Download Ch. 19 Viruses

Ch. 19
Viruses 1
Viruses
­early experiments on tobacco mosaic virus ­ stunts tobacco plants and discolors leaves
­much smaller than bacteria (and ribosome)
http://en.wikipedia.org/wiki/Tobacco_mosaic_virus
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genetic material = double stranded DNA, single stranded DNA, double stranded RNA or single stranded RNA
capsid= protein shell around genome
­made of proteins called capsomeres
­most complex capsids are in bacteriophages
some have accessory structures:
viral envelopes ­ contain phospholipids and membrane proteins from host cell
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http://micro.magnet.fsu.edu/cells/viruses/images/virus.jpg
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­are parasites when it comes to reproduction
­lack enzymes, ribosomes and other protein making substances
­need a host cell
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Shapes of Viruses
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Reproductive cycle
identify hosts ­ have receptor molecules on surface of cell
­can have broad host range
West Nile virus ­ mosquitos, birds, humans
Cold virus ­ cells in upper respiratory tract
­identify host cells by "lock and key" between viral surface proteins and host receptor proteins
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viral reproductive cycle
1. virus binds to host cell
2. viral DNA and capsid proteins
released into cell
3. host enzymes replicate viral genome
4. host enzymes transcribe viral
genome into mRNA ­ ribosomes
make more capsids
5. viral genome/capsids reassemble
exit cell
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Two types of reproductive cycles
1. Lytic life cycle ­ kills host
­phage that reproduces only by lytic cycle = virulent phage
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Lytic cycle can wipe out bacterial population within hours
So why aren't all bacteria extinct?
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1. receptor sites are no longer recognized by a phage
2. restriction enzymes in bacteria cut up foreign DNA, bacterial DNA is immune
(Bacteria DNA is methylated so doesn't get broken up by own enzymes)
3. bacteria can coexist with phages if go through lysogenic cycle
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2. Lysogenic Cycle
­replicates phage genome without destroying host
­temperate phages can use both methods of reproduction ex. lambda phage
­viral genome gets incorporated into host genome (prophage)
­viral genome can then exit host genome and force reproduction into the lytic cycle
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Lysogenic and Lytic cycles
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Reproductive cycles of animal viruses depends on whether the genetic material is DNA or RNA, double stranded or single stranded
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viral envelopes are used to gain entry to host cell
­glycoproteins bind to receptor molecules on host cell
viral envelopes can come from the host plasma membrane or nuclear membrane (Ex. Herpes virus)
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­some genetic viral RNA can serve directly as mRNA in protein synthesis (Class IV) so translated directly into a protein
Ex. cold virus (Rhinovirus, SARS, West Nile virus, polio)
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­Class V ­ viral RNA serves as a template for mRNA synthesis; packs its own enzymes to do this in the capsid
Ex. Ebola, Measles, Mumps, Rabies
http://www.vaccineplace.com/images/learn/rabies/rabies­virus.jpg
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­Class VI ­ retroviruses use reverse transcriptase ­ transcribes RNA to DNA
Ex. HIV (human immunodeficiency virus)
causes AIDS (acquired immunodeficiency
syndrome)
­have envelopes, 2 identical molecules of ssRNA and 2 molecules of reverse transcriptase
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HIV reproductive cycle
integrated virus is called a provirus ­ stays permanently in cell
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HIV infection
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How did viruses originate?
­believed to evolve after first cells
­believed to be fragments of cellular DNA that move from one cell to another
­original sources may have been plasmids
or transposons (other mobile genetic elements)
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­some viruses have similar genetic material to host although some animal viruses can be like plant viruses
Mimivirus ­largest virus discovered yet (size of small bacteria)
­ dsRNA
­1.2 million base pairs
­ 1000 genes
­ has proteins for translation,
DNA repair
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Infections by viruses can cause:
­damage or lysis of cells due to hydrolytic enz.
­cells to produce toxins that lead to disease
­some produce toxic envelope proteins
recovery depends on ability of tissues to regenerate
Ex. cold virus ­ regenerate
polio ­ no regeneration of nerve cells
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To prevent viral infections
Vaccines = harmless variants or derivatives of pathogenic microbes that stimulate an immune response
ex. smallpox,measles, polio, rubella, hepatitis B, mumps
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smallpox
measles
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Polio
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hepatitis
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Treatment for viral infections:
­ antibiotics do not work
­antivirals drugs ­ interfere with nucleic acid synthesis Ex. acyclovir for herpes ­ inhibits viral polymerase'
Ex. azidothymidine for HIV ­interfers with synthesis of DNA by reverse transcriptase
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Emerging Viruses
Ebola ­ 1976
SARS ­2002
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Emerging viruses ­ appear suddenly
caused by:
1. mutation of existing viruses (ex. flu virus)
2. spread of viral disease from one host to another
humans get them from animals
Ex. hantavirus from rodents
bird flu
SARS from bats
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flu pandemic in 1918­1919 resulted in 40 million deaths in people, originated in birds
H1N1 flu pandemic:
H = hemaglutinin (11types)
N = neuraminidase (9 types)
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3. viral infection of small population gets to larger population
Plant viral diseases;
­ 2,000 types known
­cause $ damage
­symptoms: stunted growth, spotted leaves
damaged flowers or roots
­most have RNA genome
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­spread by:
­horizontal transmission ­ infected from external source
­vertical transmission­ get from parent
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Viral infection of plants
can spread through plasmodesmata
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Viroids viroids = small circular RNA molecules that infect plants
­do not encode proteins
­replicate in host cells
­cause errors on growth pathways
­cause abnormal development and stunted growth
Ex. cadang­cadang ­ kills coconut palms
lesson: a single molecule can be an infectious agent that spreads disease
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Prions
= infectious proteins
­cause scrapie in sheep, mad cow disease, Creutzfeld­Jakob in humans, Kuru
­transmitted in food
­affects brains causing degeneration
­slow acting agents ­ long incubation period
(>10 years)
­hard to destroy or deactivate
­no known cure
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model for how prions reproduce
­ misfolded protein
­converts normal proteins to misfolded
­form complexes ­ causes disease
Stanley Prusiner won Nobel Prize in 1980's for this discovery
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