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

A = naked virusB = enveloped virus1 = capsid2 = nucleid acid3 = capsomer4 = nucleocapsid5 = virion6 = envelope7 = spikes

Icosahedral Viruses

Helical Viruses

Complex viruses

Virus Multiplication

• Attachment/Adsorption• Penetration • Uncoating • Biosynthesis • Assembly • Release

Biosynthesis Method

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

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 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.

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