Ch 5 Viruses.docx - Viruses Chapter 5 Learning Objectives Lesson 5.1 General Structure and Classification Describe the two commonly used methods of

Ch 5 Viruses.docx - Viruses Chapter 5 Learning Objectives...

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Viruses Chapter 5 Learning Objectives Lesson 5.1: General Structure and Classification Describe the two commonly used methods of classifying viruses Define and describe capsid, capsomeres, nucleocapsid, virion, and envelope Describe and differentiate the types of viral genomes General Structure Viruses are microscopic particles that infect cells Genome is DNA or RNA Cannot reproduce on their own Obligate intracellular parasites Infect both eukaryotes and prokaryotes o Bacteriophage (phage): Infects bacteria Classification Taxonomic classification remains difficult Currently viruses are classified based on: o Morphology o Nucleic acid type o Mode of replication o Host organism o Disease they cause International Committee on Taxonomy of Viruses (ICTV) Classification (Cont.) Order (-virales) o Family (-viridae) Subfamily (-virinae) Genus (-virus) o Species (-virus) Baltimore Classification System places the virus into one of seven groups distinguishing the viruses on the basis of the relationship between the viral genome and the messenger RNA o Group I o Group II o Group III o Group IV o Group V o Group VI o Group VII Morphology Size varies from very small, approximately 20 nm (parvovirus), to up to 450 nm (poxvirus) Most cannot be visualized by light microscopy Exception to the size is the mimivirus found in 2003 o Has a diameter of 750 nm and was found in free-living amoebas o Largest known viral genome (~1000 genes on a double-stranded circular DNA)
Size Comparison Viruses consist of genetic material carried in the viral coat or capsid The capsid consists of proteins coded by viral genes The capsid is a complex structure and serves as the basis for morphological distinction Protomers form capsomeres, which aggregate to form the capsid General Structure of Animal Virus Proteins associated with nucleic acids are called nucleoproteins The association of viral capsid proteins with viral nucleic acid is referred to as a nucleocapsid Spikes o Long projections from the nucleocapsid Virion o Fully assembled virus Classification by Morphology Helical viruses o Capsids with rod-shaped capsomeres o Genetic material can be single-stranded RNA (ssRNA) or single-stranded DNA (ssDNA) o Can be naked (e.g., tobacco mosaic virus) or enveloped (e.g., influenza virus) Helical Virus Icosahedral viruses o Three-dimensional, geometric figure with 12 corners, 20 triangular faces, and 30 edges o Arrangement of capsomeres varies between the viruses o Herpesviridae, Adenoviridae, Papovaviridae, Parvoviridae Icosahedral Virus Enveloped Viruses Viral envelope surrounds the nucleocapsid Typically obtained by budding through a host membrane (usually plasma membrane, or membranes from ER or nuclear membrane) Might include some viral glycoproteins Some or all host membrane proteins are replaced by viral proteins Complex Viruses Bacteriophages o

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