Introduction to virology prepared for the benefit of undergraduate medicos. Here is part 1

1. GENERAL PROPERTIES OF VIRUS – the
living chemicals!!

2. WHAT ARE THESE?
 Unicellular organisms
 Prokaryotes
 Eukaryotes
 Do not possess cellular organization
 Contain only one type of genetic material  DNA or RNA (Never both)
 Obligate intracellular parasites
 Lack enzymes for Protein & Nucleic acid synthesis – Depend on the Host
 Mechanism of Multiplication is complex
 Not affected by Antibiotics

3. WHAT ARE THESE?
• Viruses can be crystallized like chemicals
• Since they contain infectious nucleic acid 
Living Chemicals!!
• Diseases caused range from common cold to
Rabies and AIDS
• Some viruses can even cause Cancers –
Oncogenic viruses

4. Morphology
• Extracellular infectious particle  Virion
• Small in size – Ultramicroscopic
• Filterable – Can pass through filters in which bacteria find it
difficult to pass
• Virus particles seen after appropriate staining  Elementary Bodies
• Largest virus  300nm (Size of smallest Bacteria)
• Smallest virus  20nm ( Size of largest protein molecule )

5. How do we Measure??
• Historically !
– Pass through colloidal membrane of graded
porosity  Gradocol membranes
• Later on!
– Ultracentrifuge – size estimated depending on rate
of sedimentation
• Now!
– Electron Microscopy – both size and shape can be
studied

6. Shape of Virus
• Nucleic acid
• Protein coat Nucleocapsid
• Capsid Protects nucleic acid from action of
nucleases
• Composed of capsomeres  polypeptide
molecules
• Capsid attach to Host cell and introduce viral
genome into host cell

7. Symmetry
• Icosahedral Symmetry – Cubical
• Helical symmetry
• Icosahedral  polygon with 12
vertices (corners), 20 facets (Sides)
– Each facet is in shape of a
equilateral triangle
– Capsomeres in icosahedral
symmetry are of two types
• Pentagonal capsomeres at the vertices
• Hexagonal capsomeres in the facets

8. Symmetry
• Helical symmetry
– Nucleic acid and capsomeres are
wound together to form a Helical tube
• Some viruses have Complex
Symmetry  Pox virus

9. Envelope / Outer covering !
• Envelope
– Enveloped virus
– Non enveloped ( Naked Virus)
• Derived from host cell while budding
• Made up of lipoproteins
• Lipid from host + protein coded by virus
• Protein subunits projecting from the surface  Peplomers
• Virus may carry more than one type of peplomers  E.g – Influenza virus
– Triangular spike – Haemagglutinin spike
– Mushroom shaped structure – Neuraminidase
• Envelope : Chemical , Antigenic & Biological properties

11. Chemical Properties
• Only one type of genetic material
• Viruses are the only living forms where the genetic information is
carried solely by RNA.
• Can be extracted using chemicals and can initiate infection in host cells
• Viral protein in capsid  protection of nucleic acid , Antigen specific
• Lipids in envelope derived from host cell
• Viruses do not have enzymes for producing viral components or for
producing energy – Depend on host cell enzymes

12. Resistance!
• Mostly heat labile
– Destroyed in seconds @56°C
– Destroyed in minutes @37°C
– Destroyed in days @4°C
• Stable at lower temperatures
• For long term storage  store at – 70°C
• Better method – Lyophilization/ freeze drying
• Can be stored for years & reconstituted when needed by adding water
• Some viruses do not wihstand freeze drying – Polio Virus

13. Resistance!
• Resistance to acids – Vary
• Susceptible to alkaline conditions
• Inactivated by Sunlight, UV rays and ionizing radiation More resistant to
chemical disinfectants – Some of them act as preservatives for virus !
• Killed by oxidizing agents
– Hydrogen peroxide
– Potassium permagnate
– hypochlorites
• Formaldehyde & BPL are actively virucidal
• Ether, chloroform (Lipid solvents) active on enveloped viruses

15. Viral hemagglutination
• Observed first in influenza
• Viruses agglutinate RBC’s of different species
• Hemagglutination is d/t heamagglutinin spikes
• Neuroaminidase acts as a receptor destroying enzyme. (RDE)
• Destruction of receptor will lead to reversal of heamagglutination 
release of virus from red cell surface : ELUTION
• Convenient method of detection and assay of influenza virus
• When red cells are added to serial dilutions of viral suspension, the highest
dilution that produces heamagglutination – Heamagglutination titre

16. Viral Hemagglutination
 No Agglutination  Settle at bottom like a button
 Agglutination  Red cells spread in a shield like
pattern
 Inactive virus also lyse red cells : hence used to
titrate killed influenza vaccines
 Lysis is inhibited by Antibody to virus 
Heamagglutination inhibition : antiviral antibody
estimation
 Elution seen in viruses which have neuroaminidase
only

17. Viral multiplication
• Adsorption / Attachment
• Penetration
• Uncoating
• Biosynthesis
• Maturation
• Release

19. Adsorption/ Attachment
• Contact by random collision
• Cell surface contain specific receptors for attachment of virus
– Influenza – glycoprotein receptors on respiratory epithelium
– HIV – CD4 receptor on host cell, viral surface glycoprotein (gp120)
– Polio – lipoprotein receptor
Penetration
• Bacteria have thick cell wall , viruses cannot penetrate so , only nucleic acid is introduced
• Animal cells do not have rigid cell walls so the whole virus gets in
• Viruses may also be engulfed fully like phagocytosis : VIROPEXIS
• Enveloped viruses may fuse with plasma membrane of host cell releasing the nucleic
material into host cell

21. UNCOATING
• Stripping of outer layers and capsid
• Uncoating is d/t action of host lysosomal enzymes
• In some viruses (Eg.Pox Virus) uncoating happens in 2
steps
• Step 1 uncoating : Action of host lysosomal enzymes
• Step 2 uncoating : Action of viral uncoating enzymes

22. Biosynthesis
• Viral nucleic acid
• Capsid protein
• Enzymes needed for viral synthesis, assembly and release
• Regulator proteins : shut down host mechanism and direct synthesis of viral
components
• DNA virus synthesize viral nuclear material in host cell nucleus
– Exceptions – pox virus
• RNA virus synthesize viral nuclear material in cytoplasm.
– Exceptions – orthomyxo and paramyxo virus
• Viral proteins are synthesized in cytoplasm.

23. Transcription of
mRNA from
nucleic acid
Translation of
mRNA to early
proteins
Replication of
viral nucleic
acid
Synthesis of late
/ structural
proteins
Shut down host
mechanism

24. Classification based on replication mechanism
• Class I  (Adenovirus, Herpes virus, Papova virus) dsDNA enters host
nucleus & uses host cell enzymes for transcription – infectious
• (hepadena virus) Partial dsDNA become dsDNA in host cytoplasm using
viral polymerase and then moves into host cell nucleus – non infectious
• Class II  (Parvo virus) ssDNA moves into host nucles and converts into
dsDNA using host enzymes.
• Class III  (Reovirus) dsRNA transcribed to mRNA by viral polymerase
• Class IV  positive strand RNA RNA itself acts as mRNA

25. Classification based on replication mechanism
• Class V  Negative strand RNA  uses viral RNA
polymerases to produce mRNA
• Class VI  (retroviridiae) ssRNA is converted to
RNA:DNA hybrid by viral reverse transcriptase.
From this hybrid DNA is synthesised to form provirus.
Provirus is integrated into host cell chromosome for further
replication. May cause neoplasia

26. Maturation
• Nucleus : Herpes, Adeno
• Cytoplasm : Picorna, pox
• Non enveloped virus are now fully formed
• Enveloped virus: only nucleocapsid is formed.
• Envelope is derived from host cell during budding
Release
• Bacteria – lysis of bacterium
• Animals – non lytic mechanism – budding
• Infected host cell may divide forming infected daughter cells
• Progeny virions affect nearing cells
• Some viruses like poliovirus lyse the cell when released.

28. Abnormal replicative cycles
• Some virions may not be infective
• This is due to defective assembly  incomplete virus : infection will have high
heamagglutinin titre but low infectivity (Von Magnus phenomenon)
• Some hos cells do not allow viral replication. Viral components may be synthesised
by assembly and maturation is defective aborive infection
• some viruses are genetically defective: cannot form fully effective progeny 
defective viruses
• Yeild of progeny virus only with a help of “Helper Virus”
Eg – Rous sarcoma virus(RSV) cannot form viral envelope. So only when this virus infects a
cell that is already infected with avian leukosis virus RSV can form a healthy progeny