Prions are infectious agents composed exclusively of a single sialoglycoprotein called PrP 27-30. They contain no nucleic acid. PrP 27-30 has a mass of 27,000 - 30,000 dalto…ns and is composed of 145 amino acids with glycosylation at or near amino acids 181 and 197. The carboxy terminus contains a phosphatidylinositol glycolipid whose components are ethanolamine, phosphate, myo-inositol and stearic acid. This protein polymerizes into rods possessing the ultrastructural and histochemical characteristics of amyloid. Amyloid is a generic term referring to any optically homogenous, waxy, translucent glycoprotein; it is deposited intercellularly and/or intracellularly in many human diseases such as: .
Alzheimer's disease .
Creutzfeldt-Jakob disease .
Down's syndrome .
Fatal familial insomnia .
Gerstmann-Straussler syndrome .
Kuru Leprosy Viroids are infectious agents composed exclusively of a single piece of circular single stranded RNA which has some double-stranded regions. .
Because of their simplified structures both prions and viroids are sometimes called subviral particles . Viroids mainly cause plant diseases but have recently been reported to cause a human disease. Catalytic RNAs are those that have the intrinsic ability to break and form covalent bonds; Viroids are catalytic RNA's (ribozymes) that cleave RNA to produce fragments containing a 5'-hydroxyl and a 2', 3'-cyclic phosphate. ( Full Answer )