The abnormal protein itself directs the conversion of the normal host protein to the abnormal form. In other words, PrPSc converts PrPC into PrPSc. J.S. Griffith first proposed the protein-only theory in 1967 to explain how prions could replicate if they were made of protein but did not contain nucleic acids. Many have called the theory heretical because it describes replication of a pathogenic agent without a nucleic acid genome. In fact, the hypothesis is based upon known properties of proteins with the added wrinkle that a protein molecule folded in an abnormal way can alter the folding of another protein molecule and thereby change its biological properties.
People often assume that scientists are in the business of trying to prove hypotheses or theories.
A hypothesis that fails one or more tests is considered disproved and it is discarded. If it is not disproved after being tested in many different ways, we become more confident that it is correct. A hypothesis is valid as long as it explains the behavior of the system it describes, but it is always possible that it will have to be revised or discarded based on new results. Background Proteins are chains of chemicals called amino acids linked together like beads on a string. The prion protein has about 250 amino acids.