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Viruses
Virology – study of viruses
Virologist – scientist that studies viruses
Introduction to Viruses:
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In 1898, Friedrich Loeffler and Paul Frosch: found evidence that the cause of foot-and-mouth disease in
livestock was an infectious particle smaller than any bacteria.
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This was the first clue to the nature of viruses, genetic entities that lie somewhere in the grey area
between living and non-living states.
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Viruses depend on the host cells that they infect to reproduce.
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When found outside of host cells, viruses exist as:
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Protein coat or capsid, sometimes enclosed within a membrane.
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The capsid encloses either DNA or RNA which codes for the virus elements. In this form
outside the cell, the virus is metabolically inert (dormant)
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Examples of Viruses: Influenza and Tobacco Mosaic Virus
Discovery of Viruses
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Beijerinck (1897) coined the Latin name "virus" meaning poison for the substance infecting tobacco
plants
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Edward Jenner (1796) developed smallpox vaccine using milder cowpox viruses – the first ever vaccine
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1900 Walter Reed showed that an agent so small it could pass through a filter, which trapped bacteria,
caused the human disease yellow fever
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1918 a pandemic Spanish flu kills 25 million people, more deaths than caused by the first World War
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Wendell Stanley (1935) crystallized sap from tobacco leaves infected with Tobacco Mosaic Virus
(TMV) & found virus was made of nucleic acid & protein
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Viruses couldn't be seen until electron microscope invented in the 1930s
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1950s widespread use of the Salk polio vaccine – killed vaccine
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1978 last naturally occurring case of smallpox in the world
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1982 recognition of new virus Human Immunodeficiency Virus (HIV-1)
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2009 H1N1 Influenza virus – Swine Flu
Characteristics of Viruses
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Not living organisms; Noncellular
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Consist of a nucleic acid core (DNA or RNA) and a protein coat called the capsid
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Cannot grow or replicate on their own (inactive particles); Can only reproduce inside of a living host
cell using its raw materials & enzymes
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Some can cause disease (smallpox, measles, mononucleosis, influenza, colds, AIDS, Ebola Virus
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Some may also cause cancers such as leukemias
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Highly host specific (only infect certain cells)
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Viruses are classified into 2 main groups by their nucleic acid --- DNA or RNA Viruses
Bacteriophage – a virus that infects a specific bacteria
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T4 Bacteriophage – virus that attacks the bacteria E. coli
Lytic & Lysogenic Cycles: 2 ways viruses reproduce in a host cell
Retroviruses
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Contains RNA – RNA enters the cells and makes DNA
– RNA is copied backward – RNA to DNA (usually DNA to RNA)
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Virus DNA becomes part of host cell’s DNA
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Hides in the cell and copies of the virus can be made at any time.
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Causes some cancers and AIDS
http://www.chm.bris.ac.uk/webprojects2006/Kelly/influenzafigure1.jpg
Viroid – infect plants, single stranded RNA molecules that have no capsid – disrupt metabolism and destroy
plants
Prion- contain only protein (no DNA or RNA), forms protein clumps in nervous tissue (mad cow disease)
Comparing Viruses & Living Cells