Viruses do not reproduce by binary fission. Back in the 50s of the last century, it was established that reproduction is carried out by the reproduction method (translated from English reproduce - make a copy, reproduce), that is, by reproducing nucleic acids, as well as protein synthesis, followed by the collection of virions. These processes occur in different parts of the cell of the so-called host (for example, in the nucleus or cytoplasm). This disjointed method of virus reproduction 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 the reproduction of viruses and is distinguished by the successive change of some stages. Consider them separately.
Phases
Viruses cannot multiply in a nutrient medium, as 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 fission. All components of this virus include nucleic acids, as well as protein molecules that are synthesized in the "host" cell separately, 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 NA.
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 the cell, which is sensitive to this virus.
- The second is the penetration of the virus into the host cells by the viropexis method.
- The third is a kind of "undressing" of virions, the release of nucleic acid from the capsid and supercapsid. In a number of viruses, the nucleic acid enters cells by the 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 virus reproduction refers to 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 "receiving". 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 orother cells, bind them. In the most complex virions, such receptors are located on the outer shell in the form of a spike-like 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 receptive 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 adsorb directly to mucoprotein receptors, while arboviruses and picornaviruses adsorb to lipoprotein receptors.
In the myxovirus virion, neuraminidase destroys the mucogphotein 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 s alt 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 polykaryons (ecmolin, DEAE-dextran, protamine sulfate), which neutralize the negative charge from sulfated polysaccharides.
Virion getting into the "host" cell
The way a virus enters a sensitive cell will not always be the same. Many virions are able to enter cells by pinocytosis, which means "drink" in Greek."drink". With this method, the pinocytic vacuole seems to draw the virion directly into the cell. Other virions can enter the cell directly through its membrane.
Contact of the enzyme neuraminidase with cellular mucoproteins promotes 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, and other viruses that choose to live in animals. Speaking of phages, they infect cells with nucleic acid. The mechanism of infection is based on the fact that those virions that are contained in cell vacuoles are hydrolyzed by enzymes (lipases, proteases), during which DNA is released from the phage membrane and enters the cell.
For the experiment, a cell was infected with a nucleic acid that was isolated from some viruses, and one complete cycle of virion reproduction was caused. However, under natural conditions, 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 the cellular protease, then it is destroyed, simultaneously releasingNK. In some bacteriophages, free NA enters the cells. The phytopathogenic virus enters through the 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 stage of virus reproduction is the synthesis of a virus-specific protein, which occurs with the participation of the so-called messenger RNA (in some viruses they are part of virions, and in some they are synthesized only in infected cells directly on the matrix of virion DNA or RNA). Viral NK replication occurs.
The process of reproduction of RNA viruses begins after the entry of nucleoproteins into the cell, where viral polysomes are formed by complexing RNA with ribosomes. After that, early proteins are also synthesized, which should include repressors from cellular metabolism, as well as RNA polymerases that 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 complexing the parent plus chain (“+” - RNA chain) with the newly synthesized, as well as complementary with it minus chain (“-” - RNA chain). The connection of these strands of nucleic acid provokes the formation of only a single-stranded RNA structure, which is called the replicative form. Synthesis of viral RNA 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. Tothese 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 replication-type template. 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. RNA is then released from the nucleus and enters the cytoplasm, where, together with ribosomes, it begins to synthesize the viral protein.
After the virions enter the cells, the synthesis of nucleic acid and cellular proteins is suppressed in them. During the reproduction of viruses containing DNA, mRNA is also synthesized on the matrix 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 amino acid fund will be the source of construction. 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 mechanism system.
Virion morphogenesis
The formation of virions can occur only in the case of a strictly ordered connection of structural viral polypeptides, as well as their NA. And this is ensured by the so-called self-assembly of protein molecules near the NC.
Virion formation
The formation of the virion occurs with the participation of some of the structural components that make up the cell. Herpes, polio, and vaccinia viruses are produced in the cytoplasm, while adenoviruses are produced 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 that, the nucleocapsid moves from the nucleus to the cytoplasm, in which the formation of the virion envelope takes place. The nucleocapsid is covered on the outside with viral proteins, and the hemagglutinins and neuraminidase are included in the virion. This is how the formation of offspring, for example, the influenza virus occurs.
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 the reproduction of viruses occurs. Virions are released from cells, usually in two ways.
First method
The first method implies the following: after the absolute maturation of 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 called lytic.
Second method
The second method involves the process of releasing virions as they mature within 2–6 hours forcytoplasmic membrane (myxoviruses and arboviruses). The secretion of myxoviruses from the cell is facilitated by neuraminidase, which destroys the cell membrane. During this method, 75-90% of the virions are released spontaneously into the culture medium, and the cells gradually die.
