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VIRUS STRUCTURE AND CLASSIFICATION

VIRUS STRUCTURE AND CLASSIFICATION. What are the properties of viruses? They are very small. “Average” Virus 0.1 m m= 100 nm. Smallest virus. Bacteria 1 m m. 0.015 m m = 15 nm. Eukaryotic cell 10 m m. VIRUS PARTICLES ARE BUILT BY AN ASSEMBLY PROCESS. +. NUCLEOCAPSID

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VIRUS STRUCTURE AND CLASSIFICATION

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  1. VIRUS STRUCTURE AND CLASSIFICATION

  2. What are the properties of viruses? • They are very small . “Average” Virus 0.1 mm= 100 nm Smallest virus Bacteria 1 mm 0.015 mm = 15 nm Eukaryotic cell 10 mm

  3. VIRUS PARTICLES ARE BUILT BY AN ASSEMBLY PROCESS + NUCLEOCAPSID For non-enveloped viruses this is the VIRION or VIRUS PARTICLE CAPSID protein GENOME RNA or DNA ENVELOPE: protein and lipid-sometimes called VIRAL MEMBRANE For enveloped viruses this is the VIRION or VIRUS PARTICLE

  4. Functions of the capsid and envelope • Protects the genome from damage as virion passes from one cell to the next. • Contains proteins that interact with specific proteins on the surface of cells in which the viruses replicate • These cell proteins are called VIRUS RECEPTORS • The viral proteins are called RECEPTOR-BINDING PROTEINS Where would expect the receptor-binding proteins to be on a non-enveloped virus? An enveloped virus?

  5. HELICAL CAPSIDS • Identical protein subunits (CAPSOMERES) • Same intersubunit bonds used over and over • Length determined by length of RNA • Single axis of symmetry

  6. An example of a naked helical virus: tobacco mosaic virus (TMV)

  7. An example of an enveloped helical virus: a human paramyxovirus

  8. ICOSAHEDRAL CAPSIDS • 5-fold, 3-fold, 2-fold axes of symmetry 3-fold • Protein subunits (capsomeres). Two types of capsomeres: hexamers (faces) and pentamers (vertices) 2-fold • Size of capsid determined by number of capsomeres 5-fold

  9. An example of a naked icosahedral virus: adenovirus 252 capsomeres

  10. An example of an enveloped icosahedral virus: a herpesvirus A herpesvirus particle A herpesvirus nucleocapsid 162 capsomeres

  11. An example of a virus with complex morphology: a poxvirus Negative staining Thin section

  12. VIRAL GENOMES RNA DNA Double-stranded Single-stranded Double-stranded Single-stranded non-segmented linear circular ss segmented ds segmented

  13. POLARITY • Refers to “strand sense” of a single strand RNA genome • Can the RNA be directly translated, i.e., can it be used as an mRNA? If yes, it is positive stranded. If no, it is negative stranded. • If the RNA genome is negative stranded it must be first transcribed to produce the complementary positive stranded mRNA. • Negative stranded viruses must carry a polymerase in the virus particle to carry out this first step

  14. VIRUS CLASSIFICATION “Baltimore classification”-based on type of genome and polarity of RNA • Class III-ds RNA • Naked icosahedral segmented(reovirus) • Class V-ss RNA (-) • Enveloped helical; single segment (paramyxovirus) • Enveloped helical; segmented (influenza virus) • Class I-ds DNA • Naked icosahedral (adenovirus) • Enveloped icosahedral (herpesvirus) • Complex (poxvirus) • Class IV-ss RNA(+) • Naked icosahedral (poliovirus) • Enveloped icosahedral (togavirus) • Enveloped helical (coronavirus) • Class II-ss DNA • Naked icosahedral (parvovirus) • Class VI-ss RNA, ds DNA intermediate • Enveloped (retrovirus)

  15. All viruses must produce mRNA that can be translated to make viral proteins. II IV ssDNA +RNA III ds DNA mRNA dsRNA I +RNA -RNA VI V Which of the classes of viruses require the presence of a virion-associated polymerase?

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