The structure of viruses is noncellular, since they do not have any organelles. In a word, this is a transitional stage between dead and living matter. Viruses were discovered by Russian biologist D.I. Ivanovsky in 1892 while considering the mosaic disease of tobacco. The entire structure of viruses is RNA or DNA enclosed in a protein shell called a capsid. A virion is a formed infectious particle.

Influenza or herpes viruses have an additional lipoprotein envelope, which arises from the cytoplasmic membrane of the host cell. Viruses are divided into DNA-containing and RNA-containing, because they can only have 1 type. However, the overwhelming number of viruses are RNA-containing. Their genomes are single-stranded and double-stranded. The internal structure of viruses allows them to reproduce only in the cells of other organisms, and nothing else. They do not exhibit any extracellular activity at all. The sizes of widespread viruses range from 20 to 300 nm in diameter.

The structure of bacteriophage viruses

Viruses that infect bacteria from the inside are called viruses. They are able to penetrate and destroy.

The body of the E. coli bacteriophage has a head, from which emerges a hollow rod, wrapped in a sheath. At the end of this rod there is a basal plate on which 6 threads are attached. Inside the head is a DNA molecule. With the help of special processes, the bacteriophage virus attaches to the body of the E. coli bacterium. Using a special enzyme, the phage dissolves and penetrates. Next, a DNA molecule is injected from the channel of the rod due to contractions of the head, and literally after 15 minutes the bacteriophage completely changes the metabolism of the bacterial cell in the way it needs. The bacterium stops synthesizing its DNA - it now synthesizes the nucleic acid of the virus. All this ends with the appearance of about 200-1000 individuals of phages, and the bacterial cell is destroyed. All bacteriophages are divided into virulent and moderate. The latter do not replicate in the bacterial cell, while the virulent ones form a generation of individuals in an already infected area.

Viral diseases

The structure and activity of viruses is determined by the fact that they can only exist in the cells of other organisms. Having settled in any cell, the virus can cause serious illness. Agricultural plants and animals are often attacked by them. These diseases sharply worsen the fertility of crops and cause numerous deaths of animals.

There are viruses that can cause various diseases in humans. Everyone knows such diseases as smallpox, herpes, influenza, polio, mumps, measles, jaundice and AIDS. All of them arise due to the activity of viruses. The structure of the smallpox virus is almost no different from the structure of the herpes virus, since they belong to the same group - Herpes Virus, which also includes some others. In our time, the human immunodeficiency virus (HIV) is actively spreading. Nobody knows yet how to overcome it.

Structure and classification of viruses

Viruses include to the kingdomVira . This

tiny microbes (“filterable agents”),

not having a cellular structure, a protein synthesizing system,

They are autonomous genetic structures and are distinguished by a special, disconnected (disjunctive) method of reproduction (reproduction): the nucleic acids of viruses and their proteins are separately synthesized in the cell, then they are assembled into viral particles.

The formed viral particle is called virion.

The morphology and structure of viruses are studied Withusing electron microscopy, since their sizes are small and comparable to the thickness of the bacterial shell.

The shape of virions may varynoah (fig.):

rod-shaped (tobacco mosaic virus),

bullet-shaped (rabies virus),

spherical (poliomyelitis viruses, HIV),

filamentous (filoviruses),

in the form of sperm (many bacteriophages).

The size of viruses is determined by:

With using electronic microscopy,

by ultrafiltration method through filters with a known pore diameter,

method ultracentrifugation.

The smallest viruses are parvoviruses (18 nm) and poliovirus (about 20 nm), the largest is the variola virus (about 350 nm).

There are DNA- and RNA-containing virusessy. They usually haploid, i.e. they have one set of genes. Exception are retroviruses with a diploid genome. The genome of viruses contains from six to several hundred genes and is represented by various typesnucleic acids:

double-stranded,

single-stranded,

linear,

ring,

fragmented.

There are also RNA viruses with negative (minus strand RNA) genemom. The minus strand RNA of these viruses performs only a hereditary function.

There are:

just made viruses (for example, polio viruses, hepatitis A) and

complex viruses (for example, measles, influenza, herpes viruses, coronaviruses).

U simply designed viruses(Fig.) the nucleic acid is associated with a protein shell called capsid(from lat. capsa- case). The capsid consists of repeating morphological subunits - capsomers. The nucleic acid and capsid interact with each other and are collectively called nucleocapsid.

U complex viruses(Fig.) the capsid is surrounded by lipoprotein shellswhoa- supercapsid, or peplos. The virus envelope is a derived structure from the membranes of the virus-infected cell. On the virus shell are located glycoproteother"spikes" or "spikes" (ash meters, or supercapsid proteins). Under the shell of some viruses is M protein.

Thus,just made viruses consist of nucleic acid and capsid.Complex viruses consist of nucleic acid, capsid and lipoprotein shell.

Virions have:

spiral,

icosahedral(cubic) or complex type of symmetry of the capsid (nucleocapsid).

Spiral type symmetry is due to the helical structure of the nucleocapsid (for example, in influenza viruses, coronaviruses). Icosahedral type symmetry is due to the formation of an isometrically hollow body from a capsid containing viral nucleic acid (for example, in the herpes virus).

The capsid and shell (supercapsid) protect virions from environmental influences, determine selective interaction (adsorption) with certain cells, as well as the antigenic and immunogenic properties of virions.

The internal structures of viruses are called gray dcevina. In adenoviruses, the core consists of histone-like proteins associated with DNA, in reoviruses - from proteins of the internal capsid.

The following are used in virology yessonomiccategories :

family (name ends with viridae),

subfamily (name ends in virinae),

genus (name ends in virus).

However, the names of genera and especially subfamilies are not given for all viruses. The virus species did not receive a binomial name, like bacteria.

The basis for the classification of polo viruseswives the following categories:

nucleino typevoic acid (DNA or RNA), its structurenumber of threads (one or two), especiallythe reproducibility of the viral genome(Table 2.3),

size and morphology of virions,number of capsomeres and type of symmetrynucleocapsid, the presence of a shell (supercapsid).

sensitivity to ether and deoxycholate,

breeding site in the cell,

antigenic properties, etc.

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

In addition to ordinary (canonical) viruses, infectious molecules are known that are not viruses and are called prions. Prions- the term proposed by S. Prusiner is an anagram of the English words “infectious protein particle.” The cellular form of normal prion protein (PgRS) is present in the body of mammals, including humans, and performs a number of regulatory functions. It is encoded by the PrP gene, located on the short arm of human chromosome 20. In prion diseases in the form of transmissible spongiform encephalopathy (Creutzfeldt-Jakob disease, kuru, etc.), the prion protein acquires a different, infectious form, designated as PgR & (Sc - from scrapie - scrapie, a prion infection of sheep and goats). This infectious prion protein has the appearance of fibrils and differs from normal prion protein in its tertiary or quaternary structure.

Other unusual agents closely related to viruses are viroids- small molecules of circular, supercoiled RNA that do not contain

3.3. Physiology of viruses

Viruses- obligate intracellular parasites, capable only of intracellular reproduction. In a virus-infected cell, viruses can remain in various states:

reproduction of numerous new virions;

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

existence in the cytoplasm of the cell in the form of circular nucleic acids, reminiscent of bacterial plasmids.

Therefore, the range of disorders caused by the virus is very wide: from a pronounced productive infection ending in cell death, to prolonged interaction of the virus with the cell in the form of a latent infection or malignant transformation of the cell.

Distinguish three types of virus interactionwith cage: productive, abortive and integrative.

1. Productive type - ends with the formation of a new generation of virions and death (lysis) of infected cells (cytolytic form). Some viruses leave cells without destroying them (non-cytolytic form).

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.

Integrative type, or virogeny - characterized by the incorporation (integration) of viral DNA in the form of a provirus into the cell chromosome and their coexistence (co-replication).

Reproduction of viruses (productive)

Productive type of interaction sa with the cell, i.e. reproduction virus (lat. re - repetition, productio - production), takes place in 6 stages:

1) adsorption virions on the cell;

2) penetration virus into a cell;

3) "strip" and release of the viral genome (virus deproteinization);

4) synthesisviral components;

5) formation virions;

6) virion yield from the cell.

These stages differ for different viruses.

Adsorption of viruses. The first stage of viral reproduction is adsorption, i.e., attachment of the virion to the cell surface. It occurs in two phases. The first phase is nonspecific, caused by ionic attraction between the virus and the cell, including other mechanisms. Second phase adsorption - highly specific cheskaya, due to homology and complementarity of receptors of sensitive cells and the viral protein ligands that “recognize” them. Proteins on the surface of viruses that recognize specific cellular receptors and interact with them , are called attach telny proteins (mainly glycoprotein ines) as part of the lipoprotein membrane.

Specific receptors cells have a different nature, being proteins, lipids, carbohydrate components of proteins, lipids, etc. Thus, the receptors for the influenza virus are sialic acid in the composition of glycoproteins and glycolipids (gangliosides) of respiratory tract cells. Rabies viruses are adsorbed on acetylcholine receptors of nervous tissue, and human immunodeficiency viruses are adsorbed on CO4 receptors of T-helpers, monocytes and dendritic cells. One cell contains from ten to one hundred thousand specific receptors, so tens and hundreds of virions can be adsorbed on it.

The presence of specific receptors underlies the selectivity of viruses to damage certain cells, tissues and organs. This is the so-called tropism (Greek tropos - turn, direction). For example, viruses that reproduce primarily in liver cells are called hepatotropic, in nerve cells - neurotropic, in immunocompetent cells - immunotropic, etc.

Penetration of viruses into cells. Viruses enter cells by receptor-dependent endocytosis (viropexis), or fusion of the viral envelope with the cell membrane, or as a result of a combination of these mechanisms.

1 . Receptor-dependent endocytosis occurs as a result of the capture and absorption of the virion by the cell: the cell membrane with the attached virion is invaginated to form an intracellular vacuole (endosome) containing the virus. Due to the ATP-dependent “proton” pump, the contents of the endosome are acidified, which leads to the fusion of the lipoprotein shell of the complex virus with the endosome membrane and the release of the viral nucleocapsid into the cell cytosol. Endosomes combine with lysosomes, which destroy the remaining viral components. The process of release of non-enveloped (simply organized) viruses from the endosome into the cytosol remains poorly understood.

2. Fusion of the virion shell with the cell membranewound characteristic only of some enveloped viruses (paramyxoviruses, retroviruses, herpesviruses), which contain fusion proteins. A point interaction of the viral fusion protein with the lipids of the cell membrane occurs, as a result of which the viral lipoprotein envelope integrates with the cell membrane, and the internal component of the virus enters the cytosol.

A) “Undressing” (deproteinization) of viruses. As a result, its internal component is released, which can cause an infectious process. The first stages of “undressing” of the virus begin during its penetration into the cell through the fusion of viral and cellular membranes or when the virus exits the endosome into the cytosol. The subsequent stages of “undressing” the virus are closely related to their intracellular transport to the sites of deproteinization. Different viruses have their own specialized “undressing” areas in the cell: for picornaviruses, in the cytoplasm with the participation of lysosomes and the Golgi apparatus; for herpes viruses - perinuclear space or pores of the nuclear membrane; for adenoviruses - first the cytoplasmic structures, and then the cell nucleus. The end products of “undressing” can be a nucleic acid, a nucleoprotein (nucleocapsid) or a virion core. Thus, the final product of picarnovirus stripping is a nucleic acid covalently linked to one of the internal proteins. And for many enveloped RNA-containing viruses, the final products of “undressing” can be nucleocapsids or cores, which not only do not interfere with the expression of the viral genome, but, moreover, protect it from cellular proteases and regulate subsequent biosynthetic processes.

B) Synthesis of viral components. Synthesis of proteins and nucleic acids of the virus, which is divided in time and space. Synthesis occurs in different parts of the cell, so this method of virus reproduction is called disJunctive(from lat. disjunctus - disunited).

WITH)Synthesis of viral proteins . In an infected cell, the viral genome encodes the synthesis of two groups of proteins:

1. non-structural proteins, serving the intracellular reproduction of the virus at its different stages;

2. structural proteins, which are part of the virion (genomic proteins associated with the virus genome, capsid and supercapsid proteins).

TOnon-structural white cam include: 1) enzymes for RNA or DNA synthesis (RNA or DNA polymerases), which ensure transcription and replication of the viral genome; 2) regulatory proteins; 3) precursors of viral proteins, characterized by their instability as a result of rapid cutting into structural proteins; 4) enzymes that modify viral proteins, for example, proteinases and protein kinases.

Protein synthesis in the cell is carried out in accordance with well-known processes transcriptions (from lat. transcriptio - rewriting) by “rewriting” genetic information from nucleic acid into the nucleotide sequence of messenger RNA (mRNA) and broadcasts(from lat. translation - transmission) - reading mRNA on ribosomes to form proteins. The transmission of hereditary information regarding mRNA synthesis varies among different groups of viruses.

I . DNA-containing viruses implement genetic information in the same way as like the cellular genome, according to the scheme:

genomicVirus DNA-» transcriptionmRNA-» broadcastvirus protein.

Moreover, DNA-containing viruses use cellular polymerase for this process (viruses whose genomes are transcribed in the cell nucleus - adenoviruses, papovaviruses, herpesviruses) or their own RNA polymerase (viruses whose genomes are transcribed in the cytoplasm, for example poxviruses).

II . Plus-strand RNA viruses (for example, picornaviruses, flaviviruses, then gaviruses) have a genome that performs mRNA function; it is recognized and translated by ribosomes. Protein synthesis in these viruses occurs without the act of transcription according to the following scheme:

genomic RNA virus-> viral protein translation .

III. Minus-single-stranded RNA genome viruses (orthomyxoviruses, paramyxoviruses, rhabdoviruses) and double-stranded (reoviruses) serves as a template from which mRNA is transcribed with the participation of RNA polymerase associated with the nucleic acid of the virus. Their protein synthesis occurs according to the following scheme:

genomic RNA virus-» transcription and- RNA- broadcast virus protein.

IV. Retroviruses (human immunodeficiency viruses, oncogenic retroviruses) have a unique way of transmitting genetic information. The genome of retroviruses consists of two identical RNA molecules, i.e. it is diploid. Retroviruses contain a special virus-specific enzyme - reverse transcriptase, or revertase, with the help of which the process of reverse transcription is carried out, i.e., complementary single-stranded DNA (cDNA) is synthesized on the genomic RNA matrix. The complementary strand of DNA is copied to form double-stranded complementary DNA, which integrates into the cellular genome and is transcribed into mRNA by the cellular DNA-dependent RNA polymerase. Protein synthesis for these viruses is carried out according to the following scheme:

genomic RNA virus-> complementary DNA-» transcription mRNA

-»broadcast virus protein.

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

The replication mechanism is different for viruses that have:

1) double-stranded DNA;

2) single-stranded DNA;

3) plus single-stranded RNA;

4) minus single-stranded RNA;

5) double-stranded RNA;

6) identical plus-strand RNAs (retroviruses).

1. Double-stranded LNA viruses . Replication of double-stranded viral DNA occurs by the usual semi-conservative mechanism: after the DNA strands unwind, new strands are complementarily added to them. Each newly synthesized DNA molecule consists of one parent and one newly synthesized strand. These viruses include a large group of viruses that contain double-stranded DNA in linear form (for example, herpesviruses, adenoviruses and poxviruses) or in a circular form, like papillomaviruses. In all viruses except poxviruses, transcription of the viral genome occurs in the nucleus.

A unique replication mechanism is characteristic of hepadnaviruses (hepatitis B virus). The genome of hepadnaviruses is represented by double-stranded circular DNA, one strand of which is shorter (incomplete plus strand) than the other strand. Initially it is being completed (Fig. 3.7). The complete double-stranded DNA is then transcribed by the cell's DNA-dependent RNA polymerase to produce small molecules of mRNA and complete single-stranded plus RNA. The latter is called pregenomic RNA; it is the template for viral genome replication. Synthesized mRNAs are involved in the process of protein translation, including viral RNA-dependent DNA polymerase (reverse transcriptase). With the help of this enzyme, the pregenomic RNA migrating into the cytoplasm is reverse transcribed into the minus strand of DNA, which, in turn, serves as a template for the synthesis of the plus strand of DNA. This process ends with the formation of double-stranded DNA containing an incomplete plus strand of DNA.

Single-stranded DNA viruses . The only representatives of single-stranded DNA viruses are parvoviruses. Parvoviruses use cellular DNA polymerases to create a double-stranded viral genome, the so-called replicative form of the latter. In this case, a minus strand of DNA is complementarily synthesized on the original viral DNA (plus strand), which serves as a template for the synthesis of the plus strand DNA of the new virion. In parallel, mRNA is synthesized and viral peptides are translated.

Plus single-stranded RNA viruses . These viruses include a large group of viruses - picornaviruses, flaviviruses, togaviruses (Fig. 3.8), in which the genomic plus-strand RNA performs the function of mRNA. For example, poliovirus RNA, after entering the cell, binds to ribosomes, working as mRNA, and on its basis a large polypeptide is synthesized, which is split into fragments: RNA-dependent RNA polymerase, viral proteases and capsid proteins. Polymerase based on the genomic plus-strand RNA synthesizes minus-strand RNA; a temporary double RNA is formed, called a replication intermediate. This replication intermediate consists of a complete plus strand of RNA and numerous partially completed minus strands. Once all the minus strands are formed, they are used as templates for the synthesis of new plus strands of RNA. This mechanism is used both for the propagation of the genomic RNA of the virus and for the synthesis of a large number of viral proteins.

Minus single-stranded RNA viruses. Minus single-stranded RNA viruses (rhabdoviruses, paramyxoviruses, orthomyxoviruses) contain an RNA-dependent RNA polymerase. The genomic minus-strand RNA that has entered the cell is transformed by the viral RNA-dependent RNA polymerase into incomplete and complete plus-strand RNA. Incomplete copies act as mRNA for the synthesis of viral proteins. Complete copies are a template (intermediate stage) for the synthesis of minus strands of the genomic RNA of the offspring

Double-stranded RNA viruses. The mechanism of replication of these viruses (reoviruses and rotaviruses) is similar to the replication of minus-single-strand RNA viruses. The difference is that the plus strands formed during transcription function not only as mRNA, but also participate in replication: they are templates for the synthesis of minus strands RNA. The latter, in combination with plus-strand RNA, form genomic double-stranded RNA virions. Replication of the viral nucleic acids of these viruses occurs in the cytoplasm of cells.

6 . Retroviruses (plus-strand diploid RNA viruses). Retroviral reverse transcriptase synthesizes (on an RNA virus template) a minus strand of DNA, from which the plus strand of DNA is copied to form a double strand of DNA closed in a ring (Fig. 3.10). Next, 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.

Formation of viruses. Virions are formed by self-assembly: the constituent parts of the virion are transported to the site of virus assembly - areas of the nucleus or cytoplasm of the cell. The connection of the virion components is determined byleno the presence of hydrophobic, ionic, hydrogen bonds and steric conformity.

There are the followinggeneral principles virus assemblies :

The formation of viruses is a multi-stage process with the formation of intermediate forms that differ from mature virions in the composition of polypeptides.

Assembly of simple viruses consists in the interaction of viral nucleic acids with capsid proteins and in the formation of nucleocapsids.

In complex viruses First, nucleocapsids are formed, which interact with modified cell membranes (the future lipoprotein envelope of the virus).

Moreover, the assembly of viruses replicating in the cell nucleus occurs with the participation of the nuclear membrane, and the assembly of viruses whose replication occurs in the cytoplasm is carried out with the participation of the endoplasmic reticulum or plasma membrane, where glycoproteins and other proteins of the virus envelope are embedded.

In a number of complex minus-strand RNA viruses (orthomyxoviruses, paramyxoviruses) the assembly involves the so-called matrix protein (M protein), which is located under the modified cell membrane. Possessing hydrophobic properties, it acts as an intermediary between the nucleocapsid and the viral lipoprotein envelope.

□ Complex viruses during the formation process, they include some components of the host cell, such as lipids and carbohydrates.

Exit of viruses from the cell. The full cycle of viral reproduction is completed in 5-6 hours (influenza virus, etc.) or after several days (hepatoviruses, measles virus, etc.). The process of viral reproduction ends with their exit from the cell, which occurs explosively or by budding or exocytosis.

Blasting path: A large number of virions are simultaneously released from a dying cell. Simple viruses that do not have a lipoprotein shell emerge from the cell along the explosive path.

Budding, exotshpt inherent in viruses that have a lipoprotein envelope, which is a derivative of cell membranes. First, the resulting nucleocapsid or virion core is transported to cell membranes, into which virus-specific proteins are already embedded. Then, in the area of ​​​​contact of the nucleocapsid or the virion core with the cell membrane, protrusion of these areas begins. The formed bud is separated from the cell in the form of a complex virus. In this case, the cell is able to maintain viability for a long time and produce viral offspring.

Budding of viruses formed in the cytoplasm can occur either through the plasma membrane (for example, paramyxoviruses, togaviruses) or through the membranes of the endoplasmic reticulum with their subsequent release to the cell surface (for example, bunyaviruses).

Viruses that form in the cell nucleus (for example, herpesviruses) bud into the perinuclear space through a modified nuclear membrane, thus acquiring a lipoprotein envelope. Then they are transported as part of cytoplasmic vesicles to the cell surface.

Viruses- these are the smallest living organisms, the sizes of which vary from 20 to 300 nm; on average, they are fifty times smaller than bacteria. They cannot be seen with a light microscope and pass through filters that do not allow bacteria to pass through.

Origin of viruses

Researchers often wonder whether viruses? If we consider any structure that has genetic material (DNA or RNA) and is capable of self-reproduction to be alive, then the answer must be affirmative: yes, viruses are alive. If the presence of a cellular structure is considered a sign of living things, then the answer will be negative: viruses are not living. It should be added that outside the host cell, viruses are incapable of self-reproduction.

For a more complete view about viruses it is necessary to know their origin in the process of evolution. There is an assumption, although unproven, that viruses are genetic material that once “escaped” from prokaryotic and eukaryotic cells and retained the ability to reproduce when returning to the cellular environment.

Viruses outside the cell are in a completely inert state, but they have a set of instructions (genetic code) necessary to re-enter the cell and, subordinating it to their instructions, force it to produce many copies identical to itself (the virus). Therefore, it is logical to assume that in the process of evolution, viruses appeared later than cells.

Structure of viruses

Structure of viruses very simple. They consist of the following structures:
1) core - genetic material represented by either DNA or RNA; DNA or RNA can be single-stranded or double-stranded;
2) capeid - a protective protein shell surrounding the core;
3) nucleocapsid - a complex structure formed by the core and capsid;
4) envelopes - some viruses, such as HIV and influenza, have an additional lipoprotein layer originating from the plasma membrane of the host cell;
5) capsomeres - identical repeating subunits from which capsids are often built.

The general shape of the capsid is characterized by a high degree of symmetry, causing ability of viruses to crystallization. This makes it possible to study them using both X-ray crystallography and electron microscopy. Once viral subunits are formed in the host cell, they can immediately self-assemble into a complete viral particle. A simplified diagram of the structure of the virus is shown in the figure.

For structure virus capsid Certain types of symmetry are characteristic, especially polyhedral and helical. A polyhedron is a polyhedron. The most common polyhedral shape in viruses is the icosahedron, which has 20 triangular faces, 12 corners, and 30 edges. In Figure A we see a regular icosahedron, and in Figure B we see a herpes virus, in a particle of which 162 capsomeres are organized into an icosahedron.

A clear illustration of spiral symmetry can be seen in the figure, RNA virus tobacco mosaic (TM). The capsid of this virus is formed by 2130 identical protein capsomeres.

VTM was the first virus, isolated in its pure form. When infected with this virus, yellow specks appear on the leaves of a diseased plant - the so-called leaf mosaic (Fig. 2.18, B). Viruses spread very quickly either mechanically when diseased plants or plant parts come into contact with healthy plants, or through the air through smoke from cigarettes made from infected leaves.

Viruses phages that attack bacteria form a group called bacteriophages or simply phages. Some bacteriophages have a clearly defined icosahedral head and a tail with spiral symmetry). The figure shows schematic images of some viruses, illustrating their relative sizes and general structure.

All viruses are divided into two groups: simple and complex. Simple viruses contain a nucleic acid and several polypeptides encoded by it. Complex viruses consist of nucleic acid, lipids and carbohydrates, which are of cellular origin, i.e., in most viruses they are not encoded by the viral genome. In exceptional cases, cellular nucleic acids or polypeptides are included in the virion.

Viruses contain nucleic acids and proteins. Proteins and nucleic acids are inextricably linked. Protein synthesis is not possible without nucleic acids, and acid synthesis is not possible without the active participation of proteins and enzymes. It is known that nucleic acids and proteins consist of C, O, H, N, P, S. The genome of the virus is represented by DNA or RNA. Based on their genome structure, mature viral particles are divided into the following groups:

1. Viruses whose genome is a single-stranded RNA molecule with template activity;

2. Viruses whose genome is single-stranded RNA that does not have template activity;

3. Viruses with single-stranded fragmented RNA that does not have template activity;

4. Viruses whose genome consists of several RNA molecules with template activity;

5. Viruses with double-stranded fragmented RNA;

6. Viruses with linear single-stranded DNA;

7. Viruses with double-stranded circular DNA;

8. Viruses with double-stranded linear infectious DNA;

9. Viruses with double-stranded linear non-infectious DNA.

In terms of nucleotide composition, the DNA of invertebrate animal viruses is more diverse than the DNA of vertebrates. Nucleic acids of virions in most cases are of viral rather than cellular origin. The infectivity of viruses is associated with the nucleic acid, and not with the protein that is part of them. This was proven by German scientists G. Schramm and A. Gierer (1956). Nucleic acids are the custodian of all genetic information of the virus. Their chemical composition and structure are not fundamentally different from the nucleic acids of more highly organized creatures (bacteria, protozoa, animals). Most of the viral particle is made up of proteins that contain the same amino acids as proteins of other organisms. The viral protein is represented mainly by polypeptides of one to three types. Proteins on the surface of the virus particle are antigens responsible for the production of antibodies in infected animals. The main part of the proteins are proteins synthesized in a susceptible cell according to information from the genome of the virus. In rare cases, it is possible that the proteins of an infected cell can be included in the lipoprotein shells and core of some viruses (avian myeloblastosis virus, icosahedral viruses).

Viral proteins are divided into capsid proteins, core proteins, envelope proteins, and enzymatic proteins. In addition to proteins, lipids and carbohydrates are found in the lipoprotein membrane. Carbohydrates are predominantly contained in glycoprotein peplomers on the surface of the viral particle.

The minerals K, Na, Ca, Mg, and Fe were found in the viruses. They are involved in the formation of protein bonds with nucleic acid.

Viral proteins perform protective (protect against adverse environmental influences) and targeting (have receptors for a specific sensitive cell) functions. In addition, viral proteins facilitate their penetration into a susceptible cell.

The functions of viral nucleic acids are as follows. They program the heredity of viruses, participate in protein synthesis, and are responsible for the infectious properties of viral particles.

The individual viral particle is called a virion. The protein shell of the virion is called the capsid. Capsids consist of surface protein subunits, which in turn are formed by protein molecules. There are the following levels of complexity of the capsid structure. The first level is individual polypeptides (chemical units), the second is capsomeres (morphological units), which consist of one or more protein molecules, the third is peplomeres (molecules that form protrusions on the lipoprotein shell of the virion).

Viruses are characterized by two types of capsid structure symmetry: cubic and helical. Viruses with a cubic type of symmetry are called isometric. All known DNA-containing animal viruses have isometric capsids. Crystallographic data indicate three types of figures with a cubic type of symmetry: tetrahedron, octahedron and icosahedron. Icosahedral symmetry is preferable for viruses, since this type of symmetry is the most economical.

Viruses with a helical type of symmetry in the capsid structure are characterized by the fact that their capsid is built from identical, spirally arranged protein subunits (capsomers).

Bacteriophages (bacterial viruses) are structurally a combination of two types of symmetry: cubic and helical. Their head is a cubic structure, and the process is spiral-shaped.

The nature of the interaction between nucleic acid and capsomeres differs in viruses with different types of symmetry of the capsid structure. In viruses with a helical capsid structure, protein subunits closely interact with the nucleic acid. In icosohedral viruses, the most pronounced regular interaction between each protein subunit and nucleic acid does not exist.

Video: Hepatitis C virus in the liver

Viruses are characterized by uniformity of shape and size; they are also not mobile to individual growth and during their ontogenesis they have the same size.
The morphological forms of viruses are smaller than those of bacteria.
The main components of a virion (a virus outside a cell) are the protein shell - the capsid - and the NK enclosed in it - the nucleocapsid. The morphological units of the capsid - capsomeres - are built from one or more proteins. These capsomeres are connected by a type of symmetry and are arranged in a unique order:
- spiral symmetry - forms cylindrical structures;
- cubic symmetry - forms structures close to spheroids.
Virions according to the type of formation of their structure are divided into:
- simple virions - built according to one type of symmetry;
- complex virions - mixed type of symmetry (spiral and cubic).

Structure of simple virions

There are two types of simple virions:
- spiral;
- spherical.
Spiral virions. There are:
1. Hard rod-shaped viruses having the shape of a hard, inflexible, very brittle cylinder. This includes viruses that vary in length from 1300-3150 Ǻ with the length of virions being 180-250 Ǻ (tobacco mosaic virus).
The structure of the tobacco mosaic virus (TMV). In an electron microscope, TMV has the shape of rods, 150-180 Å thick, 3000 Å (300 nm) long. They are also found with a shorter length, but they are not infectious. The capsomeres of the virion are arranged in a helical symmetry.

The chemical, structural and morphological unit is a protein with a molecular weight of 17400 D. Moreover, for every three turns of the helix there are 49 morphological units. Inside the hollow cylinder there is single-stranded RNA; the size of the RNA exceeds the size of the virion, but the RNA is compactly packaged and is also located along a helical line between capsomeres. There are 49 nucleotides per turn of the helix; each protein molecule is associated with three nucleotide residues.
2. Filamentous viruses have the form of elastic threads that easily bend and intersect with each other.
Spherical virions are built according to cubic symmetry. This structure is based on the structure of a twenty-sided structure - an icosahedron. The simplest icosahedron has 12 vertices and 20 faces, more complex ones contain 20T faces, where T is the triagulation number.
T=P×f2,
P - size, class of icosahedron, takes values ​​1, 3, 7, 13, 19, 21, 37,
f - any integer,
f 2 - indicates how many isosceles triangles are located on one face of the icosahedron.
Thus, the simplest icosahedrons of class 1 with f = 1 have 20 faces, and with f = 2 - 80 faces.
Viruses with a cubic type of symmetry have two types of capsomeres: capsomeres are located at the vertices, built from 5 identical subunits (pentomeres), and along the side faces - from 6 subunits (hexomers).
The size of the virus is determined by the number of capsomeres, the smallest spherical class 1 virus has 12 pentomers and no hescomeres, and the largest virus contains 1472 capsomeres. RNA or DNA is folded very compactly, forming invaginations into the capsomeres in a spiral.

Structure of complex viruses

Complex viruses include viruses that have a complex type of symmetry or additional lipid or carbohydrate components.
Additional shells are either lipid or carbohydrate, but the structure of these shells is not encoded in NA. These membranes are of cellular origin and it is difficult to determine their content; they are often fragments of the CPM that the virus captures when leaving the cell.
Shell functions:
protective (insensitive to some chemicals and toxic substances);
they serve as part of a mechanism that facilitates the penetration of the virus into the cell, due to the fact that these membranes easily merge with the CPM.
shells may have tubular projections that have antigenic activity and serve as receptors for virus attachment to the cell surface.
Viruses that have additional shells are polymorphic and resemble a bullet or thimble shape.

Bacteriophages are a group of viruses with a complex type of symmetry.
In 1917, De Herrel discovered the lysis of bacterial cells on the surface of a Petri dish and called this agent of unknown nature a bacteriophage - a bacteria eater.
There are both complex and simple viruses; they have 5 morphological forms:
- filamentous phages (helical type of symmetry, mainly containing DNA);
- phages with a cubic type of symmetry (they have the rudiments of a tail process, these are RNA- or single-stranded DNA-containing);
- phages with a short process;
- phages that have two types of symmetry (a head - a cubic type of symmetry and a non-contractile sheath - a tail - built according to a helical type of symmetry) with double-stranded DNA;
- the most complex type of symmetry (with a head and a contracting sheath, containing DNA).
Model of phage T2.
This is a bacteriophage containing a head and an appendage.
The head is built according to the cubic type of symmetry and contains a double chain inside. DNA that is many times larger than the size of the phage. DNA is compactly folded and is largely determined by the stabilizing function of the proteins putriscin and spermycin, which are associated with divalent metals; their function is to block repulsive forces and neutralize the negative charge of the particle.
The process has a complex structure, consisting of a collar, which is adjacent to the head, a contractile sheath built according to a spiral type of symmetry, inside of which there is a hollow cylinder, and at the end of the process there is a hexagonal basal plate, from which 6 threads extend. The basal plate serves as an adsorption factor on the cell surface, and the hollow rod ensures the transport of phage DNA into the bacterial cell.

Viroids. Viroids are a single-stranded RNA molecule covalently closed in a ring and do not contain a protein shell. Viroids are infectious objects. Some plant diseases have a viroid origin, but human and animal pathogens do not. Viroids have transmissibility - the ability to be transmitted from object to object, often from plant to plant mechanically (by wind, insects).

Virus cultivation

1. The use of laboratory animals, but due to the limited specificity for cultivating viruses, it is necessary to have certain laboratory animals, human tissue is also needed, and this is inconvenient and a violation of bioethics.
2. Cultivation of virus on chicken embryos, but this is not suitable for all viruses.
3. The use of a culture of cells or tissues of laboratory animals or humans that are permissive for the virus - the ability to reproduce viruses. Disadvantage: cells age during cultivation.
4. Cultivation using hybrid cells - a hybrid of a normal cell permissive for the virus with a cancer cell. Cancer cells exhibit uncontrolled mitosis, thereby prolonging the life of permissive cells.

Influence of environmental factors
1. Heating. Most viruses are stable at room temperature, but a decrease in infectivity occurs at 50-60o C. The rate of reproduction of the influenza virus decreases at 38-39o C, and the tobacco mosaic virus is stable at 65o C, but dies at 70o C.
2. Mechanical impact
- most viruses are resistant to osmotic pressure,
- ultrasound destroys rod-shaped viruses in a few minutes and has little effect on spherical viruses,
- drying - some viruses are easily transferred, while others are inactivated at room temperature when the humidity decreases.
3. Radiation: UV and ionizing radiation cause death and, in low doses, mutation.
4. Chemical factors:
- alcohol, iodine, hydrogen peroxide,
- antibiotics, but there are no effective ones for systemic treatment. There are prophylactic antibiotics and there are those that are used for local treatment.
The agent against viruses is the interferon system produced by the human body.

Storing viruses in laboratories
Viruses are stored in a freeze-dried state in a cryoprotector system, dried at 60°C from a frozen state. In this case, the viral particle is placed in cryoprotectors, which protect the viruses from damage by ice particles. Viruses can also be stored in blood serum in a CO2 atmosphere at -70°C; glycerin is used as a stabilizer.

Main groups of viruses

Viruses, depending on the object of influence, are divided into: viruses of bacteria, plants, insects, animals and humans.
There is an artificial classification of viruses, which lays down:
- type of NK (DNA or RNA),
- single- or double-stranded structure,
- presence or absence of an outer shell,
- if single-stranded RNA, then +RNA or -RNA,
- presence of reverse transcriptase in the structure.