Question

State the difference between virus and viroid.

• A. absence of protein coat in viroid but present in virus
• B. presence of low molecular weight $RNA$ in virus but absent in viroid
• C. both (a) and (b)
• D. none of the above

Answer 1

Answer: (A)

Virus

1. Viruses are non-cellular, non-cytoplasmic, infectious agents.
2. They are smaller than bacteria and thus can pass through a bacteria proof filter.
3. Viruses are transmissible from diseased to healthy organisms.
4. All viruses are obligate parasites and can only multiply within the living host cells.
5. Viruses contain only a single type of nucleic acid, i.e. either DNA or RNA.
6. Viruses are host specific in that they infect only a single species and definite cells of the host organisms.
7. Viruses are effective in very small doses only. Most of them are highly resistant to germicides and extremes of physical conditions.

Viroid

1. Viroids are extremely simple infectious agents discovered in 1967 by Diener and Raymer. These consist of only very small RNA genomes 240-350 nucleotides long and no other structure (no protein coat). They are smallest known agents of infectious plant diseases.
2. Diener and Raymer (1967) found that the causal agent of potato spindle tuber disease (PSTD) was a free RNA (no nucleoprotein). Diener (1971) called it viroid. Since then, a number of diseases like cadang of coconut and cucumber pale fruit have been found to be due to viroids.
3. These viroids (RNA macromolecules) contain sufficient information needed to direct their own replication but totally depend upon the host's metabolic machinery.