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A proteinaceous infectious particle, or prion, (pronounced /ˈpriː.ɒn/ ( listen)[1]) is an infectious agent composed primarily of protein in beta-sheet form (normal proteins are in alpha helices).[2] This is in contrast to viruses, which consist of two or three parts: a helical molecule, protein coat and sometimes a viral wrapper.[3] The word prion, coined in 1982 by Dr. Stanley B. Prusiner, is a portmanteau derived from the words protein and infection.[4] Prions are the cause of a number of diseases in a variety of mammals, including bovine spongiform encephalopathy (BSE, also known as "mad cow disease") in cattle and Creutzfeldt–Jakob disease (CJD) in humans. In general usage, prion refers to the theoretical unit of infection. All known prion diseases affect the structure of the brain or other neural tissue and all are currently untreatable (no natural defense) and universally fatal.[5]

Prions propagate by transmitting a mis-folded protein state: so as with viruses the protein cannot replicate by itself. Instead, when a prion enters a healthy organism, the prion form of a protein induces pre-existing normal forms of the protein to convert into the rogue form. Since the new prions can then go on to convert more proteins themselves, this triggers a chain reaction that produces large amounts of the prion form.[6] All known prions induce the formation of an amyloid fold, in which the protein polymerises into an aggregate consisting of tightly packed beta sheets. Amyloid aggregates are fibrils, growing at their ends, and replicating when breakage causes two growing ends to become four growing ends. The incubation period of prion diseases is determined by the exponential growth rate associated with prion replication, which is a balance between the linear growth and the breakage of aggregates.[7]

This altered structure is extremely stable and accumulates in infected tissue, causing tissue damage and cell death.[8] This structural stability means that prions are resistant to denaturation by chemical and physical agents, making disposal and containment of these particles difficult. Prions come in different strains, each with a slightly different structure, and most of the time, strains breed true. Prion replication is nevertheless subject to occasional epimutation and then natural selection just like other forms of replication.[9] However, the number of possible distinct prion strains is likely far smaller than the number of possible DNA sequences, so evolution takes place within a limited space.

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