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Transcript
Chapter 14 Viruses, Prions, and Viroids: Infectious Agents of Animals and Plants Animal Virus Classification Reflects _________________ relationships Inherently difficult, changing Based mainly on _______________ structure Virus ______________ structure Presence or absence of an _____________ Families of Viruses Members of families share common ______________ Relationship between families more complex Families further subdivided into genera 14 RNA-virus families infect vertebrates 7 DNA-containing families infect vertebrates Family name ends in “-viridae” Animal Virus Classification and Taxonomy Family “-______________” Named for appearance – Coronaviridae Named for place found – Bunyaviridae Genus“-______________” All one word - Enterovirus Species Named for disease caused Poliovirus polio Named for place found Marburg; Ebola Types Akin to subspecies, strains Some types should be separate species ____________ Binomial nomenclature Informal Groupings of Animal Viruses Non-evolutionary groupings Based on ______________ ______________ between individuals Enteric viruses Respiratory viruses Zoonoses Sexually transmitted viruses Enteric Viruses Ingested on ______________ -contaminated material “Fecal-oral route” Often cause gastroenteritis Inflammation of stomach and intestine Some cause systemic disease rather than gastroenteritis e.g., poliovirus Respiratory Viruses Inhaled, ______________ in respiratory tract Remain ______________ in respiratory tract e.g., rhinovirus Not included Inhaled viruses causing systemic diseases (mumps virus, measles virus) Zoonoses Transmitted from one animal species to another (including humans) e.g., rabies bat Old Yeller humans (humans cannot transmit rabies to other humans) e.g., canine distemper dogs lions e.g., arboviruses Arthropod borne viruses Infect arthropods, replicate, transmitted to vertebrates e.g., W est Nile Virus Sexually Transmitted Viruses Transmitted during ______________ activity Many cause lesions in genital tract e.g., herpesviruses, papillomaviruses Some cause ______________ infections e.g., HIV, hepatitis viruses Methods of Studying Viruses Cultivation of host cells Viruses multiply only ______________ host cells Living animals Embryonated chicken eggs Cell culture (tissue culture) Limited life span Immortal cells from tumor Quantitation Determining the number of virions present Viral Quantitation Determining numbers Plaque assay Electron microscopic counting Quantal assays Hemagglutination ______________ Assay Used for virus that ______________ host cell Known volume of virus-containing solution added to tissue culture cells Each plaque represents one virion – Plaque Forming Units (PFUs) Similar to CFUs in bacteria Viral Quantitation Electron microscopic ______________ Can often distinguish between infective and non-infective virions Helpful in identifying type of virus Quantal assays Several ______________ administered to cells ______________ = dilution at which 50% of host cells are infected or killed ID50 (infective dose) LD50 (lethal dose) Hemagglutination Some viruses agglutinate (clump) red blood cells e.g., influenza virus Serial ______________ of virus added to RBCs Highest dilution showing maximal hemagglutination determined Host-Virus Interactions Bacteriophage host organism is single cell Possess rudimentary defense mechanisms Animal virus host organism is multicellular Possesses various defense mechanisms Host can develop ______________ Virus – Host Coevolution Host more resistant Virus less pathogenic “______________ ______________” “Normal” host is often asymptomatic Disease results when transmitted to a susceptible host e.g., measles & smallpox in New W orld indigenous populations Acute Infections Short duration Host may develop immunity Life cycle similar to that of virulent phage Attachment Entry Targeting Uncoating Replication Maturation Release Shedding Transmission Characteristics of Acute Infections ______________ Similar to bacteriophages ______________ Entire virion enters cell Naked viruses by endocytosis Enveloped viruses by either endocytosis or membrane fusion Characteristics of Acute Infections Targeting and uncoating ______________ Virion targeted to site where it will multiply e.g., most DNA viruses multiply in nucleus ______________ Nucleic acid separates from protein coat Replication and Protein Production in Acute Infections Production of proteins Utilizes ______________ cell ______________ and other machinery Replication of nucleic acid Genetic material varies between families Methods of nucleic acid replication variable Often involves viral enzymes Acute Infections Maturation ______________ ______________ of virions Virions exit host Multistep process Generally use same opening or Release May involve lysis or ___________ surfaces used to gain entry ______________ Transmitted to new host Comparison of Replication Cycles Persistent Infections Viruses continually present in the body Released by ______________ May or may not cause disease Can be transmitted to others Four categories Late complications following acute infection Latent infections Chronic infections Slow infections (Some overlap in these categories) Persistent Infections Latent infections Acute infection _________________ period ______________ Infectious virions undetectable until reactivation Initial vs. reactivated symptoms may differ e.g., herpesviruses (HSV-1, HSV-II, varicella) Sometimes involves integration into host DNA Persistent Infections Chronic infections Infectious virus ______________ at ______________ times Disease may be either present or absent over extended period of time e.g., Hepatitis B virus Slow infections Amount of virus gradually __________________ over long periods of time Asymptomatic over long periods of time e.g., lentiviruses, retroviruses (e.g. HIV) Complex Infections Do not fit other categories well e.g. Retroviruses Single-stranded RNA viruses Some cause tumors or leukemia Best known is HIV HIV preferentially destroys helper T lymphocytes Cd4+ protein on surface Need T cells to fight off other diseases Result is immune deficiency - AIDS HIV – a Retrovirus ______________– backwards ______________ genome (HIV has two strands) RNA DNA RNA protein Uses reverse transcriptase to go RNA DNA Then makes dsDNA and integrates into host chromosome: now called provirus HIV – Entry and Uncoating HIV virion binds to cell with ______________ Why are receptors there? Loses envelope to fusion with host membrane Naked nucleocapsid enters Uncoating to release ssRNA genome HIV – Replication and Gene Expression ssRNA ssDNA dsDNA dsDNA ______________ into host chromosome dsDNA ssRNA more virions dsDNA ssRNA message long polypeptide cut with ______________ to make smaller proteins Reverse ______________ Critical to understanding HIV Copies info on RNA DNA No ______________ many mistakes Mistakes ______________ changes in proteins Host makes antibodies to coat proteins Coat proteins change before host can make enough antibodies Rapid ______________– even faster than bacteria! Tumors Tumors result from abnormal growth of cells “______________cells” Most NOT caused by viruses Control of cell division / differentiation altered Tumor suppressor gene inactivated form Proto-oncogene mutation oncogene Types of tumors Benign tumor remains in defined area Malignant tumor (cancer) Isolated Metastasizing – spread to other parts of body ______________ sometimes carried on viruses Effects of Viruses on Animal Cell Hosts Viral Host Range Includes all cell ______________ able to be infected e.g., species, strains, cell types within a species These cells possess receptors recognized by virus Host range can be altered Phenotypic mixing Genetic reassortment ______________ Mixing Animal cells sometimes simultaneously infected by two different viruses Host ranges overlap, but differ Viral genetic material and viral capsids ______________ Host range temporarily altered Can facilitate interspecies gene transfer Genetic ______________ in Segmented Genomes Some viruses possess segmented genomes e.g., influenza virus Many strains of these viruses exist Host ranges overlap, but differ Two different strains can infect a single cell e.g., bird and human viruses can both infect pigs RNA segments mixed and matched “______________ ______________” New strain avoids immunity already in place Plant Viruses Many plant diseases are caused by viruses Especially prevalent in ________________ plants Yield can be severely reduced Without plants (food), we are dead Viruses occasionally produce desirable effects e.g., color variegation in tulips Eventually plant becomes excessively weak Plant Viruses Enter via ______________ sites, not via receptors Source of virus Soil Vectors (insect, human, etc.) Tobacco Virus-resistant crops genetically engineered Dodder, a parasitic plant that may vector viruses between its host plants Effects of Plant Viruses Viroids Infectious __________ -stranded ______________ 246 – 375 nucleotides long ~10% of size of smallest known RNA virus Replicate autonomously ______________, resistant to digestion by nucleases Cause disease Mechanism unknown All identified viroids infect plants Prions Proteinaceous infectious agents ______________ only, no nucleic acid Linked to diseases of humans and other animals e.g., mad cow disease, kuru, scrapie, etc. “transmissible spongiform encephalopathies” Slow, always fatal Prion converts normal host protein into prion