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VIRUS
A little bit of history
• 10th century: Muhammad ibn Zakarīya Rāzi (Rhazes) wrote the Treatise on
Smallpox and Measles
• 1020’s: Avicenna described virus diseases in The Canon of Medicine
• 1392 : “Virus” (from Latin which means poison) first used in English
• 1400’s: “Virulent” (means poisonous) first used
• 14th century: Ibn Al-Khtib wrote about Black Death Plague in On the
Plague
• 1892: Dmitry Ivanovsky discovered tobacco mosaic virus
• Late 18th century: Edward Jenner found smallpox vaccine
• Early 20th century: Frederick Twort discovered bacteriophage
• 1931: Ernest William Goodpasture demonstrated the growth of influenza
and several other viruses in fertile chicken eggs
• 1935: Wendell Stanley crystallized the tobacco mosaic virus
• 1949: John Franklin Enders, Thomas H. Weller and Frederick Chapman
Robbins together developed a technique to grow polio virus in cultures of
living animal cells
What is virus?
• Virus is a genetic element containing either RNA
or DNA that replicate only in specific host cells
but is characterized by having an extracellular
state
• In the extracellular state, virus is a
submicroscopic particle, also called virion,
containing nucleic acid surrounded by protein
and occasionally other macromolecular
component
• Virion is only a structure which carries virus
genome between host cells and it has no
metabolic activity
Virus Structural Properties
• Nucleic acid located within particle: dsDNA, ssDNA,
dsRNA, or ssRNA
• Capsid: protein coat surrounding nucleic acid
• Morphological units or capsomeres: units which form
capsid, visible under electron microscope
• Structural subunits: protein molecules which form
capsomeres
• Nucleocapsid: a complete complex of nucleic acid and
protein coat
• Some viruses have lipid bilayer which enveloped its
nucleocapsid, and are called enveloped virus. The
ones which don’t have envelope are called naked virus
Icosahedral Viruses
A = naked virus
B = enveloped virus
1 = capsid
2 = nucleid acid
3 = capsomer
4 = nucleocapsid
5 = virion
6 = envelope
7 = spikes
Helical Viruses
Complex viruses
Virus Multiplication
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Attachment/Adsorption
Penetration
Uncoating
Biosynthesis
Assembly
Release
Biosynthesis Method
Virus Classification
• ICTV = International Committee on Taxonomy of
Viruses
• Hierarchy of virus classification:
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Ordo: -virales
Family: -viridae
Subfamily: -virinae
Genus: -virus
Species
• Species concept: A polythetic class of viruses that
constitute a replicating lineage and occupies a
particular ecological niche
Virus Classification Basis
• morphology (size, shape, enveloped/naked)
• physicochemical properties (molecular mass,
buoyant density, pH, thermal, ionic stability)
• genome (RNA, DNA , segmented sequence,
restriction map, modifications etc)
• macromolecules (protein composition and
function)
• antigenic properties
• biological properties (host range, transmission
tropism etc)
Current Data
• According to 7th Report of the ICTV (2000),
there are:
– 3 orders,
– 56 families,
– 9 subfamilies,
– 233 genera
– and 1550 virus species
Other Methods of Virus Classification
• Baltimore classification: based on nucleic acid type and
replication method --> divide viruses into seven groups
• Holmes classification: based on host --> divide viruses
into three groups
• LHT classification: based on chemical and physical
characters like nucleic, Symmetry, presence of
envelope, diameter of capsid, number of capsomers
• Casjens and Kings classification: based on type of
nucleic acid ,presence of envelope, symmetry and site
of assembly --> divide viruses into four groups
Baltimore Classification
Plant Viruses
Plant Viruses Infection
• Plant cells have thick cell wall and
plasmodesmata
• Plant viruses do not appear to specifically interact
with host cell membranes or cell walls, as do
bacterial and animal viruses
• Infection mechanism appear to be:
1. passive carriage through breaches in the cell wall in
the first instance
2. followed by later cell-to-cell spread in a plant by
means of specifically-evolved "movement"
functions, and perhaps spread via conductive tissue
as whole virions
Passive Carriage of Plant Viruses
• a purely mechanical injury that breaches the cell wall and
transiently breaches the plasma membrane of
underlying cells;
• similar gross injury due to the mouthparts of a herbivorous
arthropod, such as a beetle;
• injection directly into cells through the piercing
mouthparts of sap-sucking insects or nematodes;
• carriage into plant tissue on or in association with cells of a
fungal parasite;
• vertical transmission through infected seed or by
vegetative propagation;
• transmission via pollen; and
• grafting of infected tissue onto healthy tissue.
Animal Viruses
Animal Viruses Infection
Fate of Infected Cells
Animal Virus Release
Bacteria Viruses
Bacteriophage Infection
• The phage tail fibres are the attachment sites; these
individually bind the bacterial cell surface - specifically to
certain lipopolysaccharides and to the surface outer
membrane protein.
• After tail fibre binding has consolidated, the baseplate
then settles down onto the surface and binds firmly to it.
• After this occurs, a comformational changes takes place in
baseplate and sheath protein structures, and the tail
sheaths contracts, pushing the tail core through the cell
wall, possibly in an ATP-driven process: this is aided by a
lysozyme activity associated with the baseplate assembly.
• DNA is then extruded from the phage head.
Bacteriophage Cycles
TERIMA KASIH