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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
A = naked virusB = enveloped virus1 = capsid2 = nucleid acid3 = capsomer4 = nucleocapsid5 = virion6 = envelope7 = spikes
Icosahedral Viruses
Virus Multiplication
• Attachment/Adsorption• Penetration • Uncoating • Biosynthesis • Assembly • Release
Virus Classification• ICTV = International Committee on Taxonomy of
Viruses• Hierarchy of virus classification:– 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
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 ofunderlying 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.
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.