The Viruses!Viruses!§Definitions: —Obligate intracellular parasites (pathogens) Infectious particles Non-living §Composition —Protein Nucleic Acid §Culture —Need host cells first must culture host cells then infect with virus §Small particles !Main Types of Viruses!žBased on general structure —Complex Enveloped Naked or Non-enveloped !Naked vs. Enveloped!Structural Organization!§Basic Structures —Covering ••Capsid all viruses —protein ••Envelope some but not all viruses —lipid —Central core (nucleic acid core) ••Hollow space/area that contains nucleic acid Capsids!§Two basic types based on shape Helical Capsids!Icosahedral Capsids!!Envelopes!§Lipid bilayer —Flexible outer layer Obtained from host Often has viral proteins on surface ••Called spikes Envelopes!Irregular/Complex Coverings!!!!Medical Importance of Viral Coverings!§Covering Protects virus —Relates to how easy disinfectants can destroy viruses §Covering used by virus to attach to host cell Covering is recognized by immune system of host Nucleic Acid Core!§Area that contains viruses nucleic acid —DNA or RNA (never both) May be single stranded (ss) or double stranded (ds) Linear, circular, or segmented ssRNA may be either (+) sense or (-) sense ••(+) sense functions as mRNA can be translated immediately •(-) sense complimentary strand [(+) strand ] must be transcribed, then (+) strand acts as mRNA Nucleic Acid Core!§Nucleic acid contains genes to instruct host cell how to make more viruses —Small genome (4 to several hundred genes) §Other core substances —Enzymes (polymerases, nucleases, others) Basic Viral Taxonomy!!!!!Viral Families!§DNA Viruses; ~6 important families of human pathogens ••Poxviridae smallpox virus •Papillomaviradae benign tumors (warts) •Parvoviridae - gasteronteritis •Hepadnaviridae hepatitis B •Herpesviridae chicken pox, herpes simplex •Adenoviridae respiratory infections Viral Families!§RNA Viruses; ~12 important families of human pathogens ••Orthomyxoviridae - influenza •Paramyxoviridae mumps, measles, respiratory infections •Rhabdoviridae - rabies •Togaviridae yellow fever, rubella, encephalitis •Retroviridae AIDS, T-cell leukemia •Reoviridae - rotavirus diarrheal disease, encephalitis •Picornaviridae polio, hepatitis A •Coronaviridae respiratory infections •Caliciviridae Norwalk virus gasteroenteritis,•Filoviridae Ebola virus hemorrhagic fever •Arenaviridae Lassa virus hemorrhagic fever •Bunyaviridae - encephalitis !Viral Replication!Viral Replication!§Basic Stages —Adsorption Penetration Synthesis Maturation (assembly) Release Bacteriophages!§Viruses which infect bacteria Easy to study —T-Even Coliphages §Two types of replication —Lytic Cycle Lysogenic Cycle Typical Bacteriophage!Lytic Cycle!Adsorption !Penetration!Synthesis!Assembly!Maturation Release!!Lysogenic Cycle!§Cycle carried out by: temperate phages Viral nucleic acid - incorporated into bacterial chromosome —Viral nucleic acid now called a prophage §No immediate synthesis, maturation or release Prophage duplicated along with bacterial chromosome as bacteria divide Lysogeny!§Medical significance —Corynebacterium diphtheriae••Bacterial chromosome only weakly pathogenic •Bacterial chromosome + prophage very pathogenic —Prophage codes for toxin !!Animal Viruses!§More varied replication cycles than bacteriophages —Still have same basic stages Adsorption ••Very similar capsid or envelope proteins —Penetration and Uncoating ••Several methods endocytosis, membrane fusion, translocation (direct penetration) !Absorption, Penetration and Uncoating!Animal Viruses Replication Stages (cont.)!—Synthesis and Maturation ••Quite varied, depending on type of viral nucleic acid —Release ••Several methods lysis, exocytosis (budding) Budding or Exocytosis!Special Types of Animal Viruses!The persistant viruses and oncogenic viruses Persistent Animal Viruses!§Persistent virus causes long lasting infection —Chronic infection long lasting but virus always detectable!••Hepatitis B, HIV!—Latent infection (latency) long lasting but virus dormant (undetectable) !••no symptoms, no antibody production! •Herpesviruses (simplex, zoster, Epstein-Barr)!!!§viruses remain inside host cells!—relatively inactive!little or no immediate host cell damage! can become reactivated!••virus begins to replicate!•host cell damage >> disease! !!Herpes zoster!Oncogenic viruses!§persistent animal viruses!viral nucleic acid incorporated into host genome !! —redirect normal growth patterns of host cells!leads to neoplasia (cancer)!§examples!—Burkitt’s lymphoma (Epstein-Barr virus)!some leukemias (retroviruses) Human papillomavirus (cervical cancer) Other Infectious Particles!§Prions!—infectious protein particles (PrP)!pathogens of animals cause chronic persistent infections! Examples!••scrapie(sheep), BSE (cattle), Creutzfelt-Jacob disease (humans), Kuru (humans)! §Viroids!—infectious RNA particles!pathogens of plant cells End!§Go to: Taxonomy and
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