Replication of Viruses
As viruses are obligate intracellular pathogens they cannot replicate without the machinery and metabolism of a host cell.
Although
the replicative life cycle of viruses differs greatly between species and
category of virus, there are six basic stages that are essential for viral
replication.
1. Attachment:
a. Viral proteins on the capsid or
phospholipid envelope interact with specific receptors on the host cellular
surface.
b. This specificity determines the
host range (tropism) of a virus.
2. Penetration: The process of attachment to a
specific receptor can induce conformational changes in viral capsid proteins,
or the lipid envelope, that results in the fusion of viral and cellular
membranes.
a. Some DNA viruses can also enter
the host cell through receptor-mediated endocytosis.
3. Uncoating: The viral capsid is removed
and degraded by viral enzymes or host enzymes releasing the viral genomic
nucleic acid.
4. Replication: After the viral genome has
been uncoated, transcription or translation of the viral genome is initiated.
a. It is this stage of viral
replication that differs greatly between DNA and RNA viruses and viruses with
opposite nucleic acid polarity.
b. This process culminates in
the de novo synthesis of viral proteins and genome.
5. Assembly: After de novo synthesis
of viral genome and proteins, viral proteins are packaged with newly replicated
viral genome into new virions that are ready for release from the host cell.
6. Virion release: There are two methods of viral
release: lysis or budding.
a. Lysis results in the death of an
infected host cell, these types of viruses are referred to as cytolytic.
An example is variola major also known as smallpox.
b. Enveloped viruses, such as
influenza A virus, are typically released from the host cell by budding. It is
this process that results in the acquisition of the viral phospholipid
envelope. These types of virus do not usually kill the infected cell and are termed cytopathic
viruses.
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