viroids

Meaning: A viroid is a small circular, single-stranded RNA, a few hundred nucleotides long, with no capsid and no envelope. It typically does not code for any protein, so it relies on host enzymes: host RNA polymerase copies it, often by a rolling-circle mechanism. Viroids are known mainly as plant pathogens. They are thought to cause disease by disturbing host gene regulation, for example through RNA silencing. They spread through damaged tissue, seeds, pollen or contaminated tools.

On the MCAT: Expect a classification question that sorts infectious agents by composition. A virus has nucleic acid inside a capsid, a viroid is RNA alone, and a prion is protein alone. A passage may describe an agent that is destroyed by RNase but not by protease, which points to a viroid. Another question may ask whether a drug that blocks capsid assembly or viral protein synthesis would work. It would not, because the viroid has neither. Viroids also show that RNA can carry infectious information without being translated.

Common confusion: Students call a viroid a very small virus or mix it up with a prion. A viroid has nucleic acid but no protein coat. A prion has protein but no nucleic acid. Also, being RNA does not make a viroid an mRNA. It is not usually translated.