UPSC Prelims 2002 Q134: Science & Technology – Health & Diseases
Which one of the following statements is correct?
- (a) Prions are the smallest free - living cells
- (b) The cell wall of Mycoplasmas is made up of amino sugars
- (c) Viroids consist of a single - stranded RNA molecule
- (d) Rickettsia’s lack cell wall
Answer: (c) Viroids consist of a single - stranded RNA molecule
Explanation
Microbes: prions, mycoplasma, viroids and rickettsia
| Statement | Verdict |
|---|---|
| Prions are the smallest free-living cells | False — prions are infectious proteins, not cells |
| Mycoplasma cell walls are of amino sugars | False — mycoplasmas have no cell wall |
| Viroids consist of a single-stranded RNA molecule | True — circular RNA without a protein coat |
| Rickettsia lack a cell wall | False — they have a cell wall like bacteria |
Viroids are the smallest known infectious agents and consist of a single-stranded circular RNA molecule without a protein coat. Prions are proteins, mycoplasmas lack a cell wall, and rickettsias are small bacteria that do have a cell wall.
Answer: (c) Viroids consist of a single-stranded RNA molecule.
Why each option is right or wrong
- (a) Incorrect. Prions are proteins and not cells.
- (b) Incorrect. Mycoplasmas have no cell wall at all.
- (c) CORRECT. Viroids are single-stranded RNA.
- (d) Incorrect. Rickettsias have a cell wall.
Key point
Mycoplasma: smallest free-living cells, no cell wall; viroids: naked RNA (potato spindle tuber); prions: misfolded proteins (mad cow disease).
Remember
Viroid = naked single-stranded RNA; mycoplasma = no cell wall.
Common mistakes
- Calling prions cells because they cause disease.
- Giving the mycoplasmas a cell wall of amino sugars.
- Thinking rickettsias have no cell wall like mycoplasmas.
Exam tip
Only the viroid statement describes its true structure.
Sources
- NCERT Class XI Biology — The Living World
- NCERT Class XI Biology — Biological Classification