19_Lecture_Post

19_Lecture_Post - Viruses Overview: A Borrowed Life Viruses...

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Viruses Overview: A Borrowed Life Viruses called bacteriophages can infect and set in motion a genetic takeover of bacteria, such as Escherichia coli Viruses lead “a kind of borrowed life” between life - forms and chemicals The origins of molecular biology lie in early studies of viruses that infect bacteria
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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 In 1935, Wendell Stanley confirmed this hypothesis by crystallizing the infectious particle, now known as tobacco mosaic virus (TMV)
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Figure 19.2 Extracted sap from tobacco plant with tobacco mosaic disease RESULTS Passed sap through a porcelain filter known to trap bacteria Healthy plants became infected Rubbed filtered sap on healthy tobacco plants 1 2 3 4
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Structure of Viruses Viruses are not cells A virus is a very small infectious particle consisting of nucleic acid enclosed in a protein coat and, in some cases, a membranous envelope Viral genomes may consist of either Double- or single-stranded DNA, or Double- or single-stranded RNA Depending on its type of nucleic acid, a virus is called a DNA virus or an RNA virus
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Capsids and Envelopes A capsid is the protein shell that encloses the viral genome Capsids are built from protein subunits called capsomeres A capsid can have various structures Some viruses have membranous envelopes that help them infect hosts These viral envelopes surround the capsids of influenza viruses and many other viruses found in animals Viral envelopes, which are derived from the host cell’s membrane, contain a combination of viral and host cell molecules
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Figure 19.3 Capsomere of capsid RNA Capsomere DNA Glycoprotein Glycoproteins Membranous envelope RNA Capsid Head DNA Tail sheath Tail fiber 18 250 nm 80 225 nm 70–90 nm (diameter) 80–200 nm (diameter) 20 nm 50 nm 50 nm 50 nm (a)Tobacco mosaic virus (b) Adenoviruses (c) Influenza viruses (d) Bacteriophage T4
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Bacteriophages , also called phages , are viruses that infect bacteria They have the most complex capsids found among viruses Phages have an elongated capsid head that encloses their DNA A protein tail piece attaches the phage to the host and injects the phage DNA inside
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Viruses replicate only in host cells Viruses are obligate intracellular parasites, which means they can replicate only within a host cell Each virus has a host range , a limited number of host cells that it can infect
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General Features of Viral Replicative Cycles Once a viral genome has entered a cell, the cell
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19_Lecture_Post - Viruses Overview: A Borrowed Life Viruses...

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