E. VIROIDS AND PRIONS
The overall purpose of this Learning Object is to introduce viroids and prions and several diseases they may cause.
LEARNING OBJECTIVES FOR THIS SECTION
Viroids and Prions
Viroids (def) are even more simple than viruses. They are small, circular, single-stranded molecules of infectious RNA lacking even a protein coat. They are the cause of a few plant diseases such as potato spindle-tuber disease,cucumber pale fruit, citrus exocortis disease, and cadang-cadang (coconuts).
Prions (def) are infectious protein particles thought to be responsible for a group of transmissible and/or inherited neurodegenerative diseases including Creutzfeldt-Jakob disease, kuru, and Gerstmann-Straussler- syndrome in humans as well as scrapie in sheep and goats. The infections are often referred to as transmissible spongiform encephalopathies (def). Most evidence indicates that the infectious prion proteins are modified forms of normal proteins coded for by a host gene in the brain. It is thought that the normal prion protein, expressed on stem cells in the bone marrow and on cells that will become neurons, plays a role in the maturation of neurons. In the case of the disease scrapie, the normal prion protein in an animal without the disease has alpha-helices in the proteins secondary structure (see Fig 1) while the scrapie prion protein in diseased animals has beta-sheets for the secondary structure (see Fig. 2). When the scrapie prion protein contacts the normal protein it causes it to change its configuration to the scrapie beta-sheet form. This suggests that the conversion of a normal prion protein into an infectious prion protein may be catalyzed by the prion protein itself upon entering the brain. Inherited forms may be a result of point mutations that make the prion protein more susceptible to a change in its protein structure.
There is growing evidence that other probable protein misfolding diseases initiated by prions include Alzheimer's disease, Hunington's disease, Parkinson's disease, frontotemporal dementias, amyotrophic lateral sclerosis, and certain cancers.
E-Medicine article on infections associated with organisms mentioned in this Learning Object. Registration to access this website is free.
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Updated: July 12, 2006
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