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An Introduction to the Viruses Chapter 6 Copyright © The McGraw-Hill Companies, Inc) Permission required for reproduction or display. Learning Objectives: • Differentiate a virus from a bacterium. • Define viral species. Give an example of a family, genus, and a common name for a virus. • Describe the structure of an enveloped and a nonenveloped virus. • Describe viral infectious cycle including lytic and lysogenic cycles. • Describe how bacteriophages and animal viruses are cultured • Describe the relationship between viruses and cancer. • Provide an example of a latent viral infection • Discuss how a protein can be infectious Why are viruses important? • Health—HIV is a leading cause of death in Africa • Economics—Banana Bunchy Top Virus (BBTV) damages banana crops in Asia and the Pacific annually costing millions • Important participant in the evolution of life forms35-90% of human genome came from viruses! • Gene Therapy—Viruses are efficient at introducing foreign DNA into a cell The Scale of Viruses • Too small for the light microscope • Range from about 20 to 200 nm (0.02-0.2 µm) Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. BACTERIA CELLS Rickettsia 0.3 µm Viruses 450nm 1. Mimivirus 2. Herpes simplex 150 nm 3. Rabies 125 nm 4. HIV 110 nm 100 nm 5. Influenza 6. Adenovirus 75 nm 7. T2 bacteriophage 65 nm 8. Poliomyelitis 30 nm 9. yellow fever 22 nm Protein Molecule 15 nm 10. Hemoglobin molecule Streptococcus 1 µm (1) (2) E. coli 2 µm long (10) (9) (8) (7) (3) (6) (4) (5) Viral Classification • Type of nucleic acid (DNA, RNA) • Strandedness (single vs double) • Capsid morphology • Presence or absence of envelope • Host range Virus taxonomy • International Committee on the Taxonomy of Viruses (ICTV): 3 orders, 73 families, 287 genera • Order names end in virales • Family names end in viridae • Genus names end in virus • As an example, consider the virus from the family Herpesviridae belonging to the genus Simplexvirus, human herpesvirus 2 Viral Components 7 Generalized Structure of Viruses 8 Helical Capsid • Rod-shaped capsomeres assembled into a series of hollow disks Naked: Tobacco mosaic virus (rigid) • Enveloped : Influenza virus (more flexible) 9 Icosahedral Viruses • Ring- or domeshaped capsomeres • Spherical or cubical capsid with nucleic acid packed inside 10 Complex Virus Structure • Found in bacteriophages 11 Viral Envelope • Found in some, but not all viruses Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. • Derived from the membranes of the host cell • Formed by budding • Viral proteins (spikes) • Functions: protection, host penetration, generation of immune response Envelope Nucleocapsid (c) Viral Enzymes • Found In nucleocapsid • Polymerases • Reverse transcriptase • Found in envelope • Hemagglutinin • Neuraminidase Viral Genome • DNA or RNA, but not both • Single or double stranded • RNA: sense or anti-sense, segmented or nonsegmented • RNA virus with encoded reverse transcriptase (retrovirus) Virus Infectious Cycle • Adsorption – specific attachment • Penetration – entry of viral genome • Uncoating – release of viral genome • Synthesis – new viral products made • Assembly – new viruses are made in the cell • Release – often causes the host cell to lyse Virus Infectious Cycle 16 Virus Adsorption • Receptors are glycoproteins necessary for cell’s normal function • Enveloped viruses use envelope spikes • Naked viruses use capsid proteins • Host range Penetration/Uncoating (a) Endocytosis (b) Fusion with cell membrane Replication of DNA Viruses Release Lytic Bacteriophage Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Head • Virus binds • DNA injected • Capsid stays outside • Expression • Assembly • Cell destroyed Bacterial cell wall Tube Viral nucleic acid Cytoplasm Lytic and Lysogenic Cycles E. coli host 7 Release Bacteriophage Lysogenic State BacterialViral DNA DNA 1 Viral DNA becomes latent as prophage. 2 Adsorption Penetration Lysis 6 Lytic Cycle DNA splits Viral DNA Spliced viral genome 3 5 Replication Maturation Bacterial DNA molecule The lysogenic state in bacteria. Capsid Tail 4 + DNA Tail fibers Assembly Sheath Infection followed by integration into the genome (prophage) Lysogenic conversion Bacteriophage Growing Bactriophages in the Lab • Bacteria cultures • Plaque assays (used to visualize the presence of virus) Growing Animal Viruses in the Lab • Living animals • Embrionated eggs (pocks, embrio death) • Cell cultures (cytopathic effects) Oncogenic Viruses • Oncogene: cancer causing alteration of cellular DNA • Transformation • HPV-16: cervical cancer • Epstein-Barr virus: Burkitt’s lymphoma • HBV: liver cancer • HTLV-1 and 2: T-cell leukemia Latent Viral Infections • Latency: the ability of a pathogenic virus to lie dormant (latent) within a cell . • Herpes simplex virus • Chicken pox virus Viroids • Plant pathogens • Naked RNA • Single-stranded • Circles • Very small (100 to 300 nucleotides) Prions • Animal pathogens • “Slow viruses” • Spongiform encephalopathy • Kuru • Mad Cow Disease • Creutzfeld-Jakob Disease • Defectively folded host protein