It is a process in which a virus enters susceptible host cell and induces a living host cell to synthesize essential components for the synthesis of new viral particles. The particles are then assembled and newly formed virions escape from host cell to infect other cells.
In viral replication, following events occur in sequence,
- Recognition of host surface proteins & attachment of virus to host cell.
- Penetration into host cell membrane by endocytosis.
- In case of enveloped virus, Merging of envelope with the host cell membrane
- Entry of capsid along with genome into the interior of host cell.
- Removal of capsid to free the genome containing DNA
- Entry of viral genome to cell nucleus & transcription of its DNA into viral m‐RNA by the host cell’s RNA polymerase.
- Synthesis of viral proteins and enzymes by host cell’s ribosomes utilizing viral m‐RNA.
- During this process, regulatory proteins are synthesized first, which initiate the transcription of early genes responsible for viral DNA replication.
- After DNA replication late genes are transcribed & translated to produce structural proteins required for assembly of new virion.
- Assembly of viral components to form a mature virus particle.
- Release of progeny virus through budding of host cell.
Steps in Viral Replication:
The replication of virus may differ depending upon type of virus species however basic process of viral replication involves following steps:
- Viral attachment & entry
- Early protein synthesis
- Nucleic acid synthesis
- Late protein synthesis & processing
- Packaging & assembly
- Viral release
Adsorption/attachment of virus to host cell occurs in two steps. The first step is preliminary attachment involving ionic bond and charge. It is weak, can be reversed easily by shift in pH or salt concentration. The second step involves specific, non-reversible attachment through molecular entities known as receptors. High specificity exists between virus & cell, the envelope spikes may unite with cell surface receptors. Receptors may exist on bacterial pili or flagella or on the host cell membrane. However plant viruses do not seem to require receptor sites, while animal & bacterial viruses requires marked specificities of adsorption.
Penetration i.e. Entry of virus to host cell can occur through various mechanisms. This step may occur by phagocytosis, or envelope of the virus may blend with the cell membrane, or penetration of host cell through transient pores or the virus may “inject” its genome into the host cell.
Injection of genome by virus occurs with the bacteriophage when the tail of the phage unites with the bacterial cell wall and enzymes open a hole in the wall. The DNA of the phage penetrates through this hole. Plant virus penetrates through the transient pores, also called as ectodesmata.
Uncoating refers to removal of capsid to release the genomic material which usually occurs due to the action of lysosomal proteases of phagocytic vacuoles in host cell.
Replication & Biosynthesis
If the genome freed from capsid is RNA, it acts as a messenger RNA molecule and provides the genetic codes for the synthesis of enzymes. The enzymes are used for the synthesis of viral genomes and capsomeres and the assembly of these components into new viruses. If the viral genome consists of DNA, it provides the genetic code for the synthesis of messenger RNA molecules, and the biosynthetic process proceeds.
In some cases, such as in HIV virus, RNA of the virus serves as a template for the synthesis of a DNA molecule. The enzyme reverse transcriptase catalyzes the DNA production. The DNA molecule then remains as part of the host cell chromosome for an unspecified period. From this location, it encodes messenger RNA molecules for the synthesis of enzymes and viral components.
Biosynthetic process can be divided into early and late events with the proteins synthesized named as early protein and late protein respectively. In between these protein synthesis, nucleic acid i.e. genomic material of virus is also replicated.
Assembly & Release
When multiple copies of viral components are synthesized, they are assembled to form mature virus particle in the nucleus or cytoplasm of host cell. DNA viruses except pox virus are assembled in nucleus while RNA virus are usually assembled in cytoplasm.
For the release of new viral particles, number of processes may occur. In some cases, host cell is lysed releasing virions. Naked virions are undergo such process and gets released in burst-like fashion form infected cells.
For enveloped viruses, nucleocapsid move toward the membrane of the host cell, where they force themselves through that membrane in a process called budding. During budding, a portion of cell membrane pinches off and surrounds nucleocapsid as an envelope.
In few cases, virions exit infected cell through tubules (special channels) over a period of time.
The replication process in which the host cell experiences death is called the lytic cycle of reproduction. The viruses so produced are free to infect and replicate in other host cells in the area. However, some virus replicate through lysogenic cycle of replication.