Question

How are viroids different from viruses?

Answer 1

Concept

Both viroids and viruses are infectious, acellular agents that require a living host cell to replicate. However, they differ markedly in their structure, type of genetic material and hosts they infect.

Key Differences in a Table

Feature	Viruses	Viroids
Basic nature	Infectious particles made of nucleic acid plus protein coat (capsid)	Infectious agents made only of a short RNA strand, no protein coat
Genetic material	Either DNA or RNA (may be single- or double-stranded)	Only RNA; small, circular, single-stranded, low molecular weight
Protein coat (capsid)	Present; capsid protects nucleic acid and helps in host recognition	Absent; viroids are naked RNA molecules without a capsid
Size	Very small, but larger than viroids	Even smaller than viruses (few hundred nucleotides long)
Hosts infected	Infect plants, animals, humans and microorganisms (e.g., bacteria)	Known to infect only higher plants (angiosperms)
Examples of diseases	Influenza, measles, polio, AIDS, COVID‑19, mosaic diseases of plants	Potato spindle tuber disease, cucumber pale fruit, coconut cadang‑cadang

Point-wise Answer

• Viroids are smaller than viruses and consist only of a short, circular, single-stranded RNA molecule without any protein coat.
• Viruses have either DNA or RNA as genetic material, enclosed in a protein coat called capsid; viroids have only naked RNA and lack a capsid.
• Viruses can infect plants, animals, humans and microorganisms, whereas viroids are known to infect only plants.
• Viroids were first discovered as pathogens causing certain plant diseases (e.g., potato spindle tuber), while viruses cause a wide range of plant as well as animal diseases.

Short Exam-style Answer

Viroids are infectious, naked RNA particles that are smaller than viruses and lack a protein coat. They consist of a short, circular, single-stranded RNA of low molecular weight and infect only plants. Viruses, on the other hand, have either DNA or RNA as genetic material enclosed in a protein coat (capsid) and infect plants, animals and microorganisms. Thus, viroids differ from viruses mainly in the absence of a capsid, their simpler structure and their restricted host range.