Viroids – Molecular nature: Viroids, which cause several plant diseases, are composed of what type of nucleic acid?

Introduction:
Viroids are the smallest known infectious agents, responsible for several economically significant plant diseases. Unlike viruses, viroids do not encode proteins. Understanding their molecular composition is key to distinguishing them from viruses and prions.

Given Data / Assumptions:

• Viroids infect plants and replicate in host cells without coding for proteins.
• They are circular molecules with extensive base pairing and secondary structure.
• The question asks for the nature of the nucleic acid they contain.

Concept / Approach:
Viroids are composed of small, circular single-stranded RNA molecules, typically 250–400 nucleotides long. Their pathogenicity arises from RNA structure and host interaction, not from expressed proteins.

Step-by-Step Solution:
Step 1: Exclude DNA options because viroids are RNA-only agents.Step 2: Exclude protein-only agents (that would be prions).Step 3: Identify the correct form as ssRNA with circular topology and high intramolecular base pairing.Step 4: Select “Single-stranded RNA (ssRNA).”

Verification / Alternative check:
Molecular analyses and replication mechanisms (rolling-circle) validate viroids as small circular ssRNA pathogens of plants.

Why Other Options Are Wrong:

• ssDNA/dsDNA: not consistent with viroid biology.
• dsRNA: some RNA viruses have dsRNA genomes, but viroids do not.
• Protein-only: describes prions, not viroids.

Common Pitfalls:
Confusing viroids with viruses due to similar names; assuming all infectious agents encode proteins.

Final Answer:
Single-stranded RNA (ssRNA).