Understanding Viruses: Structure, Replication, and Impact on Health

1. Ch. 19 Warm-up • Why do many scientists classify viruses as non-living? • Draw the basic structure of a virus. Label and define capsid, viral envelope and nucleic acid.

2. Ch. 19 Warm-up • Draw the lytic/lysogenic cycle. • What stage of the lytic-lysogenic cycle is a virus virulent? Temperate? • What determines a host range?

3. Viruses Chapter 19

4. What you must know: • The components of a virus. • The differences between lytic and lysogenic cycles.

5. Bacteria Virus • Prokaryotic cell • Most are free-living (some parasitic) • Relatively large size • Antibiotics used to kill bacteria • Not a living cell (genes packaged in protein shell) • Intracellular parasite • 1/1000 size of bacteria • Vaccines used to prevent viral infection • Antiviral treatment Bacteria vs. Viruses

6. Viruses • Very small (<ribosomes) • Components = nucleic acid + capsid • Nucleic acid: DNA or RNA (double or single-stranded) • Capsid: protein shell • Some viruses also have viral envelopes that surround capsid • Limited host range (eg. human cold virus infects upper respiratory tract) • Reproduce within host cells

10. Simplified viral replicative cycle

11. Bacteriophage • Virus that infects bacterial cells

12. Video: t4 Phage infection

13. Lytic Cycle of T4 Phage

14. Bacteriophage Reproduction • Lytic Cycle: • Use host machinery to make copies of virus • Death of host cell by rupturing it (lysis) • Virulent phages replicate by this method • Lysogenic Cycle: • Phage DNA incorporated into host DNA and replicated along with it • Phage DNA = prophage • Temperate Phage: uses both methods of replication

15. Lytic Cycle vs. Lysogenic Cycle

16. Animal viruses have a membranous envelope • Host membrane forms around exiting virus • Difficult for host immune system to detect virus

17. Video: How Dengue Virus enters a cell

18. Retrovirus • RNA virus that uses reverse transcriptase(RNA  DNA) • Newly made viral DNA inserted into chromosome of host • Host transcribes viral DNA (= provirus) to make new virus parts • Example: HIV (Human Immunodeficiency Virus)

19. HIV = Retrovirus

20. HIV • Infects white blood cells • HIV+: provirus (DNA inserted) • AIDS: active viral reproduction

21. Video: HIV Life Cycle

22. Other Human Viruses • Herpes virus • Smallpox Herpes Simplex Virus 1 (HSV-1) Herpes Simplex Virus 2 (HSV-2) Eradicated in 1979 due to worldwide vaccination campaigns

23. Ebola • Transmission: contact with contaminated blood or bodily fluids • Symptoms: fatigue, fever, severe headache, vomiting, diarrhea, rash, bleeding • May appear 2-21 days after exposure

24. Ebola • Treatment: • Intravenous fluids, balance electrolytes • Experimental: antiviral drugs, plasma transfusions from survivors, antibodies (Zmapp) • No vaccine (yet) • Statistics: • 2014 Ebola Outbreak (worldwide): 21,382 cases, 8474 deaths (*as of 1/19/15) • Seasonal Influenza: estimated 36,000 deaths in U.S. each year (2015: mutated H3N2 strain)

25. 2014 Ebola Outbreak

26. Cumulative Ebola Cases

27. Vaccines • Weakened virus or part of pathogen that triggers immune system response

28. Emerging viruses = mutation of existing viruses

29. Viroids • Small, circular RNA molecules that infect plants • Cause errors in regulatory systems that control plant growth • Eg. coconut palms in Philippines

30. Prions • Misfolded, infectious proteins that cause misfolding of normal proteins • Eg. mad cow disease (BSE),Creutzfeldt-Jakob disease (humans), scrapie (sheep)

31. Diseases caused by prions • Prions act slowly – incubation period of at least 10 years before symptoms develop • Prions are virtually indestructible (cannot be denatured by heating) • No known cure for prion diseases