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10-chapt27_lecture_mmw (1) - Viruses Chapter 27 The"flu up...

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Viruses Chapter 27 The “flu” – up close!
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2 Nature of Viruses All viruses have the same basic structure -Nucleic acid core surrounded by capsid Nucleic acid can be DNA or RNA; Circular or linear; Single- or double-stranded Some viruses store specialized enzymes inside their capsids Many animal viruses have an envelope
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3 Nature of Viruses Viruses are obligate intracellular parasites - Host range = Types of organisms infected - Tissue tropism = Types of cells infected Viruses can remain dormant or latent for years More kinds of viruses exist than organisms -a few thousand described; perhaps trillions exist!
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4 Viral Replication Viruses can only reproduce inside cells -Outside, they are metabolically inert and known as virions Virus hijacks the cell’s transcription and translation machineries to express: - Early genes (expressed 1 st ) - Middle genes - Late genes (expressed late, as a cascade) End result is assembly and release of viruses
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5 Viral Shapes Most viruses come in two simple shapes - Helical - Icosahedral Some viruses are complex -T-even phages = Binal symmetry -Poxviruses = Multilayered capsid Enveloped viruses are polymorphic
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6 Viral Shapes Icosahedral: 20 equilateral triangle facets (like “soccer balls”) Binal Symmetry: T4 Phage, neither purely icosahedral nor helical
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7 Viruses vary in size, as well as in shape
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8 Table 27.1
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9 Viral Genomes Vary greatly Most RNA viruses are single-stranded -Replicated in the host cell’s cytoplasm - Retroviruses (HIV) employ reverse transcriptase - to transcribe RNA into DNA, - often integrated into host DNA -Most DNA viruses are double-stranded -Replicated in nucleus of eukaryotic host cell
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