Chapter 19 Viruses 1 Discovery of Viruses a 1883 Mayer investigating tobacco mosaic disease i Gives leaves a mottle appearance stunts growth disease could be spread by spraying sap could not find a microbe in infected sap b 1890 Ivanowski c 1897 Beijerinck i Filtered infected sap for microbes ii Sap still contagious Small microbe Toxin product produced i Did serial infections to attempt to dilute this toxin ii Reasoned the pathogen could reproduce iii Could not be cultured on media like bacteria needed a host iv Simple reproducing particle i Crystallized the particle Tobacco Mosaic Virus TMV cells don t crystallize like ii Now TEM is used Transmission Electron Microscopy iii Viral infection of plants mosaic virus d 1935 Stanley this 2 What is a Virus a Very small infectious particles b Obligate intracellular parasites requires a host c Have nucleic acids enclosed in a protein coat some with a membrane envelope i Genetic material varies considerably 1 Single or double stranded DNA or RNA 2 Usually only one molecule not all 3 4 100 s of genes 3 Virus structure a Capsid protein shell of virus varies in shape some with only protein covering i Viral envelopes found in influenza viruses and other that infect animal cells ii Membrane cloaks the capsid like an Cloak of Invisibility iii Membrane has derived from host cell 1 Bacteriophages viruses that infect bacteria 2 Have very complex capsids 3 Tail piece injects DNA contained in head into bacterium 4 Virus reproduction a Host range limited number of potential host cells i Some may infect several species some are species specific Rabies dogs ii Viruses that infect Eukaryotes are typically tissue specific human cold viruses raccoons skunks humans infect cells of upper respiratory tract 1 Process a Virus infects cell takes over cell machinery cell begins to produce virus DNA and proteins b Components typically can self assemble DNA and proteins c 100 1000s of new virus particles emerge from cell a Reproductive cycle leads to the death of host cell 2 3 Lytic cycle Lysogenic cycle a Replication of viral genome without killing host cell inserts into host genome is replicated with host DNA b Examples of viral disease i Small pox chicken pox measles mumps common cold flu polio herpes rabies AIDS Ebola west Nile hanta virus 5 Result of Infection Varies a Recovery from colds cells of upper respiratory tract can be repaired b Polio virus attacks nerve cells causes permanent damage c Ebola a Vaccines variants or derivatives of the infectious agent vacca cow b First vaccine used cowpox as vaccine for smallpox disease was thought to be c New threats new host species or increased access to a larger population i Example Dr Andrew Wakefield suggested a link between MMR vaccine to 6 Vaccines eliminated 7 Evolution of Viruses autism a Can we classify them as a life form most complex molecules Or simplest life form b Likely evolved after evolution of life true cells i Viral DNA often contains genes similar to its host c Plasmids and Transposons i Plasmids small circular pieces of DNA found in bacteria and yeast a eukaryote can replicate independently of cells genome ii Transposons DNA segments that move within the host genome d Prions a molecule can be an infectious agent infectious proteins misfolded i Mad cow disease creutzfeldt jacob disease in humans prions are smaller than a virus ii Kuru 1 People in papua new Guinea honored the dead by consuming them 2 Kuru to tremble with fear developed in the 1920s 3 Mood swings shaking behavior then death 4 2500 died between 1957 1975 a Men consumed muscles women and children everything else 5 Disease had a long incubation time a No children born after cannibalism was banned got kuru b c d Adults got the disease even in to the 1990s In 4 years not found in 4 9 year olds In 10 years it was not found in 10 19 year olds iii BSE Bovine Spongiform Encephalopathy 1 Symptoms aggression lack of coordination with inability to stand or walk muscle tremors weight loss abnormal posture fatal but proceeds slowly few months to a year Incubation time may last several years 2 3 Discovered 1984 1986 in British cattle 4 180 000 confirmed cases as high as 2 million 5 Cattle feed a Rendering carcasses of sheep goats and cattle used to make meat and bone meal Ill non productive cattle sent to rendering plants b c Banned from cattle and sheep feed in 1988 d Caused brain lesions likely from scrapie in sheep e Over 4 6 million cattle slaughtered f Ban of British beef e Creutzfeldt Jacob disease in humans i 1 1 million per year usually 55 75 years in age ii Sporadic 85 cases no causal link iii Familial 10 inherited dominant disorder iv Iatrogenic 5 from medical procedure person person 1 New variant in humans a Strikes people 16 30 years old b Human form of mad cow disease c Over 150 cases 2 Species barrier a Sheep humans no disease for centuries b Sheep cows humans species barrier easier to cross 3 Chronic wasting disease is classified as a transmissible spongiform encephalopathy Likely caused by Prions causes numerous symptoms a b Characteristic weight loss and death c Causes lesions in the brain