How are viroids different from viruses?
Viroids differ from viruses in structure and composition. Discovered by T.O. Diener in 1971, viroids are smaller than viruses and caused potato spindle tuber disease.
The key difference is that viroids are free RNA of low molecular weight and lack the protein coat that is found in viruses. In contrast, viruses contain genetic material that could be either RNA or DNA, enclosed within a protein coat called capsid made of small subunits called capsomeres.
Thus, while viruses are nucleoproteins with infectious genetic material protected by a protein covering, viroids are simply infectious free RNA molecules without any protein coat.
Explanation
The textbook clearly distinguishes viroids from viruses. Viroids (discovered by T.O. Diener in 1971) are smaller than viruses and consist only of free RNA without a protein coat. Viruses, on the other hand, are nucleoproteins containing either RNA or DNA enclosed in a protein coat (capsid). This structural difference is fundamental and exam-important.