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VirusesChapter 5Learning ObjectivesLesson 5.1: General Structureand ClassificationDescribe the two commonly used methods of classifying virusesDefine and describe capsid, capsomeres, nucleocapsid, virion, and envelopeDescribe and differentiate the types of viral genomesGeneral Structure Viruses are microscopic particles that infect cellsGenome is DNA or RNACannot reproduce on their ownObligate intracellular parasitesInfect both eukaryotes and prokaryotesoBacteriophage (phage): Infects bacteriaClassificationTaxonomic classification remains difficultCurrently viruses are classified based on:oMorphologyoNucleic acid typeoMode of replicationoHost organismoDisease they causeInternational Committee on Taxonomy of Viruses (ICTV)Classification (Cont.)Order (-virales)oFamily (-viridae)Subfamily (-virinae)Genus (-virus)oSpecies(-virus)Baltimore ClassificationSystem places the virus into one of seven groups distinguishing the viruses on the basis of the relationship between the viral genome and the messenger RNAoGroup IoGroup IIoGroup IIIoGroup IVoGroup VoGroup VIoGroup VIIMorphologySize varies from very small, approximately 20 nm (parvovirus), to up to 450 nm (poxvirus)Most cannot be visualized by light microscopyException to the size is the mimivirus found in 2003oHas a diameter of 750 nm and was found in free-living amoebasoLargest known viral genome (~1000 genes on a double-stranded circular DNA)
Size Comparison Viruses consist of genetic material carried in the viral coat or capsidThe capsid consists of proteins coded by viral genesThe capsid is a complex structure and serves as the basis for morphological distinctionProtomers form capsomeres, which aggregate to form the capsidGeneral Structure of Animal VirusProteins associated with nucleic acids are called nucleoproteinsThe association of viral capsid proteins with viral nucleic acid is referred to as a nucleocapsidSpikesoLong projections from the nucleocapsidVirionoFully assembled virusClassification by MorphologyHelical virusesoCapsids with rod-shaped capsomeres oGenetic material can be single-stranded RNA (ssRNA) or single-stranded DNA (ssDNA)oCan be naked (e.g., tobacco mosaic virus) or enveloped (e.g., influenza virus)Helical VirusIcosahedral virusesoThree-dimensional, geometric figure with 12 corners, 20 triangular faces, and 30 edgesoArrangement of capsomeres varies between the virusesoHerpesviridae, Adenoviridae, Papovaviridae, ParvoviridaeIcosahedral VirusEnveloped VirusesViral envelope surrounds the nucleocapsidTypically obtained by budding through a host membrane (usually plasma membrane, or membranes from ER or nuclear membrane)Might include some viral glycoproteinsSome or all host membrane proteins are replaced by viral proteinsComplex VirusesBacteriophageso