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Naked nucleic acid
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Smallest pathogens
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Copied by host RNA polymerase
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Many cause infections/damage to plants
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Hepatitis D in humans
 Liver cell death
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Circular RNA molecules
 Few hundred nucleotides long
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Don’t encode proteins
 Can replicate in host cells
 Cause errors in regulation of growth
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First discovered in 1982
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“Proteinaceous infectious particle”
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Rogue proteins
 Normal protein = PrP gene
 Prion = misfolded version of this protein
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Prion folding
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Can arise spontaneously
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Trigger NO IMMUNE RESPONSE
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Characteristics of prions:
 Long incubation period
 Indestructible
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Group of neurodegenerative disorders
 transmissible spongiform encephalopathies (TSEs)
 Proteins aggregate & form tough, fibrous deposits
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Always fatal
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Rapidly progressive once symptoms appear
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Animal Prion Diseases:
 Scrapie
 Mad cow
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Creutzfeldt-Jakob Disease (CJD)
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Variant Creutzfeldt-Jakob Disease (vCJD)
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Gerstmann-Straussler-Scheinker Syndrome
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Fatal Familial Insomnia
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Kuru
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Proteins are self-propetuating
 No genetic material
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Have distinct strains that produce different
phenotypes
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Normal proteins have many alpha helices;
prions have many more beta-pleated sheets
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New evidence shows prions can:
 Maintain long-term memory (snails, mice, fruit
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flies)
Speed up evolution in yeast
Aid in synthesis of melanin in mammals
Form biofilms
Store hormones
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Are prion diseases caused by gain of function
or loss of function?
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How does the amino acid sequence of the
prion determine its configuration?
 How can the same sequence determine different
“packing” arrangements?