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Chapter 13

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  1. Chapter 13 Viruses, Viroids, and Prions

  2. General Characteristics of Viruses Differentiate a virus from a bacterium. • Obligatory intracellular parasites • Contain DNA or RNA • Contain a protein coat • Some are enclosed by an envelope • Some viruses have spikes • Most viruses infect only specific types of cells in one host • Host range is determined by specific host attachment sites and cellular factors

  3. Virus Sizes How could the small size of viruses have helped researchers detect viruses before the invention of the electron microscope? Figure 13.1

  4. Virion Structure • Nucleic acid • DNA or RNA • Capsid • Capsomeres • Envelope • Spikes Figure 13.2a

  5. Morphology of an Enveloped Virus Figure 13.3

  6. Enveloped Viruses Figure 13.16b

  7. Morphology of a Helical Virus Figure 13.4

  8. Morphology of a Complex Virus Figure 13.5

  9. Taxonomy of Viruses • Family names end in -viridae. Herpesviridae • Genus names end in -virus. Herpesvirus • Viral species: A group of viruses sharing the same genetic information and ecological niche (host). Common names are used for species. • Subspecies are designated by a number. Human herpes virus HHV-1, HHV-2, HHV-3

  10. Growing Viruses • Viruses must be grown in living cells • Bacteriophages form plaques on a lawn of bacteria Figure 13.6

  11. Growing Viruses • Animal viruses may be grown in living animals or in embryonated eggs Figure 13.7

  12. Growing Viruses • Animal and plant viruses may be grown in cell culture • Continuous cell lines may be maintained indefinitely Figure 13.8

  13. Virus Identification • Cytopathic effects: Detectable changes in the host cell due to viral infection (p. 441) • Blood serum (serology) tests • Detect antibodies against viruses in a patient • Use antibodies to identify viruses in neutralization tests, viral hemagglutination, and Western blot • Nucleic acids: Studies of the DNA/RNA

  14. Virus Identification: Cytopathic Effect Figure 13.9

  15. The Lytic Cycle • Attachment: Phage attaches by tail fibers to host cell • Penetration: Phagelysozyme opens cell wall; tail sheath contracts to force tail core and DNA into cell (analogy hypodermic syringe) • Biosynthesis: Production of phage DNA and proteins (What is the eclipse period?) • Maturation: Spontaneous assembly of phage particles • Release: Phagelysozyme breaks cell wall

  16. Lytic Cycle of a T-Even Bacteriophage 1 2 3 Figure 13.11

  17. Lytic Cycle of a T-Even Bacteriophage 4 Figure 13.11

  18. The Lysogenic Cycle Figure 13.12

  19. Results of Multiplication of Bacteriophages • Lytic cycle • Phage causes lysis and death of host cell • Lysogenic cycle: Prophage DNA incorporated in host DNA may result in… • Lysogenic (bacterial) cells become immune to reinfection of the same virus (but not other types of phage) • Phage conversion: Bacterial cells that have new genetic properties (Examples: Clostridium botulinium, Corynebacteriumdiphtheriae, Streptococcus pyogenesscarlet fever toxin genes are brought into the cell & incorporated into the cell DNA by the virus) • Specialized transduction: Bacterial DNA next to the phage DNA that is accidentally excised out of the bacterial DNA and packaged with the phage DNA (See slides 20 & 21)

  20. Generalized Transduction 2 3 4 5 6 Figure 8.28

  21. Specialized Transduction Figure 13.13

  22. Multiplication of Animal Viruses • Attachment: Viruses attach to cell membrane • Penetration by endocytosis or fusion (HIV) • Uncoating by viral or host enzymes • Biosynthesis: Production of nucleic acid and proteins • Maturation: Nucleic acid and capsid proteins assemble • Release by budding (enveloped viruses) or rupture

  23. Attachment, Penetration, Uncoating • By pinocytosis Figure 13.14a

  24. Attachment, Penetration, Uncoating • By fusion Figure 13.14b

  25. Budding of an Enveloped Virus Figure 13.20

  26. Multiplication of DNA Virus Figure 13.15

  27. Sense Strand (+ Strand) RNA Virus Figure 13.17a

  28. Antisense Strand (– Strand) RNA Virus Figure 13.17b

  29. Double-Stranded RNA Virus Figure 13.17c

  30. Multiplication of a Retrovirus Figure 13.19

  31. Prions • Proteinaceous Infectious particle • Inherited and transmissible by ingestion, transplant, and surgical instruments • Spongiform encephalopathies: Sheep scrapie, Creutzfeldt-Jakob disease, Gerstmann-Sträussler-Scheinker syndrome, fatal familial insomnia, mad cow disease • PrPC: Normal cellular prion protein, on cell surface • PrPSc: Scrapie protein; accumulates in brain cells, forming plaques

  32. Filoviridae • Single-stranded RNA, – strand, one RNA strand • Filovirus • Enveloped, helical viruses • Ebola and Marburg viruses Figure 23.21

  33. Retroviridae • Single-stranded RNA, 2 RNA strands, produce DNA • Use reverse transcriptase to produce DNA from viral genome • Lentivirus (HIV) • Oncogenic viruses • Includes all RNA tumor viruses Figure 19.13