A viroid is a tiny infectious agent made of only free RNA. It has NO protein coat, and its RNA is of low molecular weight. T.O. Diener discovered it in 1971, and it causes potato spindle tuber disease. Memory hook: "viroiD = Diener, naked RNA, low weight, potato."
A virus has nucleic acid protected by a protein coat (capsid). A viroid is just free, low-molecular-weight RNA with no coat at all — the missing coat is why it is named "viroid."
Your doubts, answered
What is a viroid in simple words?
A viroid is just a short, naked piece of RNA that can infect a host and cause disease. Unlike a virus, it has no protein coat around it. Think of a virus as a nut with a shell (RNA/DNA + protein coat), and a viroid as the same seed with NO shell at all. This is the exact line NCERT uses, and NEET loves to test it.
Do viroids have a protein coat or not?
No. Viroids do NOT have a protein coat. The whole name comes from this fact: it 'lacked the protein coat that is found in viruses, hence the name viroid'. If a NEET option says a viroid 'has a protein coat', that option is wrong. Viroid = free RNA, coat absent.
Is the RNA of a viroid high or low molecular weight?
It is LOW molecular weight. This is a classic trap. NCERT says 'The RNA of the viroid was of low molecular weight.' In NEET 2016, the WRONG statement was 'Their RNA is of high molecular weight' — so remember: viroid RNA = low, not high.
Who discovered viroids and in which year?
T.O. Diener discovered viroids in 1971. Easy hook: viroi-D = Diener. He found an infectious agent smaller than viruses that caused potato spindle tuber disease. The year 1971 and the name Diener are both examination-worthy points.
Which disease is caused by a viroid?
Potato spindle tuber disease. This is the standard NCERT example. When a question links 'potato spindle tuber' with an agent, the answer is a viroid (free RNA, no coat). Do not confuse it with prion diseases like mad cow disease.
Why is a viroid smaller than a virus?
Because a viroid is only a small strand of free RNA, while a virus carries both nucleic acid AND a protein coat (capsid). Removing the coat and having low molecular weight RNA makes the viroid smaller. NCERT directly says the agent Diener found was 'smaller than viruses'.
⚠️ The NEET trap ✗ The RNA of a viroid is of high molecular weight. ✓ The RNA of a viroid is of LOW molecular weight; it is free RNA with no protein coat, causing potato spindle tuber disease. 🧠 Viroid = small, naked, LOW-weight RNA. If an option says 'high molecular weight', it is the WRONG statement (NEET 2016 answer).
Real NEET questions
NEET 2016
Which of the following statements is wrong for viroids?
A · They lack a protein coat
B · They are smaller than viruses
C · They cause infections
D · Their RNA is of high molecular weight ✓
Solution: The wrong statement is (D). A viroid's RNA is of LOW molecular weight, not high. The other three are all true: viroids lack a protein coat (free RNA), are smaller than viruses, and cause infections such as potato spindle tuber disease. NCERT Class XI, Biological Classification: 'The RNA of the viroid was of low molecular weight.'
NEET 2017
Viroids differ from viruses in having:
A · DNA molecules with protein coat
B · DNA molecules without protein coat
C · RNA molecules with protein coat
D · RNA molecules without protein coat ✓
Solution: Answer (D). A viroid is free RNA that lacks the protein coat (capsid) found in viruses — that is the very reason for the name 'viroid'. Viruses have nucleic acid enclosed in a protein coat, so viroids differ by being RNA without a coat.
NEET 2020
Which of the following is correct about viroids?
A · They have DNA with protein coat
B · They have free DNA without protein coat
C · They have RNA with protein coat
D · They have free RNA without protein coat ✓
Solution: Answer (D). Viroids (discovered by T.O. Diener, 1971) are free RNA of low molecular weight that lack the protein coat found in viruses. So the correct description is free RNA without a protein coat.
Solved Biological Classification NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Viroids are made of RNA only — free, single-stranded RNA. They never contain DNA. This is a common NEET distractor: any option saying a viroid has DNA is wrong.
What is the difference between a viroid and a prion?
A viroid is free RNA with no protein coat (causes potato spindle tuber disease). A prion is an abnormally folded PROTEIN with no nucleic acid (causes mad cow disease/BSE). RNA vs protein is the key split.
Are viroids considered living?
Like viruses, viroids are placed among acellular, non-truly-living infectious agents. NCERT introduces them briefly along with viruses and prions because they have no cell structure.
Do viroids infect humans?
The standard NCERT example is a plant disease (potato spindle tuber disease). For NEET, remember viroids mainly as plant pathogens made of free RNA.
Why is the name 'viroid' important for NEET?
The name itself is the clue: 'viroid' means virus-like but without the coat. NCERT says it 'lacked the protein coat that is found in viruses, hence the name viroid.' The name encodes the answer.
