REPLICATION OF VIRUS
⇒ Genetic information for viral replication is contained in the viral nucleic acid but lacking the biosynthetic enzymes.
⇒ The virus depends on the synthetic machinery of the host cell for replication.
⇒ The viral multiplication cycle can be divided into six sequential phases as:-
- Adsorption or Attachment
- Biosynthesis (Replication & Protein synthesis)
- Maturation or Assembly
- Virions may come into contact with cells by random collision but adsorption takes place only if there is an affinity between host cell and virus.
- The cell surface should contain specific receptor sites to which the virus can gain attachment.
- For e.g. – in HIV the attachment is between the CD4 receptor on the host cells and viral surface glycoprotein gp 120.
- Animal cells do not have rigid cell wall & the whole virus can enter into them.
- Virus particles may be engulfed by a mechanism resembling phagocytosis, a process known as “viropexis”.
- In enveloped viruses, the viral envelope may fuse with the plasma membrane of the host cell & release the nucleocapsid into the cytoplasm.
- A process in which the outer layer and capsid of a virus is removed, leading to the release of viral genetic material into the host cell.
- In most of the viruses, Uncoating is affected by the action of lysosomal enzymes of the host cells.
- After Uncoating viral nucleic acid incorporates itself into host cell’s genetic material.
- This phase includes the synthesis not only of viral nucleic acid & capsid protein but also of enzymes necessary in various stages of viral synthesis, assembly, and release.
- Steps in Biosynthesis:-
- Transcription of messenger RNA (mRNA) from the viral nucleic acid. This process is called as “Early Transcription”.
- Translation of the mRNA into “early proteins”. These Early protein are enzymes which initiate and maintain the synthesis of virus components. This process is called as “Early Translation”. They may also shut down the synthesis of host protein & nucleic acid.
- Replication of viral nucleic acid. In this process, the viral genome is multiplied for the synthesis of new Virions.
- Transcription of mRNA (Late Transcription) and then the formation of proteins called as ‘late’ or structural proteins (Late Translation). These proteins are the components of daughter virion capsids.
- Maturation consists of the Assembly of daughter Virions which takes place after the formation of viral nucleic acid and proteins.
- Virion assembly may take place in the host cell nucleus (e.g. Herpes & Adenovirus) or cytoplasm (e.g. Picorna & Poxvirus).
- At this stage, the non-enveloped viruses are present intracellularly as fully developed virions, but in case of enveloped viruses, only the nucleocapsid is complete.
- Envelopes are derived from the host cell membrane during the process of The host cell membrane which becomes the envelope is modified by incorporation of virus-specific antigen.
- In human viruses, the release of progeny virions usually occurs without cell lysis.
- Progeny virions are released into the surrounding medium and may infect other cells via budding.
- In some viruses (e.g. varicella), transmission occur directly from cell to cell, the very little free virus being demonstrable extracellularly in the medium.
- Some of the human viruses cause profound damage to the host cell & may be released by cell lysis (e.g. poliovirus).