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Virus
Viruses are obligate intracellular parasites that lack cell organelles and exist in two phases - an extracellular phase where they possess few enzymes, and an intracellular phase where they induce host cells to synthesize viral components. They have a nucleocapsid containing either DNA or RNA surrounded by a protein capsid. Viruses replicate through either lytic or lysogenic cycles, with the lytic cycle involving adsorption to and penetration of host cells, synthesis of viral components, assembly of new virus particles, and cell lysis. Viruses can be grown in tissue culture or through plaque assays using bacterial hosts.
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Virus
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- 2. Few definitions • Virus-obligatory parasite, lacking cell organelles • Virology- study of viruses • Virologist- the one who studies viruses Size – 10-400nm Exception- pox virus- can be seen under light microscope
- 3. General Properties • Simple,lacking cell organelles • Presence of either DNA or RNA • Obligatory in nature (can not reproduce without host) • Exist in two phases-extracellular and intracellular • Extracellular phase- possess few enzymes • Intracellular phase- has replicating nucleic acid, induce host metabolism to synthesize viral components
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- 5. • Nucleocapsid- EitherDNA or RNA within protein coat called capsid • Capsid has many copies of one or a few types of proteins • These proteins are called protomers
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- 7. Helical symmetry • Likehollow tubes with protein walls(rigid) • Eg TMV(tobacco mosaic virus) • Protomers or capsomers in spiral arrangement • So becomes a tube like structure(15-18nm in diameter by 300nm long) • An RNA as genome, wound in spiral and lies within groove formed by the protein subunits
- 9. Icosahedral symmetry • Regularpolyhedron • 20 equilateral triangular faces and 12 vertices • nature’s favorite shape • Efficient way to close a space • Capsomers are ring or knob like
- 10. Each capsomer is madeof five or six protomers (pentamers or hexamers) Pentamers are at vertices Hexamers are at edges and triangular faces
- 12. Complex virus-eg.pox virus& bacteriphage
- 13. Largest of theanimal virus 400x240x200 nm Complex Ovoid to brick shaped exterior ds DNA with protein in nucleoid(biconcave disk like structure and two elliptical and lateral bodies lie between the nucleoid and its outer envelope which is a thick layer covered by an array of tubules or fibres
- 15. Viral envelopes • Anouter membrane • Made up of lipid and carbohydrate of host • Viral genes form peplopmes (spikes) on the surface, important in virus attachment to the host • Different in different viruses so can be used for the identification • Flexible so show variable shapes (also called pleomorphic)
- 16. Viral envelopes andEnzymes
- 17. Viral envelopes andEnzymes • Neuraminidase from spike in influenza virus. Can release mature virion from the host cell • Other spike is called haemagglutinin useful for attachment • These peplomers are glycoproteins • M or matrix protein is non glycosylated protein present on the inner surface of the envelope and helps stabilize it • RNA dependent RNA polymerase
- 18. Viral Genome • dsDNA, ss DNA (double stranded and single stranded) • ds RNA, ss RNA • Plus(+) strand- when RNA identical to viral mRNA • Minus(-) strand- when RNA complementary to viral mRNA • Segmented genome-more than one RNA strands
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- 24. Steps in reproduction Adsorptionand penetration- • Bacteria have receptors for phages • Eg. LPS, proteins, techoic acid, flagella, pili • T even phages use CW and proteins as the receptors • Attachment starts when the base plates settles down on the receptor firmly and tail fibrils facilitate the attachment • The tail sheath gets rearranged and becomes shorter and wider • The central tube or core is pushed through the CW • The plate has lysozyme like activity
- 25. Steps in reproduction-contd. •So helps in penetration • Finally the linear DNA is extruded from the head, through the tail tube and into the host cell through a pore
- 27. Steps in reproduction Synthesisof phage nucleic acid and proteins- • Within 2 minutes after injection of T4 DNA into a host E.coli starts synthesizing T4 mRNA • Now there is synthesis of viral constituents Assembly of phage particles- Three proteins are required- • Phage structural proteins • Proteins that help in the assembly • Proteins that are involved in the cell lysis and phage release
- 30. Steps in reproduction Releaseof phage particles- Two proteins are required- T4 lysozyme, attacks bacterial cell wall and holin which creates holes in plasma membrane Once 150 viral particles have accumulated inside the host, lysis takes place
- 31. VIRUS REPRODUCTION LYSOGENIC CYCLEby temperate phages
- 33. Few definitions • Lysogeny-relationshipbetween a temperate phage and its host • Prophage- the form of phage in the host • Lysogen or lysogenic bacteria-infected bacteria • Lysogenic conversion- Corynebacterium diphtheriae infected by beta phage and becomes pathgenic
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- 39. Cultivation of virusesin tissue culture • Monolayer tissue culture • Growth medium for cells in the tissue culture • Antibiotic added to prevent bacterial growth • Kept in a specialized plate • Viruses inoculated in to layer of cells • The cells are covered with a thin layer of agar to limit virion growth
- 40. Detection of viralgrowth in tissue culture 1. Rounding of cell 2. Detachment of cells 3. Formation of giant cells
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- 44. Growing Bacteriophages InThe Laboratory Cellular destruction and lysis called plaques
- 46. Cultivation of bacterialviruses • The plaque method: • Virus, bacteria, and agar mixed, plated and incubated. • After replication the virus lyses the bacteria, forming plaques, or clear zones. • Each plaque is assumed to come from a single viral particle. • The titer (concentration of the stock solution) of the virus is given in plaque forming units.