Microbiology Unit 3

Microbiology Unit 3 - Viruses Chapter 10 History 1892...

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Viruses Chapter 10
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History 1892 Iwanoski (Russian) - reported a “filterable virus ” that caused Tobacco Mosaic Disease (TMV) - virus is a Latin word which means poison 1898 Beijerink (Dutch) - boiled the filtrate from TMV diseased leaves - boiled filtrate was no longer able to cause disease when applied to healthy tobacco plants - referred to the “filterable virus filtrate) as a “living contagious fluid 1900-1930 Scientists were unable to determine the nature of viruses. They believed viruses were very small, invisible living microorganisms . 1935 Wendel Stanley - crystallized TMV; believed TMV was solely protein - received Nobel Prize
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Since 1940 - Virology has made great progress due to two technological developments: 1. Electron microscopy – ability to see viruses 2. Cell culture - ability to cultivate viruses in vivo General Characteristics of Viruses Viruses are chemical particles : 1. Non-cellular - no cytoplasm, nucleus, organelles, etc. 2. Non-living - do not grow (always the same size) - no metabolism - non-motile, exhibit no self-movement
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3. Reproduce , but only inside living host cells - incapable of any activity outside a living host - Definition of a virus Viruses are obligate , intracellular parasites : that is, viruses are made strictly within host cells at the expense of the host . 4. Viral components (handout ) A. Nucleocapsid present in all viruses consists of a genome and a capsid 1. The Genome a. RNA or DNA , but never both - Function : to code for viral proteins : - capsomeres - viral enzymes - in the viral envelope
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When RNA ssRNA = single-stranded RNA dsRNA = double-stranded RNA When DNA ssDNA = single-stranded DNA dsDNA = double-stranded DNA b. Structural variations: nucleic acid (RNA or DNA) may be: - a single segment - multiple segments (influenza virus has 8 segments) strand of DNA or RNA may be: - linear - a closed loop
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2. The Capsid consists of many protein subunits called Capsomeres functions of capsids: a. determines the shape of naked (= non-enveloped) viruses b. capsid projections, called spikes , attach virus to host cell c. protects genome from environmental hazards (chemicals, acids, bases, freezing temps, etc) stimulates the host’s immune system to produce antibodies against the virus
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B. Envelopes 1. present in many (but not all ) animal viruses 2. structure of the envelope (= fluid-mosaic model of the cell membrane) bilayer of phospholipid molecules proteins 3. some envelopes have Spikes Spikes function to attach the virus to the host cell 4. Damage to the envelope - due to disinfectants, detergents, acids, freezing and thawing,…. may render the virus unable to infect the host.
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Viral Shapes 1. Naked (non-enveloped) virion shapes a. Rod-shaped / helical capsid a hollow cylinder example: TMV = tobacco mosaic virus b. Icosahedral capsid a polyhedron with 20 equal triangular planes example: = polio virus
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2. Enveloped virion shapes a. envelope encloses the capsid, and is loose fitting
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Microbiology Unit 3 - Viruses Chapter 10 History 1892...

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