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
10.
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
14.
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
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
20.
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.
27.
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