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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 2 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 3 http://micro.magnet.fsu.edu/cells/viruses/images/virus.jpg 4 are parasites when it comes to reproduction lack enzymes, ribosomes and other protein making substances need a host cell 5 Shapes of Viruses 6 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 7 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 8 Two types of reproductive cycles 1. Lytic life cycle kills host phage that reproduces only by lytic cycle = virulent phage 9 Lytic cycle can wipe out bacterial population within hours So why aren't all bacteria extinct? 10 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 11 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 12 Lysogenic and Lytic cycles 13 Reproductive cycles of animal viruses depends on whether the genetic material is DNA or RNA, double stranded or single stranded 14 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) 15 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) 16 Class V viral RNA serves as a template for mRNA synthesisÍž packs its own enzymes to do this in the capsid Ex. Ebola, Measles, Mumps, Rabies http://www.vaccineplace.com/images/learn/rabies/rabiesvirus.jpg 17 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 18 HIV reproductive cycle integrated virus is called a provirus stays permanently in cell 19 HIV infection 20 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) 21 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 22 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 23 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 24 smallpox measles 25 Polio 26 hepatitis 27 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 28 Emerging Viruses Ebola 1976 SARS 2002 29 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 30 flu pandemic in 19181919 resulted in 40 million deaths in people, originated in birds H1N1 flu pandemic: H = hemaglutinin (11types) N = neuraminidase (9 types) 31 3. viral infection of small population gets to larger population Plant viral diseasesÍž 2,000 types known cause $ damage symptoms: stunted growth, spotted leaves damaged flowers or roots most have RNA genome 32 spread by: horizontal transmission infected from external source vertical transmission get from parent 33 Viral infection of plants can spread through plasmodesmata 34 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 35 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 36 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 37 38