viral multiplication | viral replication

 

viral multiplication | viral replication

Viral Replication

The genomic information essential for viral replication is present in the viral nucleic acid but lack biosynthetic enzymes. 

The viral multiplication includes following steps.

1. Adsorption or Attachment
2. Penetration
3. Uncoating
4. Biosynthesis
5. Maturation
6. Release

1: Adsorption or attachment

Adsorption or attachment is specific. It is mediated by the binding of virion surface structure, known as ligands, to cell surface receptor.  For example, in Influenza virus case, the surface glycoprotein hemagglutinin attaches specifically to sialic acid residue of glycoprotein receptor sites of respiratory tract.

2: Penetration

After attachment, the virus particle is taken up into the cell.  It takes place by system receptor mediated endocytosis (viropexis). Most non-enveloped viruses go inside by receptor mediated endocytosis. Enveloped viruses fuse their membranes with cellular membranes to transfer the nucleocapsid or genome directly into the cytoplasm.

3: Uncoating

This is the process of removing the virus of its outer layers and capsid so that the nucleic acid is delivered into the cell. With most viruses, the action of lysosomal enzymes of the host cell effects the uncoating. The genome of DNA viruses must be delivered into the nucleus except poxviruses while most RNA viruses remain in the cytoplasm.

4: Biosynthesis

In this phase, the synthesis of the viral nucleic acid and capsid protein and enzymes necessary in the various stages of viral synthesis, assembly and release takes place. Moreover, regulator proteins are also synthesized which serve to direct the sequential production of viral components and shut down the normal cellular metabolism.

5: Maturation

Assembly of the various viral components into virions takes place shortly after the replication of the viral nucleic acid. It can occur in either the nucleus or cytoplasm.

In enveloped viruses, the envelopes are derived from nuclear membrane and plasma membrane of host cell.

6: Release

 After the lysis of cell, viruses can be released from cells, by exocytosis, or by budding from the plasma membrane. Viruses that exist naked nucleocapsid can be released by the lysis of the host cell or by reverse phagocytosis.

Release of enveloped viruses takes place after budding from the plasma membrane without killing the host cell.