Viruses Chapter 19 What you must know: The components of a virus. The differences between lytic and lysogenic cycles.
Bacteria vs. Viruses Bacteria Prokaryotic cell Most are free-living (some parasitic) Relatively large size
Antibiotics used to kill bacteria Virus Not a living cell (genes packaged in protein shell)
Intracellular parasite 1/1000 size of bacteria Vaccines used to prevent viral infection Antiviral treatment Viruses Very
Limited host range (eg. human cold virus infects upper respiratory tract) Reproduce within host cells Simplified viral replicative cycle Bacteriophage Virus
that infects bacterial cells Lytic Cycle of T4 Phage 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 Lytic Cycle vs. Lysogenic Cycle Animal viruses have a membranous envelope
Host membrane forms around exiting virus Difficult for host immune system to detect virus
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) HIV = Retroviru s HIV Infects white blood
cells HIV+: provirus (DNA inserted) AIDS: active viral reproduction 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 Vaccines Weakened virus or part of pathogen that triggers immune system response
Emerging viruses = mutation of existing viruses Viroids Small, circular RNA molecules that infect plants
Cause errors in regulatory systems that control plant growth Eg. coconut palms in Philippines Prions Misfolded,
infectious proteins that cause misfolding of normal proteins Eg. mad cow disease (BSE),Creutzfeldt-Jakob disease (humans), scrapie (sheep) 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 Ch. 19 Practice Questions 1.
Why do many scientists classify viruses as non-living? 2. Draw the basic structure of a virus. Label and define capsid, viral envelope and nucleic acid.
Ch. 19 Practice Questions 1. Draw the lytic/lysogenic cycle. 2. What stage of the lyticlysogenic cycle is a virus
virulent? Temperate? 3. What determines a host range?
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