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VIRUS. A little bit of history. 10 th 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
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A little bit of history • 10th century: Muhammad ibnZakarīyaRāzi (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? • Virusis 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 moleculeswhich 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
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: • passive carriage through breaches in the cell wall in the first instance • 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 alysozyme activity associated with the baseplate assembly. • DNA is then extruded from the phage head.
