Q.Which of the following shows single stranded RNA and lacks protein coat?

Potato spindle tuber disease is caused by viroids — the smallest infectious RNA agents, so the answer is (C).

Viroids are the smallest known infectious agents. They were discovered by T.O. Diener in 1971 as the cause of potato spindle tuber disease. Unlike a virus, a viroid is simply a short, single-stranded circular RNA molecule with NO protein coat (capsid).

Checking the options:

• Lichens are symbiotic associations of algae and fungi, not pathogens.

• Bacteria and viruses cause other diseases, but potato spindle tuber is specifically caused by a naked RNA molecule, which defines a viroid.

Three of the four statements about viruses are true: they are obligate parasites, they can be precipitated, and they can mutate. Only the claim that they have a complete cellular structure is false.

Go through each statement one at a time.

1. They are obligate parasites — TRUE. A virus has no metabolic machinery of its own (no ribosomes, no enzymes for protein synthesis). It can replicate only by taking over a living host cell's machinery, which is exactly what "obligate parasite" means.
2. They have complete cellular structure — FALSE. A virus is acellular. It consists only of genetic material (DNA or RNA) enclosed in a protein coat (capsid), sometimes with an outer envelope. It has no cytoplasm, no plasma membrane of its own, no organelles — so it does not meet the basic definition of a cell.
3. Viruses can be precipitated — TRUE. Outside a host, viruses behave like inert chemical particles rather than living organisms. Purified virus preparations can be crystallised and can be brought out of solution (precipitated), which is one of the classic pieces of evidence used historically to argue that a virus sits at the boundary between living and non-living matter.
4. Viruses can mutate — TRUE. Because viral genome replication (especially for RNA viruses) lacks the proofreading accuracy of host DNA replication, viruses accumulate mutations rapidly. This is why new viral strains/variants keep appearing. So three statements — (i), (iii), (iv) — are true, and only (ii) is false.

TMV is the Tobacco Mosaic Virus, a rod-shaped RNA virus that infects tobacco plants and served as a landmark organism in early virology.

TMV was one of the first viruses ever discovered — Dmitri Ivanovsky (1892) and later Martinus Beijerinck (1898) showed that the sap of infected tobacco plants remained infectious even after passing through bacteria-proof filters, establishing the concept of a filterable, sub-bacterial infectious agent (a virus). TMV causes a characteristic mottled, mosaic-like pattern of light and dark green patches on tobacco leaves. It also became historically important because Wendell Stanley later crystallised TMV and showed it consists largely of protein, work that contributed to the 1946 Nobel Prize.

A virus is nucleic acid plus a protein coat.

A virus consists of a core of nucleic acid (either DNA or RNA) surrounded by a protein coat called the capsid. Thus its basic composition is nucleic acid and protein. (Option D omits the protein coat, so it is incomplete.)

HIV is an RNA retrovirus → (b) RNA only.

• The AIDS virus (HIV) is a retrovirus: its genome is single-stranded RNA.

• Inside a host cell it uses reverse transcriptase to make DNA from its RNA, but the virus particle itself carries RNA only.

Viruses reproduce only inside a host cell → obligate intracellular parasites → (b).

• Viruses are acellular and inert outside a host; they can multiply only inside a living host cell using the host's machinery, making them obligate intracellular parasites.

• Bacteria, slime moulds and blue-green algae can grow independently.

Louis Pasteur first used the Latin word "virus" (poison) for disease-causing agents smaller than bacteria; later scientists (Ivanowsky, Beijerinck, Stanley) each revealed a new property of these agents.

The discovery and naming of viruses happened in stages:

1. Louis Pasteur was the first to use the term "virus" for an infectious agent that behaved differently from ordinary bacteria — too small to be seen or filtered out, yet clearly capable of causing disease.

2. D. J. Ivanowsky (1892) found that the sap of tobacco plants infected with mosaic disease remained infectious even after being passed through a bacteria-proof filter, hinting the agent was smaller than any known bacterium.

3. M. W. Beijerinck (1898) went further, showing this fluid could multiply only inside a living host, and named it "Contagium vivum fluidum" (infectious living fluid).

4. W. M. Stanley (1935) showed that viruses could be crystallised, and that these crystals were largely made of protein — the first hint of their unique, non-cellular nature.