Viruses, Viroids, and Prions

1. Viruses, Viroids, and Prions The biology behind the bugs

2. Definitions • Virus-A sub-microscopic, parasitic, filterable agent consisting of a nucleic acid surrounded by a protein coat • Viroid-Infectious RNA (no protein coat) found only in plants • Prion-Infectious, self-replicating protein with no detectable nucleic acid ex. Scrapie infection of sheep, and “Mad cow”

3. History of Viruses • Tobacco Mosaic Virus or Tobacco Mosaic Disease (Plant virus) • Scientists showed that a healthy plant could be “infected”, but they were thinking that a bacterial agent was the cause. • Using a filter designed to not allow bacteria to pass, a bacteriologist found that the agent causing the disease passed through the filter • Invention of Electron Microscope made it possible to see viruses

4. So How Big IS a Virus? Figure 13.1

5. Characteristics of Viruses • Obligate intracellular parasite • Contain either DNA or RNA • And a protein coat • Some are enclosed by an envelope • Multiply inside living cells using host proteins (enzymes) to produce more viral particles • Most viruses infect only specific types of cells in one host • Host range is determined by specific host attachment sites and cellular factors

6. Host Range • Refers to the types of cells that a virus is able to infect • In order for a virus to attach to a cell, it must have a receptor that it recognizes on that cell. This is why viruses that infect bacteria (bacteriophages) do not infect animal cells

7. Viral Structure • A Virion is a complete infectious viral particle containing a nucleic acid surrounded by a protein coat • The protein coats protect the nucleic acid and promote the virus’s attachment to a host cell Viruses are classified based on the differences in their protein coats called Capsids • Some viruses have an envelope as well • Some viruses are covered by Spikes (carbohydrate-protein complexes) which can be used to attach to the host cell

8. Viral Morphology • Helical • Polyhedral • Enveloped • Complex

9. Helical Viruses Figure 13.4a, b

10. Polyhedral Viruses Figure 13.2a, b

11. Complex Viruses Figure 13.5a

12. Viral Classification • Nucleic acid type • Strategy for replication • Morphology • Family names end in -viridae • Genus names end in -virus • 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

13. Viral Taxonomy example: • Herpesviridae (family) • Herpesvirus (genus) • Human herpes virus 1, HHV 2, HHV 3 • Species • Subspecies • Retroviridae • Lentivirus • Human Immunodeficiency Virus 1, HIV 2

14. Viral Replication • Lytic cycle Phage causes lysis and death of host cell • Lysogenic cycle Prophage DNA incorporated in host DNA

15. Bacteriophage Replication Figure 13.12

17. H1N1 • 16 Types of Hemagglutinin (HA) • 9 Types of Neuraminidase (NA)

18. Retroviruses

19. Viral Culturing In Lab • Bacteriophage- Plaque forming units (PFU) • Similar to colony formation (CFU), but bacteriophage kill bacteria, so a plaque is the absence of growth in a bacterial lawn • Animal Viruses • Living animals (mice, rabbits, or guinea pigs) • Embryonated eggs • Cell cultures

20. Viruses must be grown in living cells. • Phage form plaques on a lawn of bacteria. Figure 13.6

21. Animal and plants viruses may be grown in cell culture. • Continuous cell lines may be maintained indefinitely. Figure 13.8

22. Virus Identification • Cytopathic effects (cell damage) • Serological tests • Detect antibodies against viruses in a patient • Use antibodies to identify viruses in neutralization tests, viral hemagglutination, and Western blot • Nucleic acids • RFLPs (Restriction Fragment Length Polymorphism) • PCR (Polymerase Chain Reaction)