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19_Lectures_PPT_2008_class

19_Lectures_PPT_2008_class - Chapter 19 Viruses 0.5 m...

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0.5 µm Chapter 19 Viruses
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Summary Concept 18.1: A virus consists of a nucleic acid surrounded by a protein coat Concept 18.2: Viruses reproduce ONLY in host cells Concept 18.3: Viruses, viroids, and prions are formidable pathogens in animals and plants
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A borrowed life Viruses called bacteriophages can infect and set in motion a genetic takeover of bacteria, such as Escherichia coli E. coli and its viruses are called model systems because of their frequent use by researchers in studies that reveal broad biological principles the field of molecular biology essentially began in these systems But are they ALIVE? Historically they were thought to be biological chemicals, e.g. the latin root word for virus means ‘poison’
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Bacterial cells are much smaller and more simply organized than those of eukaryotes. Viruses a re smaller and simpler than bacteria Overview: Microbial Model Systems Virus Bacterium Animal cell Animal cell nucleus 0.25 µm
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Concept 18.1: A virus consists of a nucleic acid surrounded by a protein coat Scientists detected viruses indirectly long before they could see them The story of how viruses were discovered begins in the late 1800s with Tobacco Mosaic Virus (TMV)
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TMV phenotype
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The Discovery of Viruses: Scientific Inquiry Tobacco mosaic disease stunts growth of tobacco plants and gives their leaves a mosaic coloration In the late 1800s, researchers hypothesized that a particle smaller than bacteria caused the disease they showed that the sap from the diseased tobacco plant was sufficient to spread the disease 1883 Adolf Mayer 1893 Dimitri Ivanowsky 1898 Beijernick In 1935, Wendell Stanley confirmed this hypothesis by crystallizing the infectious particle, now known as tobacco mosaic virus (TMV) this type of crystallography is not possible for bacteria, so it was unclear what these things were ..... then ....
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Fig. 19-2 RESULTS 1 2 3 Extracted sap from tobacco plant with tobacco mosaic disease Passed sap through a porcelain filter known to trap bacteria Rubbed filtered sap on healthy tobacco plants 4 Healthy plants became infected
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Structure of Viruses Viruses are not cells Viruses are very small infectious particles consisting of nucleic acid enclosed in a protein coat and, in some cases, a membranous envelope They are not metabolically active without a host some are smaller than a ribosome, some are much bigger
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Viral Genomes Viral genomes may consist of Double- or single-stranded DNA Double- or single-stranded RNA Can be a few as four genes or as many as several hundred Depending on its type of nucleic acid, a virus is called a DNA virus or an RNA virus E.coli can be infected by all all classes
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Types of coli-phage
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Capsids and Envelopes A capsid i s the protein shell that encloses the viral genome A capsid can have various structures commonly the 20-sided shape, icosahedral Greek: eikosaedron, from eikosi (twenty) and hedron (seat)
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