Viruses replicate inside host cells using the cell's machinery. There are typically six phases to viral replication: 1) attachment to host cell receptors, 2) penetration of the host cell, 3) uncoating of the viral genome from the capsid, 4) expression of viral genes and synthesis of new viral components, 5) assembly of new virus particles, and 6) release of progeny virus from the host cell. The replication cycles of DNA and RNA viruses differ in some of the intermediate steps but generally follow this overall pattern of attachment, entry, genome expression/replication, assembly and release.

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 ???