Reproduction of viruses is not carried out by binary division. As far back as the 50s of the last century, it was found that reproduction is carried out by the method of reproduction (translated from the English. Reproduce - to make a copy, reproduce), that is, by reproducing nucleic acids, as well as protein synthesis, followed by collecting virions. These processes occur in various parts of the cell of the so-called host (for example, in the nucleus or cytoplasm). This fragmented virus reproduction method is called disjunctive. This is what we will focus on in more detail in our article.
Reproduction process
This process has its own characteristics of reproduction of viruses and is characterized by a sequential change in some stages. Let's consider them separately.
Phases
Viruses cannot multiply in a nutrient medium, since they are strict intracellular parasites. In addition, unlike chlamydia or rickettsia, during reproduction, viruses in the host cell are not able to grow and do not multiply by division. All components of this virus include nucleic acids, as well as protein molecules that are synthesized separately in the "host" cell, in different parts of the cell: in the cytoplasm and in the nucleus. In addition, protein synthesizing cell systems obey one viral genome, as well as its NK.
Viral reproduction in the cell is carried out in several phases, which are described below:
- The first phase is the adsorption of the virus, which was discussed above, on the surface of the cell, which is sensitive to this virus.
- The second is the penetration of the virus into the host cells by the method of viropexis.
- The third is a kind of “undressing” of the virions, the release of nucleic acid from the capsid and supercapsid. In a number of viruses, the entry of nucleic acid into cells occurs by the fusion of the virion membrane and the host cell. In this case, the third and second phases are combined into a single one.
Adsorption
By this stage of virus reproduction is meant the penetration of a 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 "host". They are special sensitive formations that perceive irritations. Receptors are molecules or molecular complexes located on the surface of cells, and are also able to recognize specific chemical groups, molecules or other cells, to bind them. In the most complex virions, such receptors are located on the outer shell in the form of a spike outgrowth or villus; in simple virions, they are usually located on the surface of the capsid.
The adsorption mechanism 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 receptors and cells 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 virion myxoviruses, neuraminidase destroys the mucogfothein 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 affected by temperature, the reaction of the medium, 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 polycarions (ecmoline, DEAE-dextran, protamine sulfate), which neutralize the negative charge from sulfated polysaccharides.
Entering the virion in the "host" cell
The pathway for the virus to enter a cell that is sensitive to it will not always be the same. Many virions are able to penetrate into the cells by pinocytosis, which in Greek means “drink”, “drink”. With this method, the pinocytotic vacuole seems to draw the virion directly into the cell. The remaining virions can penetrate the cell directly through its membrane.
The contact of the enzyme neuraminidase with cellular mucoproteins promotes the entry of virions into the cell among myxoviruses. Recent studies have shown that DNA and RNA of virions do not separate from the outer shell, i.e., virions penetrate entirely into sensitive cells by pinocytosis or viropexis. At the moment, this is confirmed in respect of smallpox virus, smallpox vaccine, as well as other viruses that select the animal organism as a habitat. In terms of phages, they infect cells with nucleic acid. The infection mechanism is based on the fact that those virions contained in the cell vacuoles are hydrolyzed by enzymes (lipases, proteases), during which DNA is released from the phage membrane and enters the cell.
To carry out the experiment, the cells were infected with a nucleic acid that was isolated from some viruses, and one full cycle of virion reproduction was caused. However, in vivo infection with such an acid does not occur.
Disintegration
The next stage of virus 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 breaks down, simultaneously releasing NK. In individual bacteriophages, free NK enters the cells. The phytopathogenic virus penetrates through damage in the cell wall, and then it is adsorbed on the internal cell receptor with the simultaneous release of NK.
RNA replication and viral protein synthesis
The next step in the reproduction of viruses is the synthesis of a virus-specific protein, which takes place with the participation of so-called messenger RNAs (in some viruses they are part of virions, and in some they are synthesized only in infected cells directly on the virion DNA or RNA matrix). Replication of viral NK occurs.
The process of reproduction of RNA viruses begins after nucleoproteins enter the cell, where viral polysomes are formed by combining RNA with ribosomes. After this, early proteins are synthesized, where repressors from cellular metabolism, as well as RNA polymerase, which are translated with the parent RNA molecule, should be assigned. In the cytoplasm of the smallest viruses, or in the nucleus, a double-stranded viral RNA is formed by combining the parent plus chain (“+” - the RNA chain) with the synthesized again and also the minus chain complementary to it (“-” - the RNA chain) . The combination of these strands from nucleic acid provokes the formation of only a single-stranded RNA structure, which is called the replicative form. Viral RNA syntheses are carried out by replicative 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 a replicative form directly on the plus chain matrix, as well as RNA polymerase II, which takes part in the synthesis of single-stranded viral RNA on a replicative type matrix. The synthesis of nucleic acids in small viruses occurs in the cytoplasm. As for the influenza virus, the inner protein and RNA are synthesized in the nucleus. RNA is then isolated from the nucleus and enters the cytoplasm, in which, together with the ribosomes, it begins to synthesize a viral protein.
After virions enter the cells, the synthesis of nucleic acid, as well as cellular proteins, is suppressed in them. During the reproduction of viruses containing DNA, on the matrix in the core is also synthesized i-RNA, which carries the information for protein synthesis. The mechanism of viral protein synthesis is carried out at the level of the cell ribosome, and the amino acid fund will be the source of construction. The activation of amino acids is carried out by enzymes, with the help of i-RNA they are transferred directly to the 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 mechanism.
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 NK.
Virion formation
The formation of the virion occurs with the participation of some structural components that make up the cell. Herpes viruses, polio and smallpox vaccines are formed in the cytoplasm, and adenoviruses in the nucleus. The synthesis of viral RNA, as well as the formation of nucleocapsid occurs directly in the nucleus, and hemagglutinin is formed in the cytoplasm. After this, the nucleocapsid is transferred from the nucleus to the cytoplasm, in which the formation of the virion shell occurs. The nucleocapsid is coated externally with viral proteins, and hemagglutinins and neuraminidases are included in the composition of the virion. It is in this way that offspring, such as the flu virus, are formed.
Virion release from the host cell
From the host cell, virus particles are released simultaneously (during cell destruction) or gradually (without any cell destruction).
It is in this form that the reproduction of viruses takes place. Virions are released from cells, usually 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, virions exit all at the same time and completely from the cells (picornaviruses). This method is called lytic.
Second method
The second method involves the release of virions as they mature for 2-6 hours on the cytoplasmic membrane (myxoviruses and arboviruses). Myxovirus isolation from the cell is facilitated by neuraminidases that destroy the cell membrane. During this method, 75-90% of the virions exit spontaneously into the culture medium, and the cells gradually die.