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If you are asked "How are viroids different from viruses?", the textbook answer is one line: a viroid is free RNA with no protein coat. What the line hides is a six-year detective story, in which a plant pathologist kept ruling out the obvious explanation until only an "impossible" one was left. This post walks through that reasoning, step by step, because it is a model of how to test an idea.
What does NCERT say about viroids?
The NCERT Class 11 Biological Classification chapter says that in 1971 T. O. Diener discovered a new infectious agent smaller than viruses that caused potato spindle tuber disease. It was a free RNA that lacked the protein coat found in viruses, hence the name viroid, and its RNA was of low molecular weight.
The same chapter tells the earlier virus story: Dmitri Ivanowsky in 1892 found that the cause of tobacco mosaic disease passed through bacteria-proof filters, M. W. Beijerinck in 1898 named it a virus, and W. M. Stanley in 1935 showed viruses could be crystallised and consist largely of proteins. By 1971, everyone "knew" an infectious agent needed protein. Our post on Dmitri Ivanovsky covers the first part.
How did Diener rule out a virus?
The US Department of Agriculture's Agricultural Research Service, where Diener worked, tells the story in its feature Tracking the elusive viroid. The steps are worth setting out like a lab report.
A faster test. Spindle tuber takes a couple of years to show in potatoes. ARS colleagues William Raymer and Muriel O'Brien found the agent spread easily to tomatoes, which became dramatically stunted within two weeks. Now experiments could be repeated quickly.
The standard virus method failed. High-speed centrifugation, used to purify viruses, recovered so little infectivity that it was clear this was not a typical virus.
Size and density. Using density gradient centrifugation, developed by ARS chemist Myron Brakke, Diener and Raymer found the agent was small and light. Diener said it seemed more likely to be a free nucleic acid than a conventional viral nucleoprotein.
Enzyme tests. They treated extracts of diseased tomato leaves with enzymes. An enzyme that destroys RNA abolished infectivity. Enzymes that destroy DNA or protein made no difference.
Conclusion: the essential part of the agent was RNA, and it contained no protein. Raymer left in 1966, and Diener spent the next five years isolating and characterising the agent before publishing.
Why was the result so surprising?
Size. ARS reports that the potato spindle tuber viroid weighs about 130,000 daltons, while scientists then believed the minimum for infectivity was about 1 million daltons.
Worked example: how far below the expected minimum was it?
Ratio = 1,000,000 / 130,000 ≈ 7.7.
So the viroid was nearly eight times lighter than the smallest infectious agent people thought could exist.
The same ARS feature describes the viroid as 80 times smaller than a virus. Notice that "smaller" here compares the viroid with a whole virus particle, not with the assumed minimum, so the two numbers answer different questions. A common mistake is to treat them as the same comparison.
Diener's 1971 paper in Virology, Potato spindle tuber "virus". IV. A replicating, low molecular weight RNA, carries the old word "virus" in quotation marks in its title.
What is a viroid made of?
Later work filled in the structure. In 1978 H. J. Gross and colleagues published the nucleotide sequence and secondary structure of potato spindle tuber viroid in Nature. A 2019 study in PLOS Pathogens describes the viroid as a circular non-coding RNA of 359 nucleotides that replicates and spreads through host plants, so every function it needs is carried by its own sequence and folded structure.
Non-coding matters. A viroid makes no proteins of its own. It relies entirely on the host cell's machinery.
How should you answer viroid questions in exams?
Use a three-point comparison with viruses:
Genetic material: viroid, RNA only; virus, DNA or RNA (NCERT notes no virus contains both).
Protein coat: viroid, none; virus, present (the capsid).
Size: viroid, smaller, with low molecular weight RNA.
Common mistakes:
Calling viroids "small viruses". They lack the defining protein coat.
Confusing viroids with prions. NCERT describes prions as abnormally folded protein, the opposite case: protein without nucleic acid.
Giving the wrong host. The classic viroid disease is in a plant, potato spindle tuber.
What can a student take from Diener's way of working?
Find a faster test. Switching from potatoes to tomatoes turned years into weeks.
Rule out, one variable at a time. Each enzyme removed one ingredient.
Take time to be sure. ARS says he spent five years preparing to meet the scepticism that greets "impossible" ideas.
If you want to check the difference between capsids, nucleic acids and prions, the Fermi Concept Library explains every biology concept in one place. For the whole chapter, see our guide to biological classification for NEET.
Next step: write the viroid, virus and prion comparison from memory, then check it against NCERT section 2.6.
