1. Viruses
Chapter 19

2. What you must know: The components of a virus.
The differences between lytic and lysogenic cycles.
How viruses can introduce genetic variation into host organisms.
Mechanisms that introduce genetic variation into viral populations.

3. Bacteria vs. Viruses 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

4. Amoeba Sisters: Viruses: Virus replication and the mysterious common cold

5. 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

6. Viruses Limited host range Reproduce quickly within host cells
Entry = attach to host cell membrane receptors through capsid proteins or glycoproteins on viral envelope (animal)
Eg. human cold virus (rhinovirus)  upper respiratory tract (mouth & nose)
Reproduce quickly within host cells
Can mutate easily
RNA viruses: no error-checking mechanisms

10. Simplified viral replicative cycle

11. Video: t4 Phage infection

12. Viral Reproduction Lytic Cycle:
Use host machinery to replicate, assemble, and release copies of virus
Virulent phages: Cells die through lysis or apoptosis
Lysogenic (Latent) Cycle:
DNA incorporated into host DNA and replicated along with it
Bacteriophage DNA = prophage
Animal virus DNA = provirus
UV radiation, chemicals: lysogenic  lytic cycle
Temperate Phage: uses both methods of replication

13. Bacteriophage
Virus that infects bacterial cells

14. Lytic Cycle of T4 Phage

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: HIV Life Cycle

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

19. HIV = Retrovirus

20. HIV Origin: Chimpanzee virus Infects white blood cells (helper T)
HIV+: provirus (DNA inserted), latent
AIDS: <200 WBC count, opportunistic infections

21. 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

22. Emerging Viruses = mutation of existing viruses Pandemic: global epidemic

23. Current Outbreaks Zika Virus Dengue Fever Chikungunya
Spread by Aedes mosquitoes (Aedes aegypti)
Major outbreak in Brazil and Latin America
Linked to birth defects (microcephaly)
Dengue Fever
Chikungunya

24. Zika Virus (as of Dec. 2015)

25. Drugs for Prevention/Treatment
Vaccine: weakened virus or part of pathogen that triggers immune system response to prevent infection
Ex. HPV, MMR, HepA, Flu shot
Antiviral Drugs: block viral replication after infection
Ex. Tamiflu (influenza), AZT (HIV)

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

28. Prions
Misfolded, infectious proteins that cause misfolding of normal proteins
Eg. scrapie (sheep), mad cow disease (BSE), Creutzfeldt-Jakob disease (humans), kuru (humans – New Guinea)

29. 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
Kuru in New Guinea

30. Prion Neurodegenerative Diseases
Alzheimer’s Disease
Parkinson’s Disease