mibo3500ch5Sp11 - Viruses and Other Acellular Agents...

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Viruses and Other Acellular Agents
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Brief History of Viruses Pasteur recognized that rabies was caused by an infectious agent but one not culturable by known methods He proposed the term “virus” (poison) Initially, people referred to all infectious agents (incorrectly) as viruses In 1892 Ivanoski isolated the plant pathogen Tobacco Mosaic Virus By 1900 Loeffler & Frosch showed that Hoof-and-Mouth disease in cattle caused by “filterable” agent (porcelain filters) Twort discovered bacterial viruses in 1915
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Novel Properties of Viruses 1. Ultramicroscopic particles Smallest 10-20 nm (poliovirus) About the size of a ribosome Largest 450 nm (poxviruses) Almost as large as small bacteria 2. Compact, economical structure Contain only the bare necessities and are inert outside of a host cell
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Novel Properties of Viruses 3. They do not independently fulfill the characteristics of life Lack ability to grow, reproduce, metabolize, transport, or show any type of sensory response 4. Basic structure consists of a protein capsid which surrounds nucleic acid Capsid consists of repeating protein subunits which self-assemble (protomers/capsomers) Nucleic acid component: can be either RNA or DNA ssDNA ssRNA dsDNA dsRNA
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Novel Properties of Viruses 5. Many have geometric shapes & can be crystallized
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Novel Properties of Viruses 6. Molecules on surface (spikes or peplomers) impart a high specificity (recognition) for host cell type Attach to host cells via specific receptors on host cells’ surfaces Viruses peplomers to match host cell receptors, enabling specific host recognition 7. Multiply by an assembly-line method, not by binary fission Exception: prophages or proviruses which are embedded in the host chromosome
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Novel Properties of Viruses 8. Lack enzymes for most metabolic processes BUT MAY CARRY some nucleases and/or polymerases 9. Lack machinery for protein synthesis 10. Exist in intracellular and extracellular states: Virus (or virus particle) is intracellular Virion is extracellular
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Novel Properties of Viruses 13. They are obligate intracellular parasites of virtually all cell types- prokaryote and eukaryote 14. Viruses contain genetic information that defines itself and encodes it’s own replication proteins Therefore, viruses MIGHT be considered “life” by some scientists
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Viral Origins Likely arose when Archaea and Bacteria were the sole Domains of life existing on Earth Three theories of viral evolution: Originated in primordial soup & co-evolved with more complex life forms Evolved from free-living life forms that lost functions (retrograde evolution) Viruses are escaped nucleic acids no longer under the control of the cell The vast diversity of viruses suggests they’ve evolved repeatedly and often
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General Structure of Viruses Genome and Viral Expression: Size ranges from 4 genes in Hepatitis B virus to hundreds of
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mibo3500ch5Sp11 - Viruses and Other Acellular Agents...

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