Final answer:
Purifying scrapie infectivity led to the discovery of prions, infectious proteins that cause neurodegenerative diseases by inducing misfolding of normal brain proteins. This groundbreaking work by Stanley Prusiner, who was awarded the Nobel Prize, revealed a new form of infectious agent without DNA or RNA.
Step-by-step explanation:
Discovering the Nature of Scrapie Infectivity
By purifying scrapie infectivity, researchers discovered prions, a type of infectious agent composed purely of protein. These findings by Stanley Prusiner challenged the prevailing belief that all infectious particles must contain DNA or RNA. Prions are abnormal, misfolded proteins that can induce normal proteins in the brain to also misfold, leading to diseases such as scrapie in sheep, and bovine spongiform encephalopathy (BSE) or mad cow disease in cattle. Prion diseases like Creutzfeldt-Jakob disease in humans cause severe neurodegenerative issues and are presently incurable. Modern techniques such as RT-QuIC are being used for earlier detection of prions.
PrPSC, the abnormal form of the prion protein, is responsible for the characteristic brain damage observed in affected individuals and animals, differentiating it from the normal form, PrPC. The discovery of prions validated the existence of infectious agents devoid of genetic material, expanding our understanding of disease causation and transmission.