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Understanding Viruses: Structure, Cycles, and Impact on Host Cells

Learn why viruses are classified as non-living entities, explore viral structure, lytic/lysogenic cycles, and host range determination. Understand the differences between bacteria and viruses and discover how vaccines and antiviral treatments work. Dive into the world of bacteriophages, retroviruses like HIV, animal viruses, and emerging virus mutations. Delve into viroids, prions, and diseases caused by misfolded proteins like mad cow disease. Unravel the mysteries of viral infections and the challenges they pose to human health.

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Understanding Viruses: Structure, Cycles, and Impact on Host Cells

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  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

  7. Simplified viral replicative cycle

  8. Bacteriophage • Virus that infects bacterial cells

  9. Video: t4 Phage infection

  10. Lytic Cycle of T4 Phage

  11. 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

  12. Lytic Cycle vs. Lysogenic Cycle

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

  14. 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)

  15. HIV = Retrovirus

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

  17. Video: HIV Life Cycle

  18. 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

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

  20. Emerging viruses = mutation of existing viruses

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

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

  23. 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

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