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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.
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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?
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Viruses Chapter 19
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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.
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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
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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
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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
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Simplified viral replicative cycle
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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
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Bacteriophage Virus that infects bacterial cells
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Lytic Cycle of T4 Phage
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Lytic Cycle vs. Lysogenic Cycle
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Animal viruses have a membranous envelope
Host membrane forms around exiting virus Difficult for host immune system to detect virus
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Video: HIV Life Cycle
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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)
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HIV = Retrovirus
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HIV Origin: Chimpanzee virus Infects white blood cells (helper T)
HIV+: provirus (DNA inserted), latent AIDS: <200 WBC count, opportunistic infections
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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
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Emerging Viruses = mutation of existing viruses Pandemic: global epidemic
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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
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Zika Virus (as of Dec. 2015)
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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)
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Viroids Small, circular RNA molecules that infect plants
Cause errors in regulatory systems that control plant growth Eg. coconut palms in Philippines
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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)
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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
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Prion Neurodegenerative Diseases
Alzheimer’s Disease Parkinson’s Disease
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