Viroids and Prions
விராய்டுகள் மற்றும் பிரையான்கள்
Viruses are not the smallest infectious agents in your syllabus. If you think a protein coat is mandatory for infection, you are losing marks. Viroids and prions operate below the viral threshold, bypassing the standard genetic machinery.
The idea
Viroids are infectious agents composed solely of low molecular weight RNA, lacking a protein coat. They were discovered by T.O. Diener in 1971 as the cause of potato spindle tuber disease. Prions are abnormally folded proteins that act as infectious agents. They are similar in size to viruses but contain no nucleic acids.
Why it matters
These represent the simplest forms of biological infectious agents that challenge the traditional definition of life and pathogens.
NEET lens
NEET focuses on the distinction: Viroids = RNA only; Prions = Protein only. Do not confuse them with viruses or mycoplasma. Memorize that viroids have low molecular weight RNA. Expect a matching-type question or a statement-based question on their composition.
Clinical significance
Prions cause fatal neurodegenerative disorders like Bovine Spongiform Encephalopathy (BSE) in cattle and its variant, Creutzfeldt-Jakob disease (CJD) in humans. These proteins induce misfolding in normal brain proteins, leading to rapid neuronal death. There is no cure for these protein-based infections.
Introduction
In 1971, T.O. Diener discovered that potato spindle tuber disease was caused not by a virus but by a bare RNA molecule — smaller than any known virus, no protein coat whatsoever. He called it a viroid. Two decades later, Stanley Prusiner showed that scrapie in sheep and Creutzfeldt-Jakob disease in humans were caused by misfolded proteins with no nucleic acid. He called them prions. Both discoveries won Nobel Prizes. Both overturned assumptions about what an infectious agent had to be.
Core concept
Viroids: discovered by T.O. Diener (1971); single-stranded, circular RNA only — no protein coat; smaller than viruses; cause plant diseases (e.g., Potato Spindle Tuber Viroid, PSTV); RNA interferes with host cell processes directly.Prions: discovered by Stanley Prusiner; misfolded proteins only — no nucleic acid; infectious (misfolded protein converts normal proteins into misfolded form); cause fatal neurodegenerative diseases: BSE (mad cow disease), Creutzfeldt-Jakob disease (CJD) in humans, scrapie in sheep.
Go deeper
The central dogma (DNA to RNA to protein) is disrupted by both: viroids (RNA without protein) and prions (protein without nucleic acid). Prions do not replicate genetically — they propagate by inducing conformational change in normal proteins. This is why conventional methods that destroy nucleic acids (UV, radiation) have no effect on prions.
From Class 10
In Class 10, you studied viruses as the smallest disease-causing agents. In Class 11, you encounter two entities even stranger: viroids — nothing but a naked strand of RNA; and prions — nothing but a misfolded protein with no nucleic acid. Both cause disease. Neither is alive in any conventional sense.