BARTON MBIO 201 - Viruses, Viroids, and Prions
School name Barton College
Pages 6

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Viruses, Viroids, and Prions – Chapter 13 notesA Glimpse of History Tobacco mosaic disease (1890s) • D. M. Iwanowsky, Martinus Beijerinck determined caused by “filterable virus” too small to be seen with light microscope, passed through filters for bacteria• Decade later, F. W. Twort and F. d’Herelle discovered “filterable virus” that destroyed bacteria• Previously, many bacteria, fungi, protozoa identified as infectious diseases• Virus means “poison”• Viruses have many features more characteristic of complex chemicals (e.g., still infective following precipitation from ethyl alcohol suspension or crystallization)Viruses: Obligate Intracellular Parasites Viruses simply genetic information: DNA or RNA contained within protective coat• Inert particles: no metabolism, replication, motility• Genome hijacks host cell’s replication machinery• Inert outside cells; inside, direct activities of cell• Infectious agents, but not alive• Can classify generally based on type of cell they infect: eukaryotic or prokaryotic• Bacteriophages (phages) infect prokaryotes• May provide alternative to antibioticsGeneral Characteristics of Viruses Most viruses notable for small size• Smallest: ~10 nm, ~10 genes• Largest: ~500 nm Virion (viral particle) is nucleic acid, protein coat• Protein coat is capsid: protects nucleic acids• Carries required enzymes• Composed of identical subunits called capsomers • Capsid plus nucleic acids called nucleocapsid • Enveloped viruses have lipid bilayer envelope• Matrix protein between nucleocapsid and envelope• Naked viruses lack envelope; more resistant to disinfectants• Viral genome either DNA or RNA, never both• Useful for classification (i.e., DNA or RNA viruses)• Genome linear or circular• Double- or single-stranded; affects replication strategy• Viruses have protein components for attachment• Phages have tail fibers• Many animal viruses have spikes• Allow virion to attach to specific receptor sites• Generally three different shapes: Icosahedral, helical, or complex• International Committee on Viral Taxonomy (ICVT) publishes classification of viruses• 2009 report: >6,000 viruses → 2,288 species → 348 genera → 87 families → 6 orders• Key characteristics include genome structure (nucleic acid and strandedness) and hosts infected• Other characteristics (e.g., viral shape, disease symptoms) also considered• Virus families end in suffix -viridae • Names follow no consistent pattern• Some indicate appearance (e.g., Coronaviridae from corona, meaning “crown”)• Others named for geographic area from which first isolated (e.g., Bunyaviridae from Bunyamwera in Uganda, Africa)• Genus ends in -virus (e.g., Enterovirus)• Species name often name of disease; e.g., poliovirus causes poliomyelitis, though viruses commonly referred to only by species name• Viruses often referred to informally• Groups of unrelated viruses sharing routes of infection• Oral-fecal route: enteric viruses• Respiratory route: respiratory viruses• Zoonotic viruses cause zoonoses (animal to human)• Arboviruses (from arthropod borne) are spread by arthropods; often can infect widely different species; important diseases: yellow fever, dengue fever, West Nile encephalitis, La Crosse encephalitisBacteriophages Three general types of bacteriophages based on relationship with host• Lytic phages• Temperate phages• Filamentous phages Lytic Phage Infections• Lytic or virulent phages exit host• Cell is lysed; productive infection: new particles formed • T4 phage (dsDNA) as model; entire process takes ~30 minutes• Five step process: attachment, genome entry, synthesis, assembly, release• Attachment: phage exploits bacterial receptors• Genome entry T4 lysozyme degrades cell wall Tail contracts, injects genome through cell wall and membrane• Synthesis of proteins and genome Early proteins translated within minutes; nuclease degrades host DNA; protein modifies host’s RNA polymerase to not recognize its own promoters Late proteins are structural proteins (capsid, tail); produced toward end of cycle• Assembly (maturation): some components spontaneously assemble, others require protein scaffolds• Release Lysozyme produced late in infection; digests cell wall Cell lyses, releases phage Burst size of T4 is ~200 Temperate Phage Infections • Option of lytic infection or incorporation of DNA into host cell genome• Lysogenic infection • Infected cell is lysogen • Lambda (λ) phage as model• Lambda (λ) phage: linear chromosome; complementary single-stranded overhangs at ends join inside host• Resulting circular molecule either directs lytic infection or integrates into E. coli chromosome• Phage enzyme integrase inserts DNA at specific site• Site specific recombination• Integrated phage DNA termed prophage • Replicates with host chromosome• Can be excised by phage-encoded enzyme• Results in lytic infection• A repressor prevents, maintains lysogenic state• Lambda (λ) phage: DNA excised from chromosome only about once per 10,000 divisions of lysogen • If DNA damaged (e.g., UV light exposure), SOS repair system turns on, activates a protease• Protease destroys repressor, allows prophage to be excised, enter lytic cycle• Called phage induction; allows phage to escape damaged host• Lysogen immune to superinfection (infection by same phage) and lysogenic conversion• Change in phenotype of lysogen from prophage • E.g., toxins encoded by phage genes; only strains carrying prophage produce the toxins• Repressor maintaining integrated prophage also binds to operator on incoming phage DNA, prevents gene expression: immunity to superinfection  Filamentous Phages• Single-stranded DNA phages; used to produce only single-stranded recombinant DNA• Look like long fibers• Cause productive infections; host cells not killed, but grow more slowly• M13 phage as model• Attaches to protein on F pilus of E. coli• Single stranded DNA genome enters cytoplasm• DNA polymerase synthesizes complementary strand• Called replicative form (RF); one strand used as template for synthesis of mRNA, copies of genome• M13 phage coat protein molecules inserted into cytoplasmic membrane• Other proteins form pores• As phage DNA excreted through pores, coat proteins coat the DNA,


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BARTON MBIO 201 - Viruses, Viroids, and Prions

Course: Mbio 201-
Pages: 6
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