life cycle of viruses

1. Medical Virology
Virus infectious cycle(replication of
RNA and DNA virus)

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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3. Replication of Animal Viruses
• Attach
• Entry
– Direct Penetration
– Membrane fusion
– Endocytosis
• Uncoating
• Synthesis
• Assembly
• Release

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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5. Viral Attachment

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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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.

14. Any questions ???