The genetic information regarding the replication of virus lies in the nucleic acids of the virus. But, since there are no biosynthetic enzymes present, the virus normally depends upon the host’s synthetic machinery to carry out replication.
The viral multiplication cycle can be divided into 6 successive steps. Some of the steps may overlap each other.
1. Adsorption or penetration:
Virions come in contact with cells by accidental bombarding, but they get adsorbed only if there is an affinity between the two. Specific receptor sites on the host cell enables the adsorption if viral particle.
Example, HIV virus attaches on the CD4receptors and surface glycoprotein, gp120.
Differences in susceptibility to viral cells are based on the presence or absence of receptors on the cells. If this phase can be avoided, certain in susceptible cells may become susceptible to some viruses.
2. Penetration: since the bacterial cell wall is more rigid, the virus cannot enter inside as a whole, but only the nucleic acid. Both animals and humans are susceptible to the entry of the whole virus particle.
The viral particle may be engulfed by a process similar to phagocytosis; viropexis. In the case of enveloped viruses, the envelope may fuse with the plasma membrane of the host cell and releases the nucleic acid into the cytoplasm.
3. Uncoating: the virus strips its outer coating and capsid and releases the nucleic acid into the cell. Lysosomal enzymes of the host interfere with the encoding. But however, by the help of viral uncoating enzymes, the nucleic acid is released into the cytoplasm.
4. Biosynthesis: viral nucleic acid and capsid proteins are synthesized along with several enzymes that are required for viral synthesis, assembly and release. Certain ‘regulator proteins’ which mediate the production of viral components by shutting down the cellular metabolism of the host are also produced. The site of viral synthesis varies with viruses. Some nucleic acids are produced inside the host cell nucleus itself and some in the cytoplasm.
Biosynthesis consist of 4 main steps;
a. Transcription of mRNA from viral nucleic acid
b. Translation of mRNA into early proteins(helps synthesis of viral components)
c. Replication of viral nucleic acid
d. Synthesis of late proteins (components of daughter virion capsids)
5. Maturation: assembly of daughter virions is followed by the synthesis of viral nucleic acid and proteins. The assembly takes place either inside the nucleus or in the cytoplasm.
Non-enveloped viruses attain full development intracellularly. But enveloped viruses develop only the nucleocapsid. During the process of budding, the envelopes are developed from the host cell membrane.
The incorporation of viral antigen on the cell membrane provides the cell with the property of hemadsoption.
6. Release: bacterial cells are lysed during the release of virus particles. Whereas, in the case of animal viruses, cell lysis normally does not occur. Here also there are exceptions, poliovirus lyses the host cells before getting released.
The host cell remains unaffected and may even undergo division, while the progeny virions are released into the surrounding medium. This may further infect other cells. In some case, the virus may be transmitted from one cell to another, without disseminating extracellularly.
There is an ‘eclipse phase’ or ’underground phase’, in which the whole multiplication processes, including penetration to the appearance of mature virions occurs. But the virus remains undemonstrative inside the host during this time. The time is about 15-30 hours.