complex viruses.

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

The 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 during the process of penetration into the cell by fusion of viral and cell membranes or when the virus leaves the endosome into the cytoplasm. The subsequent stages are closely related to their intracellular transport to the sites of deproteinization. For different viruses, there are specialized "undressing" areas. Transport to them is carried out with the help of 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 constituent components of the virus, which are necessary for the assembly of daughter populations. The virus uses the genetic apparatus of the cell for this, suppressing the synthetic reactions it needs. Synthesis of proteins and nucleic acids of the virus, i.e. its reproduction, separated in time and space, is carried out in different parts of the cell 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 included in the virion (genomic, capsid and supercapsid).

The synthesis of proteins in the cell is carried out in accordance with the processes transcriptions by "rewriting" genetic information from a 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 a nucleic acid sequence of an mRNA is translated into a specific amino acid sequence in a synthesized polypeptide. In this case, there is a discrimination of cellular mRNA and synthetic processes on the ribosomes come under viral control. The mechanisms for transmitting information regarding the synthesis of mRNA are not the same for different groups of viruses.

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

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

The group of RNA-containing retroviruses, which include human immunodeficiency viruses and oncogenic retroviruses, stands apart. They have a unique way of transmitting 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 revertase, with the help of which the reverse transcription process is carried out. 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 is integrated into the cellular genome and, in its composition, is transcribed into mRNA using cellular DNA-dependent RNA polymerase. The synthesis of proteins of these viruses is carried out according to the scheme: genomic RNA of the virus→complementary DNA→mRNA transcription→virus protein translation.

Regulation of transcription is carried out by cellular and virus-specific mechanisms. It consists in sequential reading of information from the so-called. "early" and "late" genes... In the first, information is encoded for the synthesis of virus-specific enzymes of transcription and replication, and in the second, for the synthesis of capsid proteins.

Synthesis of viral nucleic acids, i.e. viral genome replication, leads to the accumulation in the cell of copies of the original viral genomes, which are used in the assembly of virions. The way of replication depends on the type of nucleic acid of the virus, the presence of virus-specific and cellular polymerases, on 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 unweaving the DNA strands, new strands are complementary to them. Each newly synthesized DNA molecule consists of one parental 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, on the original + DNA strand, a –DNA strand is complementary synthesized, 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 intermediate replicative link... It consists of a complete + RNA strand and numerous partially completed –RNA strands. When all the –RNA strands are formed, they are used as templates for the synthesis of new + RNA strands.

Single-stranded RNA viruses are composed of RNA-dependent RNA polymerase. The genomic –RNA strand is transformed by viral polymerase into incomplete and complete + RNA strands. Incomplete copies play the role of mRNA for the synthesis of viral proteins, and full copies serve as 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 the + 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 replicated using viral reverse transcriptase, which synthesizes on the RNA virus template –DNA-strand from which + DNA-strand is copied to form a double DNA strand, closed in a ring. Further, the double strand of DNA integrates with the cell chromosome, forming a provirus. Numerous virion RNAs are formed as a result of transcription of one of the integrated DNA strands with the participation of cellular DNA-dependent RNA polymerase.

The fifth stage - assembly of virions occurs through an orderly 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 replicative complexes... The connection of the virion components is due to the presence of hydrophobic, ionic, hydrogen bonds and stereochemical correspondence.

The formation of viruses is a multistage, 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 replicative 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 replicative complexes, which then interact with modified cell membranes, which are the future lipoprotein membrane of the virion. In this case, the assembly of viruses replicating in the nucleus occurs with the participation of the nuclear membrane, and the assembly of viruses, replication of which occurs in the cytoplasm, is carried out with the participation of the membranes of the endoplasmic reticulum or the cytoplasmic membrane, into which 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. With its 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 case of violations of the self-assembly process, "defective" virions are formed.

Sixth stage - release of viral particles from the cell completes the process of reproduction of viruses 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 emerge from the dead cell at the same time.

Budding or exocytosis , characteristic of complex viruses, the supercapsid of which is derived from cell membranes. First, the nucleocapsid is transported to cell membranes, into which virus-specific proteins are already embedded. In the area of ​​contact, these areas begin to protrude 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 maintain viability for a long time, producing viral offspring.

The 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 with subsequent exit to the cell surface.

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

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

This type of interaction is typical for mild DNA-containing bacteriophages, oncogenic viruses, and some infectious DNA and RNA viruses.

Integration requires the presence of a circular form of double-stranded DNA of the virus. Such DNA attaches to cellular DNA at the site of homology and integrates into a specific region of the chromosome. In RNA viruses, the integration process is more complex and begins with the 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 offspring and can occur under the following conditions:

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

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

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

More often, the abortive type of interaction is observed when an insensitive cell is infected with a standard virus. Moreover, 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 cells to initiate the translation of viral mRNA, with the absence of specific proteases or nucleases necessary for the synthesis of viral macromolecules.

Changes in the conditions under which viruses reproduce can also lead to an 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 replication of the second virus within the same cell. One of the mechanisms is associated with inhibition of the adsorption of another virus by blocking or destroying specific receptors. Another mechanism is associated with inhibition of translation of mRNA by any heterologous mRNA in an infected cell.

Homologous interference typical for many defective viruses, especially for re-passaged in vitro and with a high multiplicity of infection. Their reproduction is possible only when a cell is infected with a normal virus. Sometimes the defective virus can interfere with the reproductive cycle of the normal virus and form defective interfering viral particles (ID). CI particles contain only part of the genome of a normal virus. By the nature of the defect, CI particles are deletional and can be considered as lethal mutants. The main property of DI particles is the ability to interfere with a normal homologous virus and are even capable of playing the role of replication assistants. The ability to adsorb and penetrate the cell is associated with the normal structure of the capsid. The release and expression of defective nucleic acid leads to various biological effects: it inhibits synthetic processes in the cell, due to homologous interference it inhibits the synthesis and transformation of proteins of normal viruses. Circulation of DI particles and coinfection with a normal homologous virus causes the appearance of sluggish, 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 spectrum of tropism and reproduce in a wide variety of cells and organs.

The factors of specificity of the virus, which are responsible for its tropism and the variety of cells affected, 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 rabies virus envelope G-protein has a high affinity for the acetylcholine receptors of neurons, 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, the reproduction of the pathogen makes them targets for cytotoxic immune responses.

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

Equally important are features of a susceptible macroorganism.

Age is about

Viruses are the smallest of all microorganisms. For them, the measurement is taken in millimicrons and in angstroms. Several methods are used to determine these particle sizes. So, a suspension of viruses is passed through special filters from collodion, which have very small pores of a certain size. Filtration is carried out through several filters with different pore sizes. The difference between the pore diameters of the last filter that allowed viral particles to pass and the filter that no longer passed viral particles indicates the average size of the viral particles. With ultra-high-speed centrifugation (50 and more thousand revolutions per minute), the size of viral particles is determined by a special formula, depending on the number of revolutions and the settling time of the particles. At the same time, the virus is purified from foreign substances. To do this, select such speeds at which foreign particles fall out, first large, and then the smallest. At the highest speed, only virus particles are obtained.

Man saw viruses only after 1940, when the electron microscope was built and improved. With an increase of tens and hundreds of thousands of times, it was possible to study the shape, size, and structure of the particles of some viruses.

It was found that both the size and the shape of individual individuals (elementary particles) of different types of viruses are quite diverse. There are large viruses (for example, the virus of psittacosis, smallpox, trachoma, etc.), medium-sized viruses (influenza, plague, rabies) and small (polio virus, measles, foot and mouth disease, encephalitis, viruses of many plants). The table shows the sizes of some viruses, determined by different methods, in millimicrons (according to V.M. Zhdanov and Shen).

The largest viruses are close in size to the smallest bacteria, and the smallest viruses are close to large protein molecules.

In appearance, some viruses are spherical (influenza virus), others are cuboid (smallpox virus), and still others are bacillus. The tobacco mosaic virus (TMV) has the form of a thin hexagonal rod 300 mm long and 15 mm in diameter.

In many viral infections (smallpox, rabies, trachoma, etc.), special intracellular bodies - inclusions, specific to each infection, are observed in the cytoplasm or nucleus of the host's cell. They are quite large and can be seen with a light microscope.

In most cases, inclusions are an accumulation of elementary bodies, viral particles, as it were, their colony. Their presence in cells helps in the diagnosis of certain diseases.

One of the peculiar properties of many plant viruses is their ability to form crystals. DI Ivanovsky was the first to observe in the leaves of tobacco affected by TMV inclusions, now called Ivanovsky crystals. They are composed of the elementary particles of the tobacco mosaic virus. Virus crystals can be dissolved, as sugar and salt dissolve. This virus can be isolated from solution in an amorphous, non-crystalline state. The precipitate can be redissolved, then converted back to crystals. If a crystalline virus is dissolved a thousand times, then a drop of such a solution will cause a mosaic disease in the plant. Poliomyelitis virus crystals have been obtained from human and animal viruses. Each crystal is made up of millions of viral particles.

The chemical composition of viruses was studied primarily in the causative agent of tobacco mosaic. This virus is a pure nucleoprotein, that is, it consists of protein and nucleic acid. The viral nucleoprotein of tobacco mosaic has a huge molecular weight (40-50 million).

A viral particle has a complex structure. Nucleic acid is located inside the viral particle, it is surrounded by a protein envelope. A viral particle usually contains one nucleic acid molecule.

Plant viruses contain ribonucleic acid, phages contain deoxyribonucleic acid. Viruses of humans and animals contain either RNA or DNA. RNA is contained in influenza viruses (1.6%), poliomyelitis (24%), tobacco necrosis (18%), tobacco mosaics (6%), foot and mouth disease (40%), Rous sarcoma (10%), etc. DNA is contained in vaccinia viruses (6%), papilloma (6.8%), herpes (3.8%), polyoma (12%), etc.

Now the question of how protein and nucleic acid combine, how they fit to each other is being intensively studied. To resolve this issue, use the method of X-ray crystallography. If there are subunits in the viral particle, then this method can establish their number, as well as their relative position. It turned out that most viruses are characterized by a regular, highly ordered arrangement of the elements of the viral particle.

In the poliomyelitis virus, the nucleic acid is coiled into a ball, the protein envelope consists of 60 identical subunits, which are combined into 12 groups, 5 subunits each. The virus particle is spherical.

The nucleic acid of the tobacco mosaic virus is in the form of a coil or spring. The TMV protein coat also consists of separate protein subunits of the same shape and size. There are a total of 2,200 subunits arranged in 130 turns around the nucleic acid rod. The molecular weight of such a subunit is 18,000. Each subunit is a peptide chain containing 158 specific amino acids, and the sequential arrangement of these amino acids has already been determined. At present, the sequence of the arrangement of 6500 nucleotides forming a nucleic acid is being intensively studied. When this problem is solved, then the plan will become known, which determines the type of virus that forms in the infected cell. Other small plant viruses have a structure similar to TMV and poliomyelitis particles.

Larger viruses, in addition to the nucleic acid, the protein envelope, also have outer envelopes containing proteins, lipoids, and carbohydrates. Some viruses contain enzymes. So, the influenza virus has the enzyme neuraminidase, the parainfluenza virus - sendai-lysine, the avian myeloblastosis virus contains adenovine triphosphatase. These enzymes dissolve the cell membrane for the virus to enter the body of its future host.

In a free state, in the external environment outside a living cell, viruses do not show activity, they only retain their viability, sometimes for a long time. But as soon as viruses meet with cells sensitive to them, they become active, take root in them and show all signs of vital activity.

The only method for studying the vital activity of viruses used to be infecting susceptible experimental animals: mice, rabbits, monkeys, etc. It is more convenient and economical to grow viruses in a developing embryo of a hen's egg. The material containing the virus is injected with a syringe into the embryo on the 8-12th day of its development. After a few days of the embryo being in the thermostat, pathological changes caused by the virus in the embryo are studied. Then they are subcultured into a fresh embryo of another egg. Recently, the method of single-layer cultures from isolated cells of animal tissues has received the greatest application. Crushed fresh tissue is treated with the enzyme trypsin, which destroys intercellular connections. The released cells are washed from trypsin, diluted with a nutrient composition (No. 199, containing the necessary amino acids and salts) and placed in test tubes or in special flat dishes. In a thermostat, cells multiply, forming a single layer on the glass. Then this culture of homogeneous cells is infected with a virus and the processes occurring in it are studied under a microscope or in other ways. So the laborious and expensive method, for example, the culture of the poliomyelitis virus in monkey liver, has been replaced by a rapid method of growing it in tissue culture.

In 1955 and later, unusual facts were obtained that caused bewilderment among scientists biologists. The tobacco mosaic virus was chemically separated into its constituent parts: protein and nucleic acid. Each of them individually did not cause mosaic disease in tobacco leaves. But when they were again combined together in a test tube (10 parts of protein and 1 part of nucleic acid) and infecting tobacco leaves with this mixture, we got a typical mosaic on the leaves, as from the original whole TMV. Electron microscopy revealed typical rods of the virus, consisting of a protein envelope, in which a nucleic acid strand was enclosed. Thus, the nucleic acid binds to the protein portion and takes its normal position in it. The discovery of this phenomenon - resiitisation (recovery) of the virus - is the largest achievement of modern microbiology, opening up new avenues in biology and medicine.

Further, it turned out that it was enough to rub a tobacco leaf with only one nucleic acid isolated from TMV in a soft way, as typical necrosis appeared on the leaf (of course, not in large quantities), in which there were a huge amount of typical whole viral particles.

The same results were obtained with human viruses: poliomyelitis, influenza, etc.

Even a hybrid tobacco mosaic virus was obtained from the protein of one type of virus and RNA of another type of virus, which differed in some characteristics from the virus of the first type. When this hybrid virus multiplied, it produced only the offspring of the virus whose RNA the hybrid contained.

All these facts indicate that nucleic acids play a leading role in the multiplication of viruses and their infectivity. Nucleic acids provide the transmission of hereditary properties. Acids contain hereditary information on the synthesis of high-grade viral particles inside the cell.

The protein envelope of the virus has a protective function, protecting the fragile nucleic acid strand from external influences, in addition, it helps the virus to enter the cell, and determines the specificity of viruses. But some scientists do not consider it possible to limit the value of proteins in this way. Further research is needed on the role of viral proteins.

The process of reproduction of viruses is fundamentally different from the process of reproduction of bacteria, protozoa and other cellular organisms.

There are four phases of this process: the attachment of viral particles to the host cell, the penetration of the virus into the cell, intracellular reproduction of the virus, and the release of new virus particles from the cell.

The first phase - the attachment, or adsorption, of the virus to the cell - has been studied in relation to influenza and polio viruses. The cell wall has a mosaic structure, in some places mucoprotein molecules protrude, in others lipoprotein molecules. The influenza virus is adsorbed on mucoproteins, and the poliomyelitis virus is adsorbed on lipoproteins. Adsorption can be observed with an electron microscope. At the site of adsorption of the virus, a depression is formed on the cell wall, where a particle of the virus is drawn in. The edges of the groove are closed, and the virus particle is inside the cell (viropexis). Simultaneously with viropexis, the destruction of the protein envelope of the virus occurs. The penetration of the influenza virus into the cell is facilitated by an enzyme of its membrane. Thus, the nucleic acid, freed from the protein membranes by the enzymes of the cell itself, enters the cell.

In the third phase, the viral nucleic acid that has penetrated into the cell is included in the metabolism of the cell and directs the cell synthesis apparatus for the production of protein and nucleic acid, not of the cell, but of new viral particles. The activity of enzymes involved in the synthesis of the virus is activated, and the rest of the enzymes is inhibited. In addition, new enzymes are created that the cell did not have, but which are necessary for the synthesis of viral particles. It can be assumed that at this time a new unified system virus-cell is organized, switched to the synthesis of viral material. At the beginning of this phase, it is not possible to distinguish any elements of the virus in the cell.

Usually, the nucleic acids and proteins of the virus are not synthesized simultaneously and in different parts of the cell. First, the synthesis of nucleic acid begins, and then, a little later, protein synthesis takes place. After the accumulation of these constituent parts of the virus, they are combined, assembled into full-fledged viral particles. Sometimes incomplete viral particles are formed, devoid of nucleic acid and therefore incapable of self-production (bagels).

The last phase quickly sets in - the release of viral particles from the cell. In any place of the cell, about 100 particles of the virus are released at once.More complex viruses also have the outer envelopes of the viral nucleoprotein, which they envelop during their passage through the cell and exit from it, while the host cell proteins are included in the outer membranes.

In viruses of humans and animals, the release of new offspring occurs in several cycles. So, in the influenza virus, each cycle lasts 5-6 hours with the release of 100 or more viral particles of one cell, and a total of 5-6 cycles are observed within 30 hours. After that, the cell's ability to produce the virus is depleted and it dies. The whole process of reproduction of the parainfluenza virus Sen dai from adsorption to exit from the cell lasts 5-6 hours.

Sometimes virus particles do not leave the cell, but accumulate in it in the form of intracellular inclusions, which are very characteristic of different types of viruses. Plant viruses form crystalline inclusions.

The family of microbes, which received the name "mycoplasma", is beginning to attract much attention, since recently causative agents of various diseases of humans and animals have been found in this group. As a latent infection, they often live in many tissue cultures - Hela and others. Mycoplasmas occupy an intermediate position between bacteria and viruses. Filterability through bacterial filters brings them closer to viruses, filterable forms are capable of self-reproduction, intracellular reproduction. The signs that bring viruses closer to bacteria include the ability to grow on nutrient media, form colonies on them, as well as the attitude towards antibiotics, sulfamides and their antigenic structure.

After the discovery and study of microorganisms, scientists believed that bacteria are the most primitively organized living creatures in which the limit of the simplicity of organizing life has been reached. However, at the end of the 19th century, other, more primitive organisms were discovered, called viruses (from Latin virus - poison).

A single individual, a separate viral particle, is called virion... According to the external form of the virion, viruses are divided into four groups:

Spherical (influenza virus, etc.);

Rod-shaped (tobacco mosaic disease virus);

Cuboidal (variola virus, adenoviruses);

Sperm (bacteriophages).

The virion consists of a centrally located nucleic acid (DNA or RNA) or a corresponding nucleoprotein surrounded by one or two envelopes (Fig. 20).

Rice. 20. Schematic structure of simple (A) and complex (B) viruses.

The first shell, in which the nucleic acid is enclosed, was called capsid (from the Greek capsa - box). The capsid protects the nucleic acid from external influences and consists of separate, repetitive and strictly ordered, protein subunits - capsomeres (from the Greek capsa - capsule and meron - part). The number of capsomeres in the capsid of this virus is constant (for poliomyelitis virus - 60, adenovirus - 252, tobacco mosaic virus - 2000, etc.).

The structure containing the nucleic acid and the capsid is called the nucleocapsid. A virion can have a single nucleocapsid (simple viruses) or a nucleocapsid coated with an outer lipid-containing envelope (complex viruses).

The outer shell (supercapsid) consists of a bilayer lipid or protein membrane. Virus-specific carbohydrate-containing proteins - glycoproteins - are immersed in the supercapsid, which form protrusions on the shell - thorns.

Capsomeres are arranged in a certain symmetrical order. Depending on the packing of capsomeres, viruses are divided into three groups:

1. viruses with a spiral-symmetric structure;

2. viruses with cubic symmetry;

3. viruses with combined symmetry.

Helically symmetric viruses are composed of nucleic acid coiled in a coiled coil and surrounded by closely adjacent capsomeres. Such viruses have a tubular nucleocapsid shape (Fig. 21). These include the tobacco mosaic virus.

Rice. 21. Schematic representation of the tobacco mosaic virus

A-Schematic representation of the surface of the tobacco mosaic virus

by Franklin (1956). Seven virions are arranged in a tight package.

B - Scheme of the structure of the tobacco mosaic virus according to Fraenkel - Conrat (1972).

1-axis of the spiral; 2 - nucleic acid molecule; 3 -protein subunits (capsomeres).

Viruses with cubic symmetry (Fig. 22) have a capsid in the form of an icosahedron (twenty-sided), inside which is located a nucleic acid (picornaviruses) or a nucleoprotein (adenoviruses, herpes viruses).

Rice. 22. Scheme of the virion in the form of an icosahedron.

Viruses with a combined type of symmetry (viruses of leukemia, sarcoma, bacteriophages) have a nucleocapsid, which is characterized by cubic symmetry, and the nucleoprotein located inside is folded spirally (Fig. 23).

Rice. 23. The structure of the bacteriophage.

A - phage particle; B - penetration of the phage into the cell;

B - a diagram of the structure of a phage particle

1- phage head; 2 - nucleic acid molecule; 3 - neck and collar;

4 - rod; 5 - shell; 6 - end plate; 7 - tail filaments.

Unlike all other organisms, viruses always contain only one type of nucleic acid: DNA or RNA. Depending on the type of nucleic acid, viruses are divided into DNA viruses or DNA genomic and RNA viruses or RNA genomic.

Virus sizes range from 20 nm to 350 nm. Their value is determined by filtration through ultrafilters with a known pore size, ultracentrifugation, diffusion and photographing in an electron microscope.

Viruses, unlike all pro- and eukaryotic organisms, are not able to reproduce by binary fission. Reproduction of viruses is carried out by their reproduction in the host cell, which proceeds in several stages (Fig. 24):

1. Adsorption of the virus on the surface of the host cell. This stage is a physicochemical process that depends on the difference in charges and other forces of intermolecular attraction. Virus adsorption is characterized by a pronounced specificity, which is determined by the receptor apparatus of the cell and the surface structure of the virus.

2. Penetration of viruses into the cell. This occurs by swallowing the virus by the cell membrane, or by fusing the cell and viral membranes, or by piercing the cell membrane and injecting nucleic acid into the interior.

3. Liberation of the nucleic acid from the capsid. In this case, the capsid of the virus is destroyed by the enzymes of the host cell, and the nucleic acid passes through the cytoplasm of the cell to the region of the nucleus.

4. Synthesis of virus components. This synthesis takes place inside the host cell at the expense of its resources according to the information of the viral nucleic acid.

5. After the accumulation of the necessary components of nucleic acids and virus proteins, the assembly of virions begins. It goes in strict order according to the genetic information of the viral nucleic acid.

6. Completion of the process of virus reproduction after the formation of 100 to 200 full-fledged virions. In this case, the host cell is completely destroyed and the virions come out.

Rice. 24. Virus life cycle

Stage 1 - attachment (adsorption),

Stage 2 - penetration (injection of a capsid with viral DNA),

Stage 3 - the release of the nucleic acid from the capsid,

Stage 4 - synthesis of viral particles,

Stage 5 - assembly of virions,

Stage 6 - destruction of the host cell and release of virions