But viruses cannot be too dangerous for their host. Otherwise, this can lead to the complete disappearance of the donor organism, which means that the pathogen will also be destroyed. But viruses cannot be too weak. If immunity develops too quickly in the host’s body, they will disappear as a species. It often happens that these microorganisms have one host, within which they live, without causing trouble to the latter, and at the same time have a pathogenic effect on other living beings.

They reproduce by reproduction. This means that their nucleic acids and proteins are reproduced first. And then viruses are assembled from the created components.

Types of virions and routes of infection

Before understanding how viruses multiply in a cell, we need to understand how these particles get there. For example, there are infections that are spread exclusively by humans. These include measles, herpes and partly influenza. They are transmitted by contact or airborne droplets.

Enteroviruses, reoviruses, and adenoviruses can enter the body through food. You can become infected, for example, with papillomavirus through direct contact with a person (both domestic and sexual). But there are other ways of infection. For example, some types of rhabdoviruses can be contracted through the bite of blood-sucking insects.

There is also a parenteral route of infection. For example, the hepatitis B virus can enter the human body during surgical procedures, dental procedures, blood transfusions, pedicures or manicures.

We should not forget about the vertical transmission of infections. In this case, when the mother becomes ill during pregnancy, the fetus is affected.

Description of viruses

For quite a long time, the causative agents of most diseases were judged only on the basis of the pathogenic effect on the body. Scientists were able to see these pathogenic organisms only when the electron microscope was invented. Then it was possible to find out how viruses reproduce.

These microorganisms vary markedly in size. Some of them are similar in size to small bacteria. The smallest ones are close in size to protein molecules. To measure them, a conventional value is used - a nanometer, which is equal to one millionth of a millimeter. They can be from 20 to several hundred nanometers. In appearance they look like sticks, balls, cubes, threads, polyhedrons.

Composition of microorganisms

To understand how viruses reproduce in cells, you need to understand their composition. Simple ones consist of nucleic acid and proteins. Moreover, the first component is a carrier of genetic data. They consist of only one type of nucleic acid - it can be DNA or RNA. Their classification is based on this difference.

If inside the cell viruses are components of a living system, then outside they are inert nucleic proteins called virions. Proteins are their essential components. But they differ for different types of viruses. Thanks to this, they can be recognized using specific immunological reactions.

Scientists have discovered not only simple viruses, but also more complex organisms. They may also include lipids and carbohydrates. Each group of viruses has a unique composition of fats, proteins, carbohydrates, and nucleic acids. Some of them even contain enzymes.

Beginning of the breeding process

You can understand how this process occurs if you consider in detail how the microorganism penetrates the cell and what happens in it after that. Virions can be thought of as a particle consisting of DNA (or RNA) enclosed in a protein sheath. Reproduction of viruses begins only after the microorganism attaches to the cell wall, to its plasma membrane. It should be understood that each virion can attach only to certain types of cells that have special receptors. Hundreds of viral particles can be located on one cell.

After this, the process of viropexis begins. The cell itself draws in the attached virions. Only after this does the “undressing” of viruses begin. With the help of a complex of enzymes entering the cell, the protein shell of the virus dissolves and the nucleic acid is released. It is she who passes through the channels of the cell into its nucleus or remains in the cytoplasm. Acid is responsible not only for the reproduction of viruses, but also for their hereditary characteristics. The cells’ own metabolism is suppressed, and all efforts are directed towards creating new components of viruses.

Composition process

Incorporated into the DNA of the cell. Multiple copies of viral DNA (RNA) begin to be actively created inside, this is done using polymerases. Some of the newly created particles connect to ribosomes, and the process of synthesis of new viral proteins also takes place there.

Once a sufficient number of virus components have been accumulated, the composition process begins. It passes near The essence of it is that new virions are assembled from components. This is how viruses reproduce.

Particles of the cells in which they were located can be detected in the newly formed virions. Often the process of their formation ends with them being enveloped in a cell membrane layer.

Completion of reproduction

Once the composition process is completed, the viruses leave their first host. The formed offspring leaves and begins to infect new cells. Viruses reproduce directly in cells. But in the end they are completely destroyed or partially damaged.

Having infected new cells, viruses begin to actively multiply in them. The cycle of reproduction repeats. How the process of releasing the created virions will proceed depends on the group of viruses to which they belong. For example, enteroviruses are characterized by the fact that they are rapidly released into the environment. But herpes agents, reoviruses, orthomyxoviruses come out as they mature. Before dying, they can go through several cycles of such reproduction. At the same time, cellular resources are depleted.

Diagnosis of diseases

Reproduction in some cases is accompanied by the fact that particles of pathogenic microorganisms can accumulate inside cells, forming crystal-like clusters. Experts call them inclusion bodies.

For example, with influenza, smallpox or rabies, such accumulations are found in the core. In spring-summer encephalitis, they are found in the core, and with other infections they can be both here and there. This sign is used to diagnose diseases. In this case, it is also important where exactly the process of virus reproduction occurs.

For example, when oval or round formations are detected in epithelial cells, they speak of smallpox. Cytoplasmic accumulations in brain cells indicate rabies.

The way viruses reproduce is very specific. First, virions enter the cells that suit them. After this, the process of releasing nucleic acids and creating “blanks” of parts for future pathogenic microorganisms begins. The reproduction process ends with the formation of new virions that are released into the environment. It is enough to disrupt one of the stages of the cycle so that the reproduction of viruses is stopped or they begin to produce inferior offspring.

creative work

Virus propagation method

A virus (from Latin virus - poison) is a microscopic particle that can infect the cells of living organisms.

Virology (from virus and logos - word, doctrine), the science of viruses. General virology studies the nature of viruses, their structure, reproduction, biochemistry, and genetics.

The method of reproduction of viruses also differs from the division, budding, sporulation or sexual process that occurs in unicellular organisms, in cells of multicellular organisms and in the latter in general. Reproduction, or replication, is the common term for the reproduction of viruses. The formation of virions occurs either by self-assembly (packaging of viral nucleic acid into protein capsids and formation of a nucleocapsid), or with the participation of the cell, or both (enveloped viruses). Of course, the opposition between mitotic cell division and replication is not absolute, since the methods of replication of genetic material in DNA-containing viruses are not fundamentally different, and if we take into account that the synthesis of genetic material in RNA-containing viruses is also carried out according to the template type, then the opposition is relative mitosis and replication of all viruses. And, nevertheless, the differences in the methods of reproduction of cells and viruses are so significant that the entire living world can be divided into viruses and non-viruses.

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Mechanisms of virus penetration into the host cell

Mechanisms of virus penetration into the host cell

To reproduce its own kind, the virus uses the protein synthesizing system of the host cell, that is, it needs to penetrate deep into the cell. First, the virus interacts with a surface on which there are special receptor sites. On its shell there are corresponding attachment proteins that react with these areas. Therefore, viruses are highly specific and infect only a certain type of cell of a certain type of organism. The presence of such receptor sites determines their sensitivity to a particular type of virus.

The virus can attach to other places on the cell surface, then infection may not occur.

In simple viruses, attachment proteins are contained in the protein shell. In complex viruses, they are on the surface of an additional membrane and have the shape of spikes, needles, etc.

Pathways for viruses to enter cells

There are several ways for viruses to enter a cell:

1. Viral envelopes can fuse with the cell membrane (eg, influenza virus).
2. The virus enters the cell by pinocytosis. In this case, enzymes of the host cell break down its membrane and release nucleic acid (for example, animal polio virus).
3. Through damaged areas of the cell wall of plant cells. Then they pass from one cell to another along cytoplasmic bridges.

Bacteriophages have a complex mechanism for the penetration of viruses into bacterial cells. With the help of tail filaments, they connect to the receptor sites of bacterial cells. After attachment, for example, the T4 bacteriophage, due to contraction of the sheath, injects a nucleic acid molecule deep into the cell through a hollow rod. The empty shell of the bacteriophage remains outside.

Reproduction of viruses and variants of virus residence in a cell

Viruses that enter a cell can immediately or later become active, that is, remain dormant for a certain time. The following options for the presence of viruses in a cell are possible:

1. Lytic infection(from Greek lysis– destruction, dissolution) – the formed viruses simultaneously leave the cell, tearing it apart. The cell dies.
2. Persistent(persistent) – new viruses emerge gradually. The cell lives and divides, although its functioning may change.
3. Latent(hidden) - the genetic material of the virus is integrated into the DNA of the chromosomes, and during cell division it is transmitted to daughter cells. The process of inserting nucleic acid into the DNA of the host cell chromosome is called integration. The integrated nucleic acid of a virus in the genome of the host cell, formed from RNA thanks to enzymes, is called provirus. Various factors (physical, biological and chemical nature) can activate the provirus. A lytic or persistent infection then develops.

Viral replication involves three main processes: viral nucleic acid replication, viral protein synthesis, and virion assembly. In the host cell, the nucleic acid of the virus transmits hereditary information about viral proteins to the protein synthesizing apparatus of the cell.

In viruses that contain mRNA, it immediately binds to the host's ribosomes and causes the synthesis of viral proteins. In other viruses, mRNA is synthesized from the RNA or DNA of the virus.

Some RNA viruses (for example, HIV, the human immunodeficiency virus) can cause DNA synthesis in the cell nucleus. Viral mRNA is synthesized from a DNA molecule. It's called a phenomenon reverse replication.

Some viruses (for example, some bacteriophages and viruses that cause certain cancers) integrate viral DNA into the DNA of host cells. If synthesis occurs, it is in combination with cell proteins. Viral proteins change the properties of cells and do not kill them. Thus, cancer cells divide frequently. Their number increases unlimitedly.

With the help of their own metabolic products, viruses suppress the synthesis of host cell proteins. Translation of information from DNA to mRNA of the cell stops, and the synthesis of the virus’s own proteins is stimulated. In this case, the protein synthesizing apparatus of the host cell and its energy resources are used. Virus nucleic acid molecules double. The viral nucleic acid is housed in a shell of viral protein synthesized by the cell.

Options for releasing viruses from host cells

There are several known options for releasing viruses from host cells:

1. For the most part, viruses destroy the host cell membrane, exit and penetrate other cells (for example, bacteriophages).
2. Complex viruses bud outward from the cell.
3. New generations of viruses stay for a long time in the host cell, which remains alive until its energy and biochemical resources are exhausted.

The process of virus reproduction is conventionally divided into 5 stages (Steinier, Edelberg, Ingram, 1979):

penetration into the host cell;

enzyme synthesis;

synthesis of virus components;

assembly of virus components to form mature virions;

release of mature virions from the host cell.

It has been established that the processes of virus penetration into a cell are different for bacterial, plant and animal viruses. Thus, if animal viruses are adsorbed directly on the host cell membrane, then bacterial and plant viruses must pass through the cell wall. At the same time, plant viruses do not have a special apparatus to overcome the cell wall, so they can only enter the plant through various wounds. The leaves and roots of plants often have tiny mechanical wounds through which tobacco mosaic viruses, potato virus X, etc. penetrate. However, most viruses enter plants through vectors, which are often insects with sucking mouthparts (aphids, leafhoppers) ), as well as mites, phytonematodes, and fungi. The penetration process is completed by the removal of the protein capsid (undressing of the virion) and the appearance of free viral nucleic acid inside the cell, which leads to the synthesis of virus-specific proteins and replication of the viral nucleic acid itself.

4. Transmission of plant viruses

Plant viruses can be transmitted from one plant to another only through cell sap. Sources of infection and methods of transmission can be different: mechanical transmission by sap from a diseased plant to a healthy one; transmission through soil or through seeds and pollen; transmission by vectors: insects, mites, nematodes, fungi (Gibbs, Harrison, 1978).

Transmission by mechanical contact is extremely rare, for example, when touching the leaves of healthy plants, damage to the edges of the leaves and leaf hairs occurs with the leaves of plants infected with viruses. In this case, the juice secreted by infected plants penetrates the wounds of healthy plants and thus infects them. Sometimes virus infection occurs when healthy plant roots come into contact with infected ones underground. In tree species, the roots of neighboring plants sometimes grow together. Transmission of viruses through soil involves the movement of free viral particles by a current of soil solution. Such viruses enter the soil after the decomposition of nutrient residues. Under hydroponic culture conditions, plants can release free viruses from the roots into the substrate, which infect healthy plants with a flow of nutrient solution (Minkevich, 1984). Viruses are not commonly thought to be transmitted through seeds and pollen, but there are at least thirty viruses that infect plants in this way. Moreover, according to A. Gibbs and B. Harrison (1978), the possibility of such transmission depends on many factors: temperature, host genotype, time of infection. Plants are more successfully infected at moderate temperatures than at very high or very low temperatures. The efficiency of virus transmission depends on the relationship between the moment of infection and the time of flowering, as well as on the location of the flowers on the plant. Most pollen-borne viruses are unable to infect plants once the flowers have already been pollinated.

Viruses can also be transmitted through vegetative parts and organs of plants: tubers, roots, cuttings and layering. However, most often viruses are transmitted using vectors, which are insects, mites, nematodes, and fungi. The virus remains in the carrier’s body in an infectious form for a certain time. The condition in which the vector remains infective after leaving the infected plant is called persistence. There are three main types of persistence: non-persistence, semi-persistence and persistence. Non-persistence means that the vector remains infective for several hours (up to four);

Semi-persistence observed when the vector remains infective for 10-100 hours:

Persistence– when the vector remains infective for more than 100 hours, and sometimes throughout its life. Among insects, aphids play the main role as virus carriers. The fact is that their mouthparts are very well adapted for inoculating plants. Aphids have very thin stylets with which they pierce plant tissue without gross damage, which contributes to the success of infection. In addition to aphids, the second most important group of virus carriers are leafhoppers, lightbearers and humpbacks. The viruses carried by these insects most often cause yellowing or curling of plant leaves. It has been established that these vectors feed mainly on the phloem of plants, so viruses are concentrated mainly in the phloem.

Whiteflies can be carriers of a number of viruses, especially in regions with hot climates. Just like leafhoppers, they feed mainly on phloem, so their larvae are sedentary. Most often, whiteflies are carriers of viruses that cause mosaics and deformities.

Among beetles, leaf beetles are more often carriers of viruses, and weevils are less common. Viruses transmitted by these insects cause mosaic and mottling. The beetles acquire the virus within 5 minutes, and healthy plants can become infected with the virus either immediately after ingestion by their vectors, or the next day. Viruses can remain in beetles for days or weeks

Insects and some other groups are also involved in the transmission of viruses, but only a small number of vectors have been identified for each such group. Mites can also be carriers of viruses, although their range of host plants is quite limited. Viruses transmitted by mites cause currant reversion, peach mosaic, fig mosaic, and rose rosette diseases. Mites have thin stylets that pierce plant cells. Mites are more often spread from plant to plant by wind.

The process of viral reproduction can be roughly divided into 2 phases . The first phase includes 3 stages: 1) adsorption of the virus on sensitive cells; 2) penetration of the virus into the cell; 3) deproteinization of the virus . The second phase includes the stages of implementation of the viral genome: 1) transcription, 2) translation, 3) replication, 4) assembly, maturation of viral particles and 5) virus exit from the cell.

The interaction of a virus with a cell begins with the adsorption process, i.e., with the attachment of the virus to the cell surface.

Adsorption is a specific binding of the virion protein (antireceptor) to the complementary structure of the cell surface - the cell receptor. According to their chemical nature, the receptors on which viruses are fixed belong to two groups: mucoprotein and lipoprotein. Influenza viruses, parainfluenza, and adenoviruses are fixed on mucoprotein receptors. Enteroviruses, herpes viruses, arboviruses are adsorbed on lipoprotein receptors of the cell. Adsorption occurs only in the presence of certain electrolytes, in particular Ca2+ ions, which neutralize excess anionic charges of the virus and cell surface and reduce electrostatic repulsion. Adsorption of viruses depends little on temperature. The initial processes of adsorption are nonspecific in nature and are the result of electrostatic interaction of positively and negatively charged structures on the surface virus and cell, and then a specific interaction occurs between the virion attachment protein and specific groups on the plasma membrane of the cell. Simple human and animal viruses contain attachment proteins as part of the capsid. In complex viruses, attachment proteins are part of the supercapsid. They can take the form of filaments (fibers in adenoviruses), or spikes, mushroom-like structures in myxo-, retro-, rhabdo- and other viruses. Initially, a single connection of the virion with the receptor occurs - such attachment is fragile - adsorption is reversible. For irreversible adsorption to occur, multiple connections must appear between the viral receptor and the cell receptor, i.e., stable multivalent attachment. The number of specific receptors on the surface of one cell is 10 4 -10 5. Receptors for some viruses, for example, arboviruses. are contained on the cells of both vertebrates and invertebrates; for other viruses only on the cells of one or more species.

Penetration of human and animal viruses into cells occurs in two ways: 1) viropexis (pinocytosis); 2) fusion of the viral supercapsid shell with the cell membrane. Bacteriophages have their own penetration mechanism, the so-called syringe, when, as a result of contraction of the protein appendage of the phage, the nucleic acid is injected into the cell.

Deproteinization of the virus, the release of the viral hemome from the viral protective shells occurs either with the help of viral enzymes or with the help of cellular enzymes. The end products of deproteinization are nucleic acids or nucleic acids associated with the internal viral protein. Then the second phase of viral reproduction takes place, leading to the synthesis of viral components.

Transcription is the rewriting of information from DNA or RNA of a virus into mRNA according to the laws of the genetic code.

Translation is the process of translating genetic information contained in mRNA into a specific sequence of amino acids.

Replication is the process of synthesis of nucleic acid molecules homologous to the viral genome.

The implementation of genetic information in DNA-containing viruses is the same as in cells:

DNA transcription mRNA translation protein

RNA transcription i-RNA translation protein

Viruses with a positive RNA genome (togaviruses, picornaviruses) lack transcription:

RNA protein translation

Retroviruses have a unique way of transmitting genetic information:

RNA reverse transcription DNA transcription mRNA translation protein

The DNA integrates with the genome of the host cell (provirus).

After the cell has accumulated viral components, the last stage of viral reproduction begins: the assembly of viral particles and the release of virions from the cell. Virions exit the cell in two ways: 1) by “exploding” the cell, as a result of which the cell is destroyed. This path is inherent in simple viruses (picorna-, reo-, papova- and adenoviruses), 2) exit from cells by budding. Inherent in viruses containing a supercapsid. With this method, the cell does not die immediately and can produce multiple viral offspring until its resources are depleted.

Virus cultivation methods

To cultivate viruses in laboratory conditions, the following living objects are used: 1) cell cultures (tissues, organs); 2) chicken embryos; 3) laboratory animals.

Cell culture

The most common are single-layer cell cultures, which can be divided into 1) primary (primarily trypsinized), 2) semi-continuous (diploid) and 3) continuous.

By origin they are classified into embryonic, tumor and from adult organisms; by morphogenesis- fibroblastic, epithelial, etc.

Primary Cell cultures are cells of any human or animal tissue that have the ability to grow in the form of a monolayer on a plastic or glass surface coated with a special nutrient medium. The lifespan of such crops is limited. In each specific case, they are obtained from the tissue after mechanical grinding, treatment with proteolytic enzymes and standardization of the number of cells. Primary cultures obtained from monkey kidneys, human embryonic kidneys, human amnion, and chicken embryos are widely used for the isolation and accumulation of viruses, as well as for the production of viral vaccines.

Semi-leathered (or diploid ) cell cultures - cells of the same type, capable of withstanding up to 50-100 passages in vitro, while maintaining their original diploid set of chromosomes. Diploid strains of human embryonic fibroblasts are used both for the diagnosis of viral infections and in the production of viral vaccines.

Continuous cell lines are characterized by potential immortality and a heteroploid karyotype.

The source of transplantable lines can be primary cell cultures (for example, SOC, PES, BHK-21 - from the kidneys of one-day-old Syrian hamsters; PMS - from the kidney of a guinea pig, etc.) individual cells of which show a tendency to endless reproduction in vitro. The set of changes leading to the appearance of such features from cells is called transformation, and the cells of continuous tissue cultures are called transformed.

Another source of transplantable cell lines is malignant neoplasms. In this case, cell transformation occurs in vivo. The following lines of transplanted cells are most often used in virological practice: HeLa - obtained from cervical carcinoma; Ner-2 - from laryngeal carcinoma; Detroit-6 - from lung cancer metastasis to the bone marrow; RH - from human kidney.

To cultivate cells, nutrient media are required, which, according to their purpose, are divided into growth and supporting media. Growth media must contain more nutrients to ensure active cell proliferation to form a monolayer. Supporting media should only ensure that cells survive in an already formed monolayer during the multiplication of viruses in the cell.

Standard synthetic media, such as synthetic media 199 and Eagle's media, are widely used. Regardless of the purpose, all cell culture media are formulated using a balanced salt solution. Most often it is Hanks solution. An integral component of most growth media is animal blood serum (veal, bovine, horse), without 5-10% of which cell reproduction and monolayer formation do not occur. Serum is not included in the maintenance media.

Isolation of viruses in cell cultures and methods for their indication.

When isolating viruses from various infectious materials from a patient (blood, urine, feces, mucous discharge, organ washings), cell cultures that are most sensitive to the suspected virus are used. For infection, cultures in test tubes with a well-developed monolayer of cells are used. Before infecting the cells, the nutrient medium is removed and 0.1-0.2 ml of a suspension of the test material, pre-treated with antibiotics to destroy bacteria and fungi, is added to each test tube. After 30-60 min. After contact of the virus with cells, excess material is removed, a supporting medium is added to the test tube and left in a thermostat until signs of virus replication are detected.

An indicator of the presence of a virus in infected cell cultures can be:

1) the development of specific cell degeneration - the cytopathic effect of the virus (CPE), which has three main types: round or small cell degeneration; formation of multinucleated giant cells - symplasts; development of foci of cell proliferation, consisting of several layers of cells;

2) detection of intracellular inclusions located in the cytoplasm and nuclei of affected cells;

3) positive hamagglutination reaction (RHA);

4) positive hemadsorption reaction (RHAds);

5) plaque formation phenomenon: a monolayer of virus-infected cells is covered with a thin layer of agar with the addition of a neutral red indicator (background - pink). In the presence of a virus, colorless zones (“plaques”) form on the pink agar background in the cells.

6) in the absence of CPD or GA, an interference reaction can be performed: the culture under study is re-infected with the virus that causes CPD. In a positive case, there will be no CPP (the interference reaction is positive). If there was no virus in the test material, CPE is observed.

Isolation of viruses in chicken embryos.

For virological studies, chicken embryos 7-12 days old are used.

Before infection, the viability of the embryo is determined. During ovoscoping, living embryos are mobile and the vascular pattern is clearly visible. The boundaries of the air sac are marked with a simple pencil. Chicken embryos are infected under aseptic conditions, using sterile instruments, after pre-treating the shell above the air space with iodine and alcohol.

Methods for infecting chicken embryos can be different: applying the virus to the chorion-allantoic membrane, into the amniotic and allantoic cavities, into the yolk sac. The choice of infection method depends on the biological properties of the virus being studied.

Indication of the virus in a chicken embryo is made by the death of the embryo, a positive hemagglutination reaction on glass with allantoic or amniotic fluid, and by focal lesions (“plaques”) on the chorion-allantoic membrane.

III. Isolation of viruses in laboratory animals.

Laboratory animals can be used to isolate viruses from infectious material when more convenient systems (cell cultures or chicken embryos) cannot be used. They take mainly newborn white mice, hamsters, guinea pigs, and rat pups. Animals are infected according to the principle of virus cytotropism: pneumotropic viruses are injected intranasally, neurotropic viruses - intracerebrally, dermatotropic viruses - onto the skin.

Indication of the virus is based on the appearance of signs of disease in animals, their death, pathomorphological and pathohistological changes in tissues and organs, as well as a positive hemagglotination reaction with extracts from organs.