2. Replication Of viruses
Different viral strategies
General outline of the replication cycle is similar
Nucleic acid -- necessary genetic information for
replication
Depend on host cell enzymes for replication
Six phases in replication
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4. 1. Adsorption or Attachment
Reactive sites on viruses surface interact with specific
receptors on susceptible host cells
Receptors on the virus capside or envelope irreversibly
binds to cellular receptors on the cell member
Limit the host spp. & cells infected
Receptor molecules differ for different viruses
glycoproteins
protein sequences
Oligosaccharides
e.g. Influenza virus -- haemagglutinin glycoprotein
receptor sites of host cell
HIV – gp120 CD4+ cell
Polio virus -- lipoprotein receptor of host cells
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6. 2. Penetration
The virus particle is taken up inside the cell
Engulfment: Whole virus enter into host cell by intracellular
vacuoles Endocytosis ( viropexis)
Directly penetration plasma membrane
Fusion: Viral envelope with plasma membrane, facilitates
entry of virus into host cell
interaction of a viral fusion protein with a second
cellular receptor (e.g., chemokine receptors for HIV)
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7. 3. Uncoating
• Viral capsid is broken by viral or cellular enzyme;
• Viral NA is released;
physical separation of the viral nucleic acid from the outer structural
components of the virion
free nucleic acid (Picornaviruses)
a nucleocapsid (Reoviruses)
• Viral NA transported to within the host cell
► Transcribed to form new progeny virions
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8. 4. Expression of viral genome and
synthesis of viral components
Specific mRNAs must be transcribed from the
viral nucleic acid
Viruses use cell components to translate the
mRNA
Different pathways based on types and
configurations of the viral genome
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9. DNA viruses
The replication takes place in the cell nucleus
Exception is Pox virus synthesizes DNA in host cell cytoplasm with their
own enzymes
Viral mRNA transcripts produced by the host cell enzyme-DNA-dep-
RNA polymerase II
RNA is processed (splicing, polyadenylation)
Early and late viral mRNA are transcribed from DNA strand in the case
of dsDNA viruses and are translated to give early and late proteins
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10. RNA viruses
Eukaryotic cells possess no enzymes for RNA replication
RNA virus supply the RNA-dependent RNA polymerase (replicase)
Take place in the cytoplasm of the cell
ssRNA Positive sense
+RNA used as mRNA directly
utilize the cell’s ribosomes and enzymes to translate the information contained in
this +RNA to produce viral proteins
ssRNA Negative Sense
• First converted to a +RNA strand
• Then used as an mRNA template for translation or direct transcription to the
genomic RNA
• They therefore need to carry a viral-specific RNA-dependent RNA polymerase
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11. 5. Assembly and Maturation
Assembly of daughter virions into capsids -- in host cell
nucleus or cytoplasm
Herpes and Adeno viruses -- host cell nucleus
Picorna and Pox viruses assembly occur in cytoplasm of
host cell
Enveloped viruses acquire envelope from host cell plasma
membrane, nuclear membrane etc.
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12. 12
6. Release
• Two ways assembled viruses leave host cell:
1. Budding: enveloped virus releases by Budding–
nucleocapsid binds to membrane which pinches off and
sheds the viruses gradually;
– cell is not immediately destroyed
2. Nonenveloped/naked virus and complex viruses released
when cell dies and ruptures
• Number of viruses released is variable
– 3,000-4,000 released by poxvirus
– >100,000 released by poliovirus.