Biological viruses are formed during the infection process in the target host cells through the viral replication. Most DNA viruses assemble in the nucleus while most RNA viruses develop just in cytoplasm. Replication between viruses is greatly varied and depends on the type of genes involved in them. As viruses are obligate intracellular pathogens they cannot replicate without the machinery and metabolism of a host cell. The host cell must provide the energy and synthetic machinery and the low molecular-weight precursors for the synthesis of viral proteins and nucleic acids.
The virus replication occurs in seven stages, namely;
Adsorption:
First step of viral replication is the adsorption. The virus attaches to the cell membrane of the host cell. It then injects its DNA or RNA into the host to start infection. Viral proteins on the capsid or phospholipid envelope interact with specific receptors on the host cellular surface. In animal cells these viruses get into the cell through the process of endocytosis which works through fusing of the virus and fusing of the viral envelope with the cell membrane of the animal cell and in plant cell it enters through the process of pinocytosis which works on pinching of the viruses.
Entry:
The cell membrane of the host cell invaginates the virus particle, enclosing it in a pinocytotic vacuole.
Uncoating:
Cell enzymes from lysosomes strip off the virus protein coat. This releases or renders accessible the virus nucleic acid or genome.
Transcription / mRNA production:
For some RNA viruses, the infecting RNA produces messenger RNA (mRNA). This is translation of the genome into protein products. For others with negative stranded RNA and DNA, viruses are produced by transcription then translation.
The mRNA is used to instruct the host cell to make virus components. The virus takes advantage of the existing cell structures to replicate itself.
Synthesis of virus components:
The following components are constructed by the virus through the host's existing organelles:
Viral protein synthesis: virus mRNA is translated on cell ribosomes into two types of virus protein.
Structural: the proteins which make up the virus particle are manufactured and assembled.
Non – structural: not present in particle, mainly enzymes for virus genome replication.
Viral nucleic acid synthesis (genome replication) new virus genome is synthesized, templates are either the parental genome or with single stranded nucleic acid genomes, newly formed complementary strands. By a virus called polymerate or replicate in some DNA viruses by a cell enzyme. This is done in rapidly dividing cells.
Virion assembly:
A virion is simply an active or intact virus particle. In this stage, newly synthesized genome (nucleic acid), and proteins are assembled to form new virus particles.
This may take place in the cell's nucleus, cytoplasm, or at plasma membrane for most developed viruses.
Release:
It is the liberation stage for the viral generation from the host cell. The viruses, now being mature are released by either sudden rupture of the cell, or gradual extrusion (budding) of enveloped viruses through the cell membrane.
The new viruses may invade or attack other cells, or remain dormant in the cell. In the case of bacterial viruses, the release of progeny virions takes place by lysis of the infected bacterium. However, in the case of animal viruses, release usually occurs without cell lysis.
After virion release, some viral proteins remain within the host’s cell membrane, which acts as potential targets for circulating antibodies. Residual viral proteins that remain within the cytoplasm of the host cell can be processed and presented at the cell surface on MHC class-I molecules, where they are recognized by T cells.
