The process of virion formation consists of stages. Virus-cell interaction
CONTENT
Control questions:
1. Reproduction of DNA genomic viruses: main stages, features of reproduction…………………………………………………..……........……...3
2. Signs of virus reproduction in living systems: laboratory animals, chick embryos, cell cultures………………………......…………………..… ……16
3. Task................................................... ........................................................ ...20
References……………………………...…………………...........25
1. Reproduction of DNA genomic viruses: main stages, features of reproduction
Reproduction of viruses
The process of viral reproduction can be divided into two phases. The first phase covers the events that lead to the adsorption and entry of the virus into the cell, the release of its internal component and its modification in such a way that it is capable of causing infection. Accordingly, the first phase includes three stages: 1) adsorption of the virus on cells; 2) penetration of the virus into cells; 3) stripping the virus in the cell. These stages are aimed at ensuring that the virus is delivered to the appropriate cellular structures and its internal component is freed from its protective membranes. Once this goal is achieved, the second phase of reproduction begins, during which the viral genome is expressed. This phase includes the stages: 1) transcription, 2) translation of messenger RNA, 3) genome replication, 4) assembly of viral components. The final stage of reproduction is the release of the virus from the cell.
The first phase of reproduction.
I. Adsorption of virions on the cell surface.
The interaction of a virus with a cell begins with the process of adsorption, i.e., the attachment of viral particles to the cell surface. The adsorption process is possible in the presence of appropriate receptors on the cell surface and “recognizing” substances on the surface of the virus. The very initial adsorption processes are nonspecific in nature, and they may be based on the electrostatic interaction of positively and negatively charged groups on the surface of the virus and cell. However, recognition of cellular receptors by viral proteins, leading to attachment of the viral particle to the cell, is a highly specific process. Proteins on the surface of the virus that recognize specific groups on the plasma membrane of the cell and cause the viral particle to attach to them are called attachment proteins.
Viruses use receptors designed to pass into the cell substances necessary for its life: nutrients, hormones, growth factors, etc. Receptors can have a different chemical nature and be proteins, the carbohydrate component of proteins and lipids, lipids. The receptors for influenza viruses and paramyxoviruses are sialic acid in the composition of glycoproteins and glycolipids (gangliosides), for rhabdoviruses and reoviruses - also a carbohydrate component in the composition of proteins and lipids, for picornaviruses and adenoviruses - proteins, for some viruses - lipids. Specific receptors play a role not only in the attachment of the viral particle to the cell surface. They determine the further fate of the viral particle, its intracellular transport and delivery to certain areas of the cytoplasm and nucleus, where the virus is able to initiate the infectious process. The virus can also attach to nonspecific receptors and even penetrate the cell, but only attachment to a specific receptor will lead to infection.
Attachment of the viral particle to the cell surface initially occurs through the formation of a single bond between the viral particle and the receptor. However, such attachment is fragile, and the viral particle can easily come off the cell surface - reversible adsorption. In order for irreversible adsorption to occur, multiple bonds must appear between the viral particle and many receptor molecules, i.e., stable multivalent attachment must occur. The number of cellular receptor molecules in adsorption sites can reach up to 3000. Stable binding of the viral particle to the cell surface as a result of multivalent attachment occurs due to the possibility of free movement of receptor molecules in the lipid bilayer of the plasma membrane, which is determined by the mobility, “fluidity” of the protein-lipid layer. An increase in lipid fluidity is one of the earliest events in the interaction of a virus with a cell, which results in the formation of receptor fields at the site of contact of the virus with the cell surface and stable attachment of the viral particle to the resulting groups.
The number of specific receptors on the cell surface varies between 104 and 105 per cell. Receptors for some viruses can be present only in a limited set of host cells, and this can determine the body's sensitivity to a given virus. For example, picornaviruses adsorb only to primate cells. Receptors for other viruses, on the contrary, are widely represented on the surface of cells of various species, such as, for example, receptors for orthomyxoviruses and paramyxoviruses, which are sialyl-containing compounds. Therefore, these viruses have a relatively wide range of cells on which adsorption of viral particles can occur. Cells of an extremely wide range of hosts possess receptors for a number of togaviruses: these viruses can adsorb and infect cells of both vertebrates and invertebrates.
II. Penetration of the virus into the cell.
Historically, there was an idea of two alternative mechanisms for the penetration of animal viruses into cells - by viropexis (endocytosis) and by the fusion of viral and cellular membranes. However, both of these mechanisms do not exclude, but complement each other.
The term “viropexis” means that the viral particle enters the cytoplasm as a result of invagination of a section of the plasma membrane and the formation of a vacuole that contains the viral particle.
Receptor endocytosis. Viropexis is a special case of receptor or adsorption endocytosis. This process is a common mechanism through which nutritional and regulatory proteins, hormones, lipoproteins and other substances from the extracellular fluid enter the cell. Receptor endocytosis occurs in specialized areas of the plasma membrane, where there are special pits covered on the cytoplasmic side with a special protein with a large molecular weight - clathrin. At the bottom of the pit there are specific receptors. The pits allow rapid invagination and the formation of clathrin-coated intracellular vacuoles. The half-life of penetration of the substance into the cell by this mechanism does not exceed 10 minutes from the moment of adsorption. The number of vacuoles formed in one minute reaches more than 2000. Thus, receptor endocytosis is a well-coordinated mechanism that ensures the rapid penetration of foreign substances into the cell.
The coated vacuoles fuse with other, larger cytoplasmic vacuoles, forming receptor-containing receptors but not clathrin, which in turn fuse with lysosomes. In this way, proteins that enter the cell are usually transported to lysosomes, where they are broken down into amino acids; they can bypass lysosomes and accumulate in other parts of the cell in an undegraded form. An alternative to receptor endocytosis is liquid endocytosis, when invagination does not occur in specialized areas of the membrane. Most enveloped and non-enveloped animal viruses enter the cell via the mechanism of receptor endocytosis. Endocytosis ensures intracellular transport of the viral particle within the endocytic vacuole, since the vacuole can move in any direction and merge with cell membranes (including the nuclear membrane), releasing the viral particle in the corresponding intracellular sites. In this way, for example, nuclear viruses enter the nucleus, and reoviruses enter the lysosomes. However, viral particles that have entered the cell are located within the vacuole and are separated from the cytoplasm by its walls. They have to go through a number of stages before they can cause an infectious process.
Fusion of viral and cellular membranes. In order for the internal component of the virus to pass through the cell membrane, the virus uses a mechanism of membrane fusion. In enveloped viruses, fusion is caused by a point interaction of the viral fusion protein with the lipids of the cell membrane, as a result of which the viral lipoprotein envelope integrates with the cell membrane, and the internal component of the virus appears on its other side. In non-enveloped viruses, one of the surface proteins also interacts with the lipids of the cell membranes, causing the internal component to pass through the membrane. Most animal viruses enter the cytosol from the receptosome.
If during endocytosis the viral particle is a passive passenger, then during fusion it becomes an active participant in the process. The fusion protein is one of its surface proteins. To date, this protein has been identified only in paramyxoviruses and orthomyxoviruses. In paramyxoviruses, this protein (P protein) is one of two glycoproteins located on the surface of the viral particle. The function of the fusion protein in the influenza virus is performed by the small hemagglutinating subunit.
Paramyxoviruses induce membrane fusion at neutral pH, and the internal component of these viruses can enter the cell directly through the plasma membrane. However, most enveloped and non-enveloped viruses induce membrane fusion only at low pH values, between 5.0 and 5.75. If weak bases (ammonium chloride, chloroquine, etc.) are added to the cells, which increase the pH in endocytic vacuoles to 6.0, membrane fusion does not occur, viral particles remain in the vacuoles, and the infectious process does not occur. The strict dependence of membrane fusion on pH values is due to conformational changes in viral fusion proteins.
The lysosome always has a low pH value (4.9). In the endocytic vacuole (receptosome), acidification is created by an ATP-dependent “proton pump” on the cell surface during the formation of a coated vacuole. Acidification of the endocytic vacuole is of great importance for physiological ligands entering the cell, since a low pH promotes dissociation of the ligand from the receptor and recycling of the receptors.
The same mechanism that underlies the fusion of viral and cellular membranes determines virus-induced hemolysis and fusion of plasma membranes of adjacent cells with the formation of multinucleated cells, symplasts and syncytia. Viruses cause two types of cell fusion: 1) “fusion from the outside” and 2) “fusion from the inside.” “Fusion from outside” occurs at high multiplicity of infection and is detected within the first hours after infection. This type of fusion, described for paramyxoviruses, is caused by proteins of the infecting virus and does not require intracellular synthesis of viral components. In contrast, “fusion from within” occurs at a low multiplicity of infection, is detected at relatively late stages of the infectious process, and is caused by newly synthesized viral proteins. “Fusion from within” is described for many viruses: herpes viruses, oncoviruses, pathogens of slow infections, etc. This type of fusion is caused by the same viral glycoproteins that ensure the penetration of the virus into the cell.
III. Undressing - deproteinization of the virus
Viral particles that have entered the cell must undress in order to cause an infectious process. The purpose of undressing is to remove the viral protective membranes that prevent the expression of the viral genome. As a result of undressing, the internal component of the virus is released, which can cause an infectious process. Undressing is accompanied by a number of characteristic features: as a result of the disintegration of the viral particle, infectious activity disappears, in some cases sensitivity to nucleases appears, resistance to the neutralizing effect of antibodies occurs, and photosensitivity is lost when using a number of drugs.
The end products of undressing are cores, nucleocapsids or nucleic acids. For a number of viruses, it has been shown that the undressing product is not naked nucleic acids, but nucleic acids associated with the internal viral protein. For example, the end product of picornaviruses is RNA, covalently linked to the VPg protein; the end product of adenoviruses is DNA, covalently linked to one of the internal viral proteins.
In some cases, the ability of viruses to cause an infectious process is determined by the possibility of their undressing in the cell of a given system. Thus, this stage is one of the stages that limit the infection.
The undressing of a number of viruses occurs in specialized areas inside the cell (lysosomes, structures of the Golgi apparatus, perinuclear space, nuclear pores on the nuclear membrane). When the viral and cellular membranes merge, penetration into the cell is combined with undressing.
Undressing and intracellular transport are interrelated processes: if proper intracellular transport to the undressing sites is disrupted, the viral particle enters the lysosome and is destroyed by lysosomal enzymes.
Second phase of reproduction .
I. Transcription.
Transcription is carried out using a special enzyme - RNA polymerase, which binds nucleotides by forming 3-5´phosphodiester bridges. This binding occurs only in the presence of a DNA template.
The products of transcription in the cell are mRNA. Cellular DNA itself, which is the carrier of genetic information, cannot directly program protein synthesis. The transfer of genetic information from DNA to ribosomes is carried out by an RNA messenger. This is the basis of the central dogma of molecular biology, which is expressed by the following formula:
DNA - transcription - RNA - translation - protein,
where the arrows show the direction of transfer of genetic information.
Implementation of genetic information in viruses. The strategy of the viral genome in relation to mRNA synthesis is different for different viruses. In DNA viruses, mRNA is synthesized on the template of one of the DNA strands. The formula for transferring genetic information is the same as in a cell:
DNA - transcription - RNA - translation - protein.
DNA viruses that reproduce in the nucleus use cellular polymerase for transcription. These viruses include papovaviruses, adenoviruses, and herpes viruses. DNA viruses that reproduce in the cytoplasm cannot use the cellular enzyme located in the nucleus. Transcription of their genome is carried out by a virus-specific enzyme - DNA polymerase, which penetrates the cell as part of the virus. These viruses include smallpox viruses and iridoviruses.
Enzymes that transcribe the viral genome. Transcription of a number of DNA-containing viruses - papovaviruses, adenoviruses, herpes viruses, parvoviruses, hepadnaviruses. It is carried out in the cell nucleus, and in this process the mechanisms of cellular transcription are widely used - transcription enzymes and further modification of transcripts. Transcription of these viruses is carried out by cellular RNA polymerase II, an enzyme that transcribes the cellular genome. However, a special group of adenovirus transcripts is synthesized using another cellular enzyme - RNA polymerase III. In two other families of DNA-containing animal viruses, smallpox viruses and iridoviruses, transcription occurs in the cytoplasm. Since there are no cellular polymerases in the cytoplasm, transcription of these viruses requires a special viral enzyme - viral RNA polymerase. This enzyme is a structural viral protein.
Regulation of transcription. Transcription of the viral genome is tightly regulated throughout the infectious cycle. Regulation is carried out by both cellular and virus-specific mechanisms. In some viruses, mainly DNA-containing ones, there are three periods of transcription - very early, early and late. These viruses include smallpox viruses, herpes viruses, papovaviruses, and adenoviruses. As a result of ultra-early and early transcription, ultra-early and early genes are selectively read to form ultra-early or early mRNAs. During late transcription, another part of the viral genome, the late genes, is read, producing late mRNAs. The number of late genes usually exceeds the number of early genes. Many very early genes are genes for non-structural proteins - enzymes and regulators of transcription and replication of the viral genome. In contrast, late genes are usually genes for structural proteins. Typically, late transcription reads the entire genome, but with a predominance of late gene transcription.
A factor regulating transcription in nuclear viruses is the transport of transcripts from the nucleus to the cytoplasm, to the site of mRNA functioning - polysomes.
The product of ultra-early transcription of herpes viruses is A-proteins. The function of one or more of them is necessary for the transcription of the next group of genes encoding P proteins. In turn, P proteins turn on the transcription of the last group of late genes encoding U proteins. This type of regulation is called “cascade”.
II. Broadcast.
This is the process of translating the genetic information contained in mRNA into a specific sequence of amino acids in the synthesized virus-specific proteins. Protein synthesis in the cell occurs as a result of translation of mRNA on ribosomes. In ribosomes, the flow of information (in mRNA) merges with the flow of amino acids, which bring transfer RNA (tRNA). There are a large number of different tRNAs in the cell. Each amino acid must have its own tRNA.
The tRNA molecule is a single-stranded RNA with a complex maple leaf-shaped structure.
The binding of a specific tRNA and amino acid is carried out by the enzyme aminoacyl synthetase. One end of the tRNA binds to the amino acid, and the other to the nucleotides of the mRNA to which they are complementary. The three nucleotides on an mRNA code for one amino acid and are called a “triplet” or “codon,” and the complementary three nucleotides on a tRNA are called an “anticodon.”
The transcription process consists of three phases: initiation of elongation, termination.
Translation initiation is the most critical stage in the translation process, based on the recognition of mRNA by the ribosome and binding to its special regions. The ribosome recognizes the mRNA through a cap at the 5′ end and slides toward the 3′ end until it reaches the initiation codon, which begins translation. In a eukaryotic cell, the initiation codons are AUG (adenine, uracil, guanine) codons encoding methionine. The synthesis of all polypeptide chains begins with methionine. Specific recognition of viral and RNA by the ribosome is carried out due to virus-specific initiator factors.
First, the small ribosomal subunit binds to the mRNA. The mRNA complex with the small ribosomal subunit is joined by other components necessary for the initiation of translation. These are several protein molecules called "initiator factors." There are at least three of them in a prokaryotic cell and more than nine in a eukaryotic cell. Initiator factors determine the recognition of specific mRNAs by the ribosome. As a result, a complex is formed that is necessary for the initiation of translation, which is called the “initiation complex.” The initiation complex includes: mRNA; small ribosomal subunit; aminoacyl-tRNA carrying an initiator amino acid; initiating factors; several molecules of GTP (guanosine triphosphate).
In the ribosome, the flow of information merges with the flow of amino acids. The entry of aminoacyl-tRNA into the A-center of the large ribosomal subunit is a consequence of recognition, and its anticodon interacts with the codon of the mRNA located in the small ribosomal subunit. When the mRNA moves one codon, the tRNA is transferred to the peptidyl center (P-center), and its amino acid joins the initiator amino acid to form the first peptide bond. The amino acid-free tRNA leaves the ribosome and can again function in the transport of specific amino acids. In its place, a new tRNA is transferred from the A-center to the P-center, and a new peptide bond is formed. A vacant mRNA codon appears in the A-center, to which the corresponding tRNA immediately joins, and new amino acids are added to the growing polypeptide chain.
Translation elongation is the process of lengthening, increasing the polypeptide chain, based on the addition of new amino acids using a peptide bond. The mRNA strand is constantly pulled through the ribosome and the genetic information contained in it is “decoded”. Often, mRNA functions simultaneously on several ribosomes, each of which synthesizes the same polypeptide strand encoded by this mRNA.
Termination of translation occurs at the moment when the ribosome reaches the termination codon in the mRNA (UAA, UGA, UAG). Translation stops and the polypeptide chain is released from the polyribosome. After the end of translation, polyribosomes break down into subunits, which can become part of new polyribosomes.
Each RNA functions on several ribosomes. A group of ribosomes operating on a single mRNA molecule is called a polyribosome or polysome. Polysomes can consist of 4-6 to 20 or more ribosomes.
Virus-specific polysomes can be either free or membrane-bound. Internal proteins are usually synthesized on free polysomes; glycoproteins are always synthesized on membrane-bound polysomes.
Since the genome of an animal virus is represented by a molecule encoding more than one protein, viruses are faced with the need to synthesize either a long mRNA encoding one giant precursor polypeptide, which must then be cut at specific points into functionally active proteins, or short monocistronic mRNAs, each of which encodes one protein. Thus, there are two ways to form viral proteins:
the first - the mRNA is translated into a giant precursor polypeptide, which, after synthesis, is sequentially cut into mature functionally active proteins;
second, the mRNA is translated to form mature proteins or proteins that are only slightly modified after synthesis.
The first method of translation is characteristic of RNA-containing plus-strand viruses - picornaviruses and togaviruses. Their mRNA is translated into a giant polypeptide chain, the so-called polyprotein, which slides in the form of a continuous ribbon from the ribosomal “conveyor” and is cut into individual proteins of the required size. The cutting of viral proteins is a multistep process carried out by both virus-specific and cellular proteases.
The second method of protein formation is characteristic of DNA-containing viruses and most RNA-containing viruses. With this method, short monocistronic mRNAs are synthesized as a result of selective transcription of one region of the genome (gene). However, these viruses make extensive use of the post-translational protein cutting mechanism.
In a eukaryotic cell, many proteins, including viral ones, undergo post-translational modifications; mature, functionally active proteins are often not identical to their newly synthesized precursors. Widespread post-translational covalent modifications include glycosylation, acylation, methylation, sulfonation (formation of disulfide bonds), proteolytic cutting and, finally, phosphorylation. As a result, instead of 20 genetically encoded amino acids, about 140 amino acid derivatives were isolated from various cells of different organs of eukaryotes.
Glycosylation. Complex RNA- and DNA-containing viruses contain proteins containing covalently attached carbohydrate side chains - glycoproteins. Glycoproteins are located within viral envelopes and are found on the surface of viral particles.
Glycosylation of polypeptides is a complex multi-stage process, the first stages of which begin already in the process of polypeptide synthesis, and the first carbohydrate residue is added to the polypeptide chain that has not yet left the ribosome. Subsequent stages of glycosylation occur by sequential addition of carbohydrate residues to the carbohydrate chain during transport of the polypeptide to the plasma membrane. Carbohydrate residues are added one at a time, and only when the synthesis of the oligosaccharide chain is initiated is the “block” transferred. The final formation of the carbohydrate chain may be completed at the plasma membrane before assembly of the viral particle.
Glycosylation affects transport; moreover, transport is inextricably linked for glycoproteins with staged glycosylation. Convincing evidence of this is the effect of glycosylation inhibitors on viral reproduction; they completely suppress the transport of polypeptides without disrupting or inhibiting their synthesis.
When glycosylation is suppressed by appropriate inhibitors (sugar analogues such as 2-deoxyglucose, the antibiotic tunicamycin), the assembly of virions of myxo-, rhabdo-, and α-viruses is blocked or non-infectious virions of herpes viruses and oncoviruses are formed.
Sulfonation. Some proteins of complex RNA and DNA viruses are sulfonated after translation. Most often, glycoproteins undergo sulfonation, and the sulfate group binds to the carbohydrate residues of the glycoprotein.
Acylation. A number of glycoproteins of complex RNA-containing viruses (HA2 of the influenza virus, G protein of the vesicular stomatitis virus, HN protein of the Newcastle disease virus, etc.) contain covalently linked 1-2 molecules of fatty acids.
Cutting. Many viral proteins, and primarily glycoproteins, acquire functional activity only after they are cut at specific points by proteolytic enzymes. Cutting occurs either with the formation of two functional protein subunits (for example, the large and small subunits of the influenza virus hemagglutinin, two glycoproteins (E2 and E3) of the Semliki Forest virus), or with the formation of one functionally active protein and an inactive enzyme, for example, the F and HN proteins of paramyxoviruses. Slicing is usually done by cellular enzymes. In many complex animal viruses that have glycoproteins, cutting is necessary for the formation of active attachment proteins and fusion proteins and, therefore, for the viruses to acquire the ability to infect a cell. Only after cutting up these proteins does the viral particle become infectious. Thus, we can talk about the proteolytic activation of a number of viruses, carried out using cellular enzymes.
Phosphorylation. Phosphoproteins are contained in almost all animal viruses - RNA - and DNA-containing ones, simple and complex in structure. Protein kinases are found in most viruses, but phosphorylation can be carried out by both viral and cellular enzymes. Typically, proteins associated with the viral genome and playing a regulatory role in its expression are phosphorylated. The mechanism of active action of interferon is associated with the process of phosphorylation.
III. Replication.
Replication is the synthesis of nucleic acid molecules homologous to the genome. DNA replication occurs in the cell, resulting in the formation of daughter double-stranded DNA. Replication occurs on untwisted sections of DNA and occurs simultaneously on both strands from the 5′ end to the 3′ end.
Because the two strands of DNA have opposite polarities and the replication site (the fork) moves in the same direction, one strand is built in the opposite direction in separate fragments called Okazaki fragments (named after the scientist who first proposed this model). After synthesis, the Okazaki fragments are “crosslinked” into a single strand by ligase.
DNA replication is carried out by DNA polymerases. To begin replication, preliminary synthesis of a short section of RNA on a DNA template, called a primer, is necessary. The synthesis of the DNA strand begins with the primer, after which the RNA is quickly removed from the growing site.
Replication of viral DNA. Replication of the genome of DNA-containing viruses is mainly catalyzed by cellular fragments and its mechanism is similar to the mechanism of cellular DNA replication.
Each newly synthesized DNA molecule consists of one parent and one newly synthesized strand. This replication mechanism is called semi-conservative.
In viruses containing circular double-stranded DNA (papovaviruses), one of the DNA strands is cut, which leads to unwinding and removal of supercoils in a certain section of the molecule.
The lower supercoiled part of the molecule, the untwisted part over a large area, and newly formed replication loops are visible.
During the replication of single-stranded DNA (parvovirus family), double-stranded forms are formed, which are intermediate replicative forms.
Replicative complexes. Since the resulting DNA and RNA strands remain associated with the matrix for some time, replicative complexes are formed in the infected cell, in which the entire process of replication (and in some cases also transcription) of the genome is carried out. The replicative complex contains the genome, replicase, and newly synthesized nucleic acid chains associated with the template. Newly synthesized genomic molecules immediately associate with viral proteins, so antigens are found in replication complexes. During the replication process, a partially double-stranded structure with single-stranded “tails” appears, the so-called replicative precursor.
Replication complexes are associated with cellular structures, either preexisting or virus-induced. For example, the replicative complexes of picornaviruses are associated with the membranes of the endoplasmic reticulum, of smallpox viruses - with the cytoplasmic matrix, the replicative complexes of adenoviruses and herpes viruses in the nuclei are in association with newly formed fibrous structures and are associated with nuclear membranes. In infected cells, there may be increased proliferation of cellular structures with which replication complexes are associated, or their formation from pre-existing material. For example, in cells infected with picornaviruses, proliferation of smooth membranes occurs. In cells infected with reoviruses, an accumulation of microtubules is observed; In cells infected with smallpox viruses, the formation of a cytoplasmic matrix occurs.
In replication complexes, simultaneously with the synthesis of genomic molecules, transcription occurs and the assembly of nucleocapsids and cores occurs, and in some infections, viral particles.
Regulation of replication. The newly formed genomic RNA molecule can be used in various ways. It can associate with capsid proteins and become part of the virion, serve as a template for the synthesis of new genomic molecules, or for the formation of mRNA; finally, in “plus”-strand viruses it can perform the functions of mRNA and bind to ribosomes. There are mechanisms in the cell that regulate the use of genomic molecules. Regulation follows the principle of self-regulation and is realized through the interaction of viral RNA and proteins due to the possibility of protein-nucleic acid and protein-protein recognition. For example, the role of the terminal protein of picornaviruses is to inhibit the translation of mRNA and select molecules for the formation of virions. The protein that binds to the 5′ end of the genomic RNA is, in turn, recognized by capsid proteins and serves as a signal for the assembly of the viral particle with the participation of this RNA molecule. Using the same principle, genomic RNA molecules are selected from minus-strand viruses. The RNA molecule is part of the virion or serves as a template for replication. To switch it to transcription, a prohibition of protein-nucleic acid interaction must occur. Adenovirus DNA replication involves a protein molecule that binds to the end of the viral DNA and is necessary for the initiation of replication. Thus, for replication to begin, the synthesis of viral proteins is necessary: in the presence of protein synthesis inhibitors, there is no switch from transcription to replication.
IV. Assembly of viral particles.
The synthesis of components of viral particles in the cell is separate and can occur in different structures of the nucleus and cytoplasm. Viruses that replicate in nuclei are conventionally called nuclear viruses. These are mainly DNA-containing viruses: adenoviruses, papovaviruses, parvoviruses, herpes viruses.
Viruses that replicate in the cytoplasm are called cytoplasmic. These include DNA-containing smallpox virus and most RNA-containing viruses, with the exception of orthomyxoviruses and retroviruses. However, this division is very relative, because in the reproduction of both viruses there are stages that occur in the cytoplasm and nucleus, respectively.
Inside the nucleus and cytoplasm, the synthesis of virus-specific molecules can also be separated. For example, the synthesis of some proteins is carried out on free polysomes, while others are synthesized on membrane-bound polysomes. Viral nucleic acids are synthesized in association with cellular structures away from the polysomes that synthesize viral proteins. With this disjunctive method of reproduction, the formation of a viral particle is possible only if viral nucleic acids and proteins have the ability, at sufficient concentration, to recognize each other in the diversity of cellular proteins and nucleic acids and spontaneously connect with each other, i.e., are capable of self-assembly.
Self-assembly is based on specific protein-nucleic acid and protein-protein recognition, which can occur as a result of hydrophobic, salt and hydrogen bonds, as well as steric matching. Protein-nucleic acid recognition is limited to a small region of the nucleic acid molecule and is determined by unique nucleotide sequences in the non-coding part of the viral genome. With this recognition of a region of the genome by viral capsid proteins, the process of assembling the viral particle begins. The attachment of other protein molecules is carried out due to specific protein-protein interactions or nonspecific protein-nucleic acid interactions.
Due to the diversity of the structure of animal viruses, the methods for forming virions are also varied, but the following general principles of assembly can be formulated:
In simple viruses, provirions are formed, which are then transformed into virions as a result of protein modifications. For complex viruses, assembly is carried out in multiple stages. First, nucleocapsids or cores are formed, with which the outer shell proteins interact.
The assembly of complex viruses (with the exception of the assembly of smallpox viruses and reoviruses) is carried out on cell membranes. The assembly of nuclear viruses occurs with the participation of nuclear membranes, the assembly of cytoplasmic viruses - with the participation of the membranes of the endoplasmic reticulum or plasma membrane, where all components of the viral particle arrive independently of each other.
A number of complex viruses have special hydrophobic proteins that act as intermediaries between the formed nucleocapsids and the viral envelopes. Such proteins are matrix proteins in a number of minus-strand viruses (orthomyxoviruses, paramyxoviruses, rhabdoviruses).
The assembly of nucleocapsids, cores, provirions and virions does not occur in the intracellular fluid, but in pre-existing in the cell or induced by the virus (“factories”).
Complex viruses use a number of elements of the host cell to build their particles, for example lipids, some enzymes, histones in DNA genomic 5V40, actin in enveloped RNA genomic viruses, and even ribosomes are found in arenoviruses. Cellular molecules have certain functions in the viral particle, but their inclusion in the virion can also be a consequence of accidental contamination, such as the inclusion of a number of cell membrane enzymes or cellular nucleic acids.
Assembly of DNA viruses. There are some differences in the assembly of DNA viruses from the assembly of RNA viruses. Like RNA-containing viruses, the assembly of DNA-containing viruses is a multi-step process with the formation of intermediate forms that differ from mature virions in the composition of polypeptides. The first stage of assembly involves the association of DNA with internal proteins and the formation of cores or nucleocapsids. In this case, the DNA is combined with pre-formed “empty” capsids.
As a result of DNA binding to capsids, a new class of intermediate forms appears, called incomplete forms. In addition to incomplete forms with different DNA contents, there is another intermediate form in morphogenesis - immature virions, which differ from mature ones in that they contain uncut polypeptide precursors. Thus, the morphogenesis of viruses is closely related to the modification (processing) of proteins.
Assembly of nuclear viruses begins in the nucleus, usually with association with the nuclear membrane. The intermediate forms of the herpes virus that form in the nucleus bud into the perinuclear space through the inner nuclear membrane, and the virus thus acquires an envelope, which is a derivative of the nuclear membrane. Further completion and maturation of virions occurs in the membranes of the endoplasmic reticulum and in the Golgi apparatus, from where the virus is transported as part of cytoplasmic vesicles to the cell surface.
In non-budding lipid-containing viruses - smallpox viruses, the assembly of virions occurs in the already described cytoplasmic viral “factories”. The lipid envelope of viruses in “factories” is formed from cellular lipids by autonomous self-assembly, therefore the lipid composition of the envelopes differs significantly from the composition of lipids in cellular membranes.
V. Exit of viral particles from the cell.
There are two ways for viral progeny to exit the cell:
1) by “explosion”;
2) by budding.
Exit from the cell by explosion is associated with the destruction of the cell, a violation of its integrity, as a result of which mature viral particles located inside the cell end up in the environment. This method of exiting the cell is characteristic of viruses that do not contain a lipoprotein shell (picorna-, rheo-, parvo-, papova-, adenoviruses). However, some of these viruses can be transported to the cell surface before cell death. Exit from cells by budding is characteristic of viruses containing a lipoprotein membrane, which is a derivative of cell membranes. With this method, the cell can remain viable for a long time and produce viral offspring until its resources are completely depleted.
Viruses are characterized by a disjunctive (from disjuncus - disconnected) method of reproduction and reproduction. The progeny of the virus arises as a result of the assembly of nucleic acids and protein subunits, which are synthesized separately by the host cell.
The penetration of a virus into a cell and reproduction of its own kind occurs in several phases:
1.penetration into the host cell,
2.synthesis of enzymes necessary for the replication of viral nucleic acids,
3.synthesis of viral parts,
4.assembly and composition of mature virions,
5. exit of mature virions from the cell.
Stages of viral reproduction.
1 - adsorption of the virion on the cell; 2 - penetration of the virion into the cell by viropexis;
3 - virus inside a cell vacuole; 4 - `undressing of the virus virion; 5 - replication of viral nucleic acid; 6 - synthesis of viral proteins on cell ribosomes; 7 - virion formation; 8 - exit of the virion from the cell by budding.
Phase I - adsorption of the virion on the cell surface.
It occurs in two stages: the first one is nonspecific, when the virus is retained on the cell surface using electrostatic forces, i.e., due to the appearance of opposite charges between individual sections of the cell membrane and the virus. This phase of interaction between the virus and the cell is reversible and is influenced by factors such as pH and salt composition of the medium.
The second stage is specific when specific virus receptors and cell receptors, complementary to each other, interact. By chemical nature, cell receptors can be mucoproteins (or mucopolysaccharides) and lipoproteins. Different viruses are fixed on different receptors: influenza viruses, parainfluenza, adenoviruses - on mucoproteins, and tick-borne encephalitis and polio viruses - on lipoproteins.
Phase II - penetration of the virus into the cell. Electronoscopic observations of the process of penetration of viruses into cells sensitive to them showed that it is carried out through a mechanism reminiscent of pinocytosis, or, as is more often called, viropexis. At the site of virus adsorption, the cell wall is drawn into the cell, a vacuole is formed, in which the virion appears. In parallel, cellular enzymes (lipases and proteases) cause deproteinization of the virion - dissolution of the protein shell and release of nucleic acid.
Phase III - hidden period (period of eclipse - disappearance). During this period, it is impossible to determine the presence of an infectious virus in the cell either by chemical, electron microscopic, or serological methods. Little is known yet about the essence of this phenomenon and its mechanisms. It is assumed that in the latent phase, the nucleic acid of the virus penetrates the chromosomes of the cell and enters into complex genetic relationships with them.
Phase IV - synthesis of virion components. In this phase, the virus and the cell are a single whole, the viral nucleic acid performs a genetic function, induces the formation of early proteins and changes the function of ribosomes. Early proteins are divided into:
A) inhibitor proteins(repressors) that suppress cell metabolism
b) enzyme proteins(polymerases) providing the synthesis of viral nucleic acids.
The synthesis of nucleic acids and proteins occurs non-simultaneously and in different structural parts of the cell. For viruses containing DNA or RNA, these processes have some differences and features.
Phase V - formation of mature virions. The process of “assembling” the virus is carried out as a result of the connection of the components of the viral particle. In complex viruses, cellular structures take part in this process and the lipid, carbohydrate, and protein components of the host cell are incorporated into the viral particle.
The process of virion formation begins a certain time after the synthesis of their constituent components has begun. The duration of this period is quite variable and is determined by the nature of the virus - it is usually shorter for RNA viruses than for DNA viruses. For example, production of complete vaccinia virus particles begins approximately 5-6 hours after cell infection and continues for the next 7-8 hours, i.e., after viral DNA synthesis has already been completed.
Very strong bonds are formed between the nucleic acid and the corresponding protein subunits, as evidenced by the difficulty in separating the protein from the viral nucleic acid. The carbohydrates and especially lipids that make up the virus particle give it greater strength.
The formation of virions, as well as the synthesis of virus components, occurs in different places of the cell, with the participation of various cellular structures. After completion of the formation process, a mature daughter viral particle is formed that has all the properties of the parent virion. But sometimes the formation of so-called incomplete viruses, which consist either only of nucleic acid, or of protein, or of viral particles, the formation of which has stopped at some intermediate stage.
Phase VI - release of mature virions from the cell. There are two main mechanisms for the release of mature virions from the cell:
1) release of the virion by budding. In this case, the outer shell of the virion is derived from the cell membrane, it contains both host cell material and viral material;
2) exit of mature virions from the cell through holes in the membrane. These viruses do not have an outer shell. With this mechanism of virus release, the cell, as a rule, dies and a large number of viral particles appear in the environment.
The death of an infected cell can be caused by three mechanisms:
1. the work of the virus, “depleting” the cell;
2. protective reaction of the cell, triggering the genetic program of its death (apoptosis);
3. the body's immune system, which destroys the infected cell.
In addition to the productive type of interaction between the virus and the cell, it is possible integrative coexistence or virogeny. Virogeny is characterized by the integration (incorporation) of the virus nucleic acid into the cell genome, as well as the replication and functioning of the viral genome as an integral part of the cell genome. For integration with the cellular genome, the appearance of a circular form of double-stranded DNA of the virus is necessary. The viral DNA embedded in the cell chromosome is called a provirus. The provirus replicates as part of the chromosome and passes into the genome of daughter cells, i.e. the state of virogenesis is inherited. Under the influence of certain physical or chemical factors, the provirus can enter an autonomous state with the development of a productive type of interaction with the cell. Additional genetic information of the provirus during virogenesis imparts new properties to the cell, which can cause the development of tumors, autoimmune and chronic diseases. The ability of viruses to integrate with the cell genome is the basis for the persistence (from the Latin persisto - to constantly remain, to remain) of viruses in the body and the development of persistent viral infections. For example, the hepatitis B virus can cause persistent lesions with the development of chronic hepatitis and often liver tumors.
Types of virus-cell interaction. Virus reproduction phases.
There are three types of virus-cell interaction:
Productive type- ends with the formation of a new generation of virions and the 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 joint coexistence (joint replication).
Reproduction of viruses:
1. adsorption of the virus on the cell - attachment of viruses to the cell surface. The virus is adsorbed on certain areas of the cell membrane - the so-called receptors. ;
2. penetration of the virus into the cell-two methods: viropexys and fusion of the viral envelope with the cell membrane. With viropexis, after the adsorption of viruses, invagination (invagination) of a section of the cell membrane and the formation of an intracellular vacuole, which contains a viral particle, occur. The vacuole with the virus can be transported in any direction to different parts of the cytoplasm or the cell nucleus. The fusion process is carried out by one of the surface viral proteins of the capsid or supercapsid shell ;
3. “undressing” the virus- removal of protective viral shells and release of the internal component of the virus, which can cause an infectious process. The end products of "undressing" are the core, nucleocapsid or nucleic acid of the virus. ;
3.biosynthesis of viral components in the cell- The viral nucleic acid that has entered the cell carries genetic information that successfully competes with the genetic information of the cell. It disorganizes the functioning of cellular systems, suppresses the cell’s own metabolism and forces it to synthesize new viral proteins and nucleic acids that are used to build viral offspring.
The implementation of the genetic information of the virus is carried out in accordance with the processes of transcription, translation and replication ;
4. formation of viruses-There are the following general principles for assembling viruses with different structures:
1. The formation of viruses is a multi-stage process with the formation of intermediate forms;
2. The assembly of simply arranged viruses involves the interaction of viral nucleic acid molecules with capsid proteins and the formation of nucleocapsids (for example, polio viruses). In complex viruses, nucleocapsids are first formed, with which supercapsid shell proteins interact (for example, influenza viruses);
3. The formation of viruses does not occur in the intracellular fluid, but on the nuclear or cytoplasmic membranes of the cell;
4. Complexly organized viruses during the process of formation include components of the host cell (lipids, carbohydrates) ;
5. release of viruses from the cell - The first type - explosive - is characterized by the simultaneous release of a large number of viruses. In this case, the cell quickly dies. This exit method is typical for viruses that do not have a supercapsid shell. The second type is budding. It is characteristic of viruses that have a supercapsid shell. At the final stage of assembly, the nucleocapsids of complex viruses are fixed on the cell plasma membrane, modified by viral proteins, and gradually protrude it. As a result of protrusion, a “bud” containing a nucleocapsid is formed. The “bud” is then separated from the cell. Thus, the outer shell of these viruses is formed during their exit from the cell .
The vital activity of bacteria is characterized by growth- the formation of structural and functional components of the cell and the increase in the bacterial cell itself, as well as reproduction- self-reproduction, leading to an increase in the number of bacterial cells in the population.
Bacteria multiply by binary fission in half, less often by budding. Actinomycetes, like fungi, can reproduce by spores. Actinomycetes, being branching bacteria, reproduce by fragmentation of filamentous cells. Gram-positive bacteria divide by ingrowth of synthesized division septa into the cell, and gram-negative bacteria by constriction, as a result of the formation of dumbbell-shaped figures, from which two identical cells are formed.
Cell division is preceded by replication of the bacterial chromosome according to a semi-conservative type (the double-stranded DNA strand opens and each strand is completed by a complementary strand), leading to doubling of the DNA molecules of the bacterial nucleus - the nucleoid.
DNA replication occurs in three stages: initiation, elongation, or chain growth, and termination.
Reproduction of bacteria in a liquid nutrient medium. Bacteria seeded in a certain, unchanging volume of the nutrient medium, multiplying, consume nutrients, which subsequently leads to the depletion of the nutrient medium and the cessation of bacterial growth. Cultivation of bacteria in such a system is called batch cultivation, and the culture is called batch culture. If the cultivation conditions are maintained by continuous supply of fresh nutrient medium and the outflow of the same volume of culture fluid, then such cultivation is called continuous, and the culture is called continuous.
When bacteria are grown on a liquid nutrient medium, bottom, diffuse or surface (in the form of a film) growth of the culture is observed. The growth of a periodic culture of bacteria grown on a liquid nutrient medium is divided into several phases, or periods:
1. lag phase;
2. logarithmic growth phase;
3. stationary growth phase, or maximum concentration
bacteria;
4. bacterial death phase.
Lag phase- the period between the sowing of bacteria and the beginning of reproduction. The duration of the lag phase is on average 4-5 hours. At the same time, the bacteria increase in size and prepare to divide; the amount of nucleic acids, proteins and other components increases.
Logarithmic (exponential) growth phase is a period of intense bacterial division. Its duration is about 5-6 hours. Under optimal growth conditions, bacteria can divide every 20-40 minutes. During this phase, bacteria are most vulnerable, which is explained by the high sensitivity of the metabolic components of an intensively growing cell to inhibitors of protein synthesis, nucleic acids, etc.
Then comes the stationary growth phase, at which the number of viable cells remains unchanged, constituting the maximum level (M-concentration). Its duration is expressed in hours and varies depending on the type of bacteria, their characteristics and cultivation.
The death phase completes the bacterial growth process., characterized by the death of bacteria under conditions of depletion of sources of nutrient medium and accumulation of bacterial metabolic products in it. Its duration ranges from 10 hours to several weeks. The intensity of bacterial growth and reproduction depends on many factors, including the optimal composition of the nutrient medium, redox potential, pH, temperature, etc.
Reproduction of bacteria on a solid nutrient medium. Bacteria growing on dense nutrient media form isolated round-shaped colonies with smooth or uneven edges (S- and R-forms), of varying consistency and color, depending on the pigment of the bacteria.
Water-soluble pigments diffuse into the nutrient medium and color it. Another group of pigments is insoluble in water, but soluble in organic solvents. And finally, there are pigments that are insoluble neither in water nor in organic compounds.
The most common pigments among microorganisms are carotenes, xanthophylls and melanins. Melanins are insoluble black, brown or red pigments synthesized from phenolic compounds. Melanins, along with catalase, superoxide mutase and peroxidases, protect microorganisms from the effects of toxic oxygen peroxide radicals. Many pigments have antimicrobial, antibiotic-like effects.
Not carried out by binary fission. Back in the 50s of the last century, it was established that reproduction is carried out by the method of reproduction (translated from English reproduce - make a copy, reproduce), that is, by reproducing nucleic acids, as well as protein synthesis with the subsequent collection of virions. These processes occur in various parts of the so-called host cell (for example, in the nucleus or cytoplasm). This disconnected method of virus reproduction is called disjunctive. This is exactly what we will focus on in more detail in our article.
Reproduction process
This process has its own characteristics of viral reproduction and is characterized by a sequential change of certain stages. Let's look at them separately.
Phases
Viral reproduction in a cell occurs in several phases, which are described below:
- The first phase is the adsorption of the virus, discussed above, on the surface of a cell that is sensitive to this virus.
- The second is the penetration of the virus into host cells by viropexys.
- The third is a kind of “undressing” of virions, the release of nucleic acid from the capsid and supercapsid. In a number of viruses, nucleic acid enters cells by fusion of the virion envelope and the host cell. In this case, the third and second phases are combined into a single one.
Adsorption
This stage of viral reproduction refers to the penetration of the viral particle into cells. Adsorption begins on the cell surface through the interaction of cellular as well as viral receptors. Translated from Latin, the word “receptors” means “receiver”. They are special sensitive formations that perceive irritations. Receptors are molecules or molecular complexes located on the surface of cells and are also capable of recognizing specific chemical groups, molecules or other cells and binding them. In the most complex virions, such receptors are located on the outer shell in the form of a spike-shaped outgrowth or villus; in simple virions they are usually located on the surface of the capsid.
The mechanism of adsorption on the surface of a susceptible cell is based on the interaction of receptors with the so-called complementary receptors of the “host” cell. Virion and cell receptors are some specific structures that are located on the surface.
Adenoviruses and myxoviruses are adsorbed directly on mucoprotein receptors, and arboviruses and picornaviruses are adsorbed on lipoprotein receptors.
In the myxovirus virion, neuraminidase destroys the mucogphothein receptor and cleaves N-acetylneuraminic acids from the oligosaccharide, which contains galactose and galactosamine. Their interactions at this stage are reversible, because they are significantly influenced by temperature, reaction of the environment and salt components. Adsorption of the virion is prevented by heparin and sulfated polysaccharides, which carry a negative charge, but their inhibitory effect is removed by some polykarions (ecmolin, DEAE-dextran, protamine sulfate), which neutralize the negative charge from sulfated polysaccharides.
Entry of the virion into the host cell
The path of introduction of a virus into a cell sensitive to it will not always be the same. Many virions are able to penetrate cells by pinocytosis, which in Greek means “to drink” or “drink.” With this method, the pinocytotic vacuole seems to draw the virion directly into the cell. Other virions can enter the cell directly through its membrane.
Contact of the neuraminidase enzyme with cellular mucoproteins promotes the entry of virions into the cell among myxoviruses. The results of recent studies prove that the DNA and RNA of virions are not separated from the outer shell, that is, the virions penetrate entirely into sensitive cells by pinocytosis or viropexis. To date, this has been confirmed for the smallpox virus, vaccinia virus, and other viruses that choose animals as their habitat. If we talk about phages, they infect cells with nucleic acid. The mechanism of infection is based on the fact that those virions contained in cell vacuoles are hydrolyzed by enzymes (lipases, proteases), during which DNA is released from the phage shell and enters the cell.
To carry out the experiment, a cell was infected using nucleic acid that was isolated from some viruses, and one complete cycle of virion reproduction was caused. However, under natural conditions, infection does not occur with the help of such an acid.
Disintegration
The next stage of viral reproduction is disintegration, which is the release of NK from the capsid and outer shell. After the virion enters the cells, the capsid undergoes some changes, acquiring sensitivity to cellular protease, then it is destroyed, simultaneously releasing the NK. In some bacteriophages, free NK enters the cells. The phytopathogenic virus penetrates through damage in the cell wall, and then it is adsorbed on the internal cellular receptor with the simultaneous release of NK.
RNA replication and viral protein synthesis
The next stage of viral reproduction is the synthesis of a virus-specific protein, which occurs with the participation of so-called messenger RNAs (in some viruses they are part of the virions, and in some they are synthesized only in infected cells directly on the virion DNA or RNA matrix). Viral NK replication occurs.
The process of reproduction of RNA viruses begins after nucleoproteins enter the cell, where viral polysomes are formed by complexing RNA with ribosomes. After this, early proteins are synthesized, which include repressors from cellular metabolism, as well as RNA polymerases, which are translated with the parent RNA molecule. In the cytoplasm of the smallest viruses, or in the nucleus, viral double-stranded RNA is formed by combining the parental plus strand (“+” - RNA strand) with the newly synthesized, as well as the minus strand complementary to it (“-” - RNA strand) . The connection of these strands of nucleic acid provokes the formation of only a single-stranded RNA structure, which is called the replicative form. Viral RNA synthesis is carried out by replication complexes, in which the replicative form of RNA, the RNA polymerase enzyme, and polysomes take part.
There are 2 types of RNA polymerases. These include: RNA polymerase I, which catalyzes the formation of the replicative form directly on the plus-strand template, as well as RNA polymerase II, which takes part in the synthesis of single-stranded viral RNA on the replicative type template. The synthesis of nucleic acids in small viruses occurs in the cytoplasm. As for the influenza virus, internal protein and RNA are synthesized in the nucleus. The RNA is then released from the nucleus and penetrates the cytoplasm, where, together with ribosomes, it begins to synthesize the viral protein.
After virions enter cells, the synthesis of nucleic acid, as well as cellular proteins, is suppressed. During reproduction on a matrix, i-RNA is also synthesized in the nucleus, which carries information for protein synthesis. The mechanism of viral protein synthesis is carried out at the level of the cellular ribosome, and the source of construction will be the amino acid pool. Activation of amino acids is carried out by enzymes, with the help of mRNA they are transferred directly to ribosomes (polysomes), in which they are already located in the synthesized protein molecule.
Thus, in infected cells, the synthesis of nucleic acids and virion proteins is carried out as part of a replicative-transcriptive complex, which is regulated by a certain system of mechanisms.
Virion morphogenesis
The formation of virions can occur only in the case of a strictly ordered combination of structural viral polypeptides, as well as their NK. And this is ensured by the so-called self-assembly of protein molecules near the NC.
Virion formation
The formation of a virion occurs with the participation of some structural components that make up the cell. Herpes, polio and vaccinia viruses are formed in the cytoplasm, and adenoviruses are formed in the nucleus. The synthesis of viral RNA, as well as the formation of the nucleocapsid, occurs directly in the nucleus, and hemagglutinin is formed in the cytoplasm. After this, the nucleocapsid moves from the nucleus to the cytoplasm, in which the virion envelope is formed. The nucleocapsid is covered on the outside with viral proteins, and the virion includes hemagglutinins and neuraminidases. This is how progeny, for example, the influenza virus, is formed.
Release of the virion from the host cell
Virus particles are released from the “host” cell simultaneously (during cell destruction) or gradually (without any cell destruction).
It is in this form that viruses reproduce. Virions are released from cells generally in two ways.
First method
The first method implies the following: after the absolute maturation of the virions directly inside the cell, they are rounded, vacuoles are formed there, and then the cell membrane is destroyed. Upon completion of these processes, the virions exit all at the same time and completely from the cells (picornaviruses). This method is usually called lytic.
Second method
The second method involves the process of releasing virions as they mature for 2–6 hours on the cytoplasmic membrane (myxoviruses and arboviruses). The release of myxoviruses from the cell is facilitated by neuraminidases, which destroy the cell membrane. During this method, 75-90% of virions are released spontaneously into the culture medium, and the cells gradually die.
Reproduction viruses are carried out in several stages, successively replacing each other: adsorption of the virus on the cell; penetration of the virus into the cell; “undressing” the virus; biosynthesis of viral components in the cell; formation of viruses; release of viruses from the cell.
Adsorption . The interaction of a virus with a cell begins with the process of adsorption, i.e., the attachment of viruses to the cell surface. This is a highly specific process. The virus is adsorbed on certain areas of the cell membrane - the so-called receptors. Cellular receptors can have a different chemical nature, representing proteins, carbohydrate components of proteins and lipids, lipids. The number of specific receptors on the surface of one cell ranges from 104 to 105. Consequently, tens and even hundreds of viral particles can be adsorbed on the cell. Penetration into the cell. There are two ways for animal viruses to enter a cell: viropexys and fusion of the viral envelope with the cell membrane. With viropexis, after the adsorption of viruses, invagination (invagination) of a section of the cell membrane and the formation of an intracellular vacuole, which contains a viral particle, occur. The vacuole with the virus can be transported in any direction to different parts of the cytoplasm or the cell nucleus. The fusion process is carried out by one of the surface viral proteins of the capsid or supercapsid shell. Apparently, both mechanisms of virus penetration into the cell do not exclude, but complement each other. “Undressing”. The process of “undressing” involves removing the protective viral shells and releasing the internal component of the virus, which can cause an infectious process. “Undressing” of viruses occurs gradually, in several stages, in certain areas of the cytoplasm or nucleus of the cell, for which the cell uses a set of special enzymes. In the case of virus penetration by fusion of the viral envelope with the cell membrane, the process of virus penetration into the cell is combined with the first stage of its “undressing”. The final products of “undressing” are the core, nucleocapsid or nucleic acid of the virus. Biosynthesis of virus components. The viral nucleic acid that has entered the cell carries genetic information that successfully competes with the genetic information of the cell. It disorganizes the functioning of cellular systems, suppresses the cell’s own metabolism and forces it to synthesize new viral proteins and nucleic acids used to build viral offspring. The implementation of the genetic information of the virus is carried out in accordance with the processes of transcription, translation and replication. Formation (assembly) of viruses. Synthesized viral nucleic acids and proteins have the ability to specifically “recognize” each other and, if their concentration is sufficient, they spontaneously combine as a result of hydrophobic, salt and hydrogen bonds. There are the following general principles for assembling viruses with different structures:
1. The formation of viruses is a multi-stage process with the formation of intermediate forms;
2. The assembly of simply arranged viruses involves the interaction of viral nucleic acid molecules with capsid proteins and the formation of nucleocapsids (for example, polio viruses). In complex viruses, nucleocapsids are first formed, with which the proteins of the supercapsid shells interact (for example, influenza viruses);
3. The formation of viruses does not occur in the intracellular fluid, but on the nuclear or cytoplasmic membranes of the cell;
4. Complexly organized viruses during the process of formation include components of the host cell (lipids, carbohydrates).
Exit of viruses from the cell. There are two main types of release of viral progeny from the cell. The first type - explosive - is characterized by the simultaneous release of a large number of viruses. In this case, the cell quickly dies. This exit method is typical for viruses that do not have a supercapsid shell. The second type is budding. It is characteristic of viruses that have a supercapsid shell. At the final stage of assembly, the nucleocapsids of complex viruses are fixed on the cell plasma membrane, modified by viral proteins, and gradually protrude it. As a result of protrusion, a “bud” containing a nucleocapsid is formed. The “bud” is then separated from the cell. Thus, the outer shell of these viruses is formed as they exit the cell. With this mechanism, a cell can produce a virus for a long time, maintaining to one degree or another its basic functions.