Viruses can only replicate inside living cells by going through a viral multiplication cycle. This cycle includes adsorption to the host cell, entry/penetration either through fusion with the plasma membrane or via endosomes, uncoating to release the viral genome, production of viral components in the nucleus or cytoplasm, assembly of new viral particles, and final release through cell lysis or budding through the plasma membrane. The specific steps may vary depending on if the virus has an envelope and what type of genome it contains.

2. Viruses are obligate intracellular
parasites
They can only replicate in living
cells.

3. Viral Multiplication Cycle
1) Adsorption
2) Entry
3) Uncoating
4) Component production
5) Assembly
6) release

4. 1)ADSORPTION
 requires viral attachment protein
 cellular receptors
 specie specific.
 some neutralizing antibodies are specific
for the attachment proteins
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5. 2)Penetration (enveloped viruses)
Either
• Fusion with plasma membrane
or
• Entry via endosomes, fusion with
acidic endosome membrane

6. 2)Penetration
herpes viruses, paramyxo viruses, HIV
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7. 7

8. 2)Penetration
(Enveloped Viruses)
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9. 2)Penetration (non enveloped)
Only via
endosomes (endocytosis or
viropexis)

10. 10

11. 3)Uncoating & eclipse
release of viral genome
Viral particle can no more be
detected.

12. 4)Synthesis of Viral Nucleic Acid
and Protein
Nucleic acid may be made in the
nucleus or the cytoplasm
Protein synthesis is always in the
cytoplasm
Latent viruses.
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13. Transcription/Translation/Synthes
is:
DNA viruses:
• replicate their DNA in host cell
nucleus. Exception - Poxviruses
• synthesize capsid and other
proteins in cytoplasm

14. Transcription, translation and
synthesis:
RNA viruses:
 Most RNA viruses replicate in the
cytoplasm
 Few exceptions e.g. Influenza virus,
retroviruses.

15. 5)Assembly:
Takes place in cell nucleus,
cytoplasm or (with most
enveloped viruses) at the plasma
membrane

16. 6)Release
Either
Lysis
Or
Budding through plasma membrane
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18. Virus replication: general

19. 1,000 – 100,000 viruses/cell, 5 – 24 hours

20. Adsorption (attachment)
Entry
Replication
Latency
Release