Precise presentation on Viral classification and Types of replication in Virus. Entry of virus Spread of virus General steps in a virus replication cycle Attachment, Penetration, Uncoating, Multiplication Multiplication of Single-Stranded RNA (ss RNA) Viruses Multiplication of Double-Stranded RNA (ds RNA) Viruses Multiplication of Single-Stranded DNA (ss DNA) Viruses Multiplication of Double-Stranded DNA (ds DNA) Viruses Release of new virions Common viral diseases of Bovines

1. Viral Classification &
Types of replication in virus
Submitted by:
Rakshith K
MVHK1742
Submitted to:
Dr Suguna Rao
Professor
Dept. of Veterinary Pathlogy
Veterinary college, Bangalore

2. Classification of viruses

4. Multiplication of virus
• Virus multiply only in living cells.
• Host cell must provide the energy and synthetic
machinery, and the precursors for the synthesis of viral
proteins and nucleic acids.
• Many of the pathological effects of viral infections is by
process of viral multiplication.
• Until a virus enters a cell, it is more dead than alive.

5. Entry
• Direct inoculation into the bloodstream. (Arthropod
borne) E.g.: Togaviridae
• Penetrating the skin, or mucosal barrier of the
respiratory or gastrointestinal tract.
• Reach the target organ.
• Some virus do primary multiplication in the cells at the
portal of entry & spread to target organ (for disease
production) – E.g.: Herpes.
• Some direct spread to target organ – E.g.: Reovirus.

6. Spread:
1. Lymphatic or bloodstream (e.g., With reoviruses)
2. Infected lymphocytes (e.g., With cytomegaloviruses)
3. Neuronal axons (e.g., With rhabdoviruses)
4. Cross the placenta to the foetus (e.g., With
cytomegalovirus)

7. General steps in a virus replication
cycle include:
1) Attachment:
• Interaction of a virus particle (virion) with a specific
receptor site on the surface of a cell.
• Specific binding of virus to the cell surface receptors

8. 2) Penetration
• The virus particle is taken up inside the cell.
• Penetration, engulfment, or viropexis.
• Enveloped viruses: Virion envelope fuses with the
plasma membrane allowing entry of the nucleocapsid.
• Non-enveloped viruses: Receptor-mediated endocytosis
(pinocytosis) OR translocation (change of location)

9. 3) Uncoating
• After penetration, the virion is uncoated.
• The RNA or DNA of the virus particle is exposed by
removal of the capsid (protein coat).
• The infectivity of the virus is lost at this point.
• Uncoating is followed by the "eclipse phase", when no
virions are seen.

10. • Infection of a cell, does not necessarily lead to viral
multiplication and cell damage.
• Cell must be permissive for productive infection means
appropriate receptors and be capable of supporting
replication.
• Non-productive infection – infection by Defective cells.
(defect in some aspect of replication)
• Such persistent infections may, under appropriate
circumstances, get activated to productive infection, or
lead to cell transformation.

11. 4) Multiplication

12. • Transcription varies with type of nucleic acid, enzymes
carried by virion & intracellular location of virus.
• DNA virus- nucleus, use host RNA polymerase.
• Single-stranded positive sense RNA viruses- mRNA.
• Negative-sense viruses (orthomyxo-, paramyxo- and
rhabdoviruses) - carry RNA polymerases.

13. Virus Replication Cycle
1. Interaction of a target cell
2. Attach to the target cell
membrane via ligand receptor
interactions.
3. Enter the target cell via
endocytosis/membrane fusion and
spread genomic and protein
components via “uncoating”
within the cell.
4. (a) RNA viruses most commonly
replicate in the cytoplasm.
(b) DNA viruses most commonly
replicate in the nucleus.
5. Reassemble of virus
6. Release of progeny from the target
cell by budding or cell lysis

14. Multiplication of Single-Stranded RNA (ss RNA) Viruses - I
• Positive (+) strand viruses: Togaviruses and picornaviruses.
• Viral RNA can serve directly as mRNA.
• Viral RNA links to ribosome & nucleotide sequences of RNA are
directly translated to proteins.
• Viral (+) strand RNA 
1. directly translated to proteins
2. Synthesise complementary negative (-) strand RNA.
• Complementary (-) strand RNA  serves as a template for
synthesis of additional (+) strands.
• Repeat the process and ultimately form viral progeny.
• Since replication doesn’t depend on any enzyme carried by the
virion, RNA extracted from (+) strand RNA viruses is infectious.

15. Multiplication of Single-Stranded RNA (ss RNA) Viruses - II
• Negative (-) strand viruses: Arenaviruses, bunyaviruses,
orthomyxoviruses, paramyxoviruses and rhabdoviruses.
• Viral RNA be transcribed into a (+) strand mRNA.
• Transcriptase carried within the virion.
• RNA extracted from (-) strand RNA viruses is not infectious,
since replication depends on virion transcriptase.

16. Multiplication of Single-Stranded RNA (ss RNA) Viruses - III
• Retro virus:
• These viruses have a DNA step in replication.
• Viral RNA serves as a template for the synthesis of viral DNA.
• This depends on the enzyme reverse transcriptase (RNA-
dependent DNA-polymerase) carried by virion.
• cDNA is made out of viral RNA.
• This cDNA is integrated in to cellular DNA.
• Viral multiplication is by transcription and translation of
integrated viral DNA, which codes for necessary proteins.
• Viral RNA and proteins pakaged to for viral progeny.

17. Multiplication of Double-Stranded RNA (ds RNA) Viruses
• Reoviruses
• their RNA is transcribed by virion polymerase
• (-) strand  (+) mRNA  viral proteins & enzymes.
• Same (+) mRNA complimentary to (-) strand RNA, yielding
double stranded RNA for assembly of virions.

18. DNA viruses
Multiplication of Single-Stranded DNA (ss DNA) Viruses
• Single-stranded (DNA) parvoviruses replicate in the nucleus.
• They depend on host cell enzymes for synthesis of
complementary DNA to form double-stranded DNA.
• This dsDNA transcribed into mRNA & genomic DNA.

19. Multiplication of Double-Stranded DNA (ds DNA) Viruses
• Papovavirus, Adenovirus, and Herpesvirus.
• dsDNA is transported to nucleus, cellular enzymes are
used for production of mRNA.
• Replication of DNA uses
1. host cell polymerases – Papavo
2. Virally coded polymerase – Herpes and Adeno
• Poxvirus multiplication occurs in the cytoplasm.
• virion is uncoated by both cellular enzymes and enzyme
products of an early transcribed mRNA.
• Cytoplasmic DNA replication by enzymes carried within
the virion and translation products of early and late
mRNA.

20. 5) Release of new virions
• Non enveloped virus mature in host cell in nucleus or
cytoplasm. Cell lysis cause their release.
• Enveloped virus obtain envelop by budding. Release by
budding will not cause cell lysis. (All (-) strand RNA virus)
• Retro, Arena virus – Non-cytolytic.
• Togaviruses, Paramyxo, and Rhabdoviruses – Cytolytic.

21.  EXCEPTIONS:
• Pox virus is released by both mechanism (BUDDING)
• acquire envelop by golgi complex intracellularly.
• acquire 2nd envelop by plasma membrane while budding.
• Pox virus: without envelop
• Released without envelop by disruption of cell.
• Both are infectious
• Both intracellular and intranuclear multiplication

22. • Herpes virus: (enveloped)
• Acquire envelop by inner lamella of nuclear membrane.
• These enveloped virions pass with in cisternae without
contact with cytoplasm and released.
• No cell disruption.
• But productive forms of herpes cause cell damage.

23. Common viral diseases of Bovines
• Vesicular Stomatitis
• Bovine viral diarrhoea-mucosal disease
• Foot-and-mouth Disease (Aphthous Fever)
• Cowpox
• Rabies
• Bovine Ephemeral Fever
• Bovine cutaneous papillomatosis
• Rinderpest
• Calf neonatal diarrhoea
• Lumpy Skin Disease
• Bovine Respiratory Syncytial Virus (BRSV)
• Bovine leukaemia virus
• Bovine immunodeficiency virus
• Bovine Parvovirus Infection
• Buffalopox
• Akabane Disease (of Cattle): Also known as "congenital arthrogryposis
hydranencephaly syndrome", Akabane disease caused by a bunyavirus.