Viral replication is the formation of biological viruses during the infection process in the target host cells. Viruses must first get into the cell before viral replication can occur. Through the generation of abundant copies of its genome and packaging these copies, the virus continues infecting new hosts. Replication between viruses is greatly varied and depends on the type of genes involved in them. Most DNA viruses assemble in the nucleus while most RNA viruses develop solely in cytoplasm.
Viral production / replication
Viruses multiply only in living cells. 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; Attachment Entry, Uncoating, Transcription / mRNA production, Synthesis of virus components, Virion assembly and Release (Liberation Stage).
Attachment
It is the first step of viral replication. The virus attaches to the cell membrane of the host cell. It then injects its DNA or RNA into the host to initiate infection. 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 cells 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. This protects the cell from antibodies like in the case of the HIV virus.
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), which can translate the genome into protein products. For viruses with negative stranded RNA, or 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 components are manufactured by the virus using the host's existing organelles: Viral proteins: Viral mRNA is translated on cellular ribosomes into two types of viral protein: Structural: proteins which make up the virus particle Nonstructural: proteins not found in the virus particle, mainly enzymes for virus genome replication Viral nucleic acid (genome replication): New viral genomes are synthesized; templates are either the parental genome or newly formed complementary strands, in the case of single-stranded genomes. These genomes are made by either a viral polymerase or (in some DNA viruses) a cellular enzyme, particularly 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 (liberation stage) The viruses, now being matured are released by either sudden rupture of the cell, or gradual extrusion (force out) 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.
Baltimore Classification System
Viruses are classed into 7 types of genes, each of which has its own families of viruses, which in turn have differing replication strategies themselves. David Baltimore, a Nobel Prize-winning biologist, devised a system called the Baltimore Classification System to classify different viruses based on their unique replication strategy. There are seven different replication strategies based on this system (Baltimore Class I, II, III, IV, V, VI, VII). The seven classes of viruses are listed here briefly and in generalities.