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chapter 18 -- animal viruses

chapter 18 -- animal viruses - Chapter 18 Animal Viruses...

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Chapter 18: Animal Viruses
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Cultivating Animal Viruses Must be done inside living tissues or cells Embryonated eggs Must be kept otherwise sterile (i.e. shell disinfection, inoculation hole plugged with gelatin) May have to inoculate specific region of the embryo Monitor eggs by “candling” eggs to view health of embryo Tissue culture cells Monolayers of animal cell lines attached to special flat flasks Thin layer of agar poured over to limit spread of virions after host cell lysis Formation of plaques may occur from lysis or cytopathic effects (CPE) may be observed via microscopy
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Penetration by Animal Viruses Naked animal viruses: Direct penetration Endocytosis Enveloped animal viruses: Endocytosis Membrane fusion
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Naked Virus – Endocytosis Cytoplasmic membrane of host cell encloses an naked virus Endosome forms around intact virus inside the newly ‘pinched off’ host membrane Low pH inside the endosome helps stimulate uncoating Lipid-degrading enzymes may facilitate the release
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Enveloped Virus – Endocytosis Cytoplasmic membrane of host cell encloses an enveloped virus Endosome forms with intact enveloped virus inside the newly ‘pinched off’ host membrane Enveloped virus can undergo membrane fusion to release nucelocapsid Enveloped Virus – Membrane Fusion Fusion occurs between the viral envelope and the host cell cytoplasmic membrane to release nucleocapsid into the cytoplasm
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Release of Viral Particles 1. Host cell autolysis Unlike lytic phages, most animal viruses don’t normally lyse their host cells Host cells usually die because they have been unable to synthesize own DNA/proteins and none of their normal functions have been carried out Cell death triggers release of cellular degradation components (e.g. lysozyme) New virions (naked) are released in the vicinity on healthy cells, and can spread 2. Budding from the plasma membrane releases new enveloped virions Spike proteins are sent to sites on the host cell’s cytoplasmic membrane or nuclear membrane, Golgi body or ER membranes depending on viral replication site In many enveloped viruses, matrix proteins coat the inside of the host cytoplasmic membrane Nucelocapsid moves toward the host cell’s cytoplasmic membrane and becomes enclosed Finally released outside of cell as an enveloped virion
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Virus Budding
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Consequences of Animal Virus Infections 1. Acute Cytocidal Infections - host cell death Death may be preceded by cytopathic effects (CPE) Micro/macroscopic changes or abnormalities in host cells or tissues as a result of viral infection Syncitium formation (giant fused cells) Cell detachment and rounding Host cell death 2. Conversion to cancerous cells Chromosomal disruption in proto-oncogene or tumor-suppressor genes by provirus
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Virus Infection Types Acute Rapid onset, last only a short time
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