Viroids are identified on the basis of which of the following three characteristic features ?
(a) Much smaller in size than viruses
(b) Consist of circular RNA molecule
(c) RNA molecule covered by pro

Question

Viroids are identified on the basis of which of the following three characteristic features ?
(a) Much smaller in size than viruses
(b) Consist of circular RNA molecule
(c) RNA molecule covered by protein coat
(d) Infect the plant cells

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Correct — option (1), the combination of statements (a), (b) and (d). The whole question turns on a single statement, and the fastest route through it is to notice which one. Statement (c) says that the RNA molecule of a viroid is covered by a protein coat, and that is exactly what a viroid does not have: the defining feature of a viroid is that it is naked RNA, an infectious nucleic acid with no protein capsid around it. Once statement (c) is rejected, only one of the four options omits it, and that option is (1). The other three all contain statement (c) and fall together. Statement (a) is correct, and it is a direct consequence of the same fact. Because a viroid carries no coat protein and no genes for one, its genome can be very small — a few hundred nucleotides, against the thousands of bases and the structural protein genes that even a small virus needs — so a viroid is much smaller than a virus in both the length of its nucleic acid and its physical size. Statement (b) is correct as well: the RNA of a viroid is a single-stranded molecule that is covalently closed into a circle, and it folds back on itself by internal base pairing into a rod-like structure that gives it stability in the absence of any protective coat. Statement (d) is correct: viroids are pathogens of higher plants, and the diseases attributed to them are agricultural ones — the potato spindle tuber disease in which the first viroid was found, citrus exocortis, cadang-cadang of coconut palms, avocado sunblotch and chrysanthemum stunt. No viroid is known to cause disease in animals or in human beings. The three features named in statements (a), (b) and (d) are therefore the ones by which a viroid is recognised, and they hang together as a single description: an infectious agent so reduced that it consists of nothing but a small circle of RNA, dependent entirely on the machinery of the plant cell it enters, since it encodes no protein of its own. The discovery belongs to Theodor Otto Diener, who showed in 1971 that the agent of potato spindle tuber disease was a free RNA of low molecular weight and not a conventional virus, and who coined the name viroid for it. Option (1) is the answer.

Why the other options are wrong — (b) This option takes the two statements about size and about circular RNA, both of which are correct, and adds the one statement in the list that is false. A viroid has no protein coat; that absence is the feature which separates it from a virus, and it is the reason a viroid can be so much smaller than a virus in the first place, since it carries no genes for capsid protein and indeed encodes no protein at all. Choosing this option usually means a candidate has assimilated the viroid to the virus, remembering the general picture of an infectious particle as nucleic acid wrapped in protein and applying it here. The correction to hold is that infectious agents form a series of decreasing complexity: a virus is nucleic acid within a protein coat, a viroid is nucleic acid alone, and a prion is protein alone with no nucleic acid at all. The option also drops statement (d), and viroids are in fact plant pathogens, so it fails at both ends.  (c) This option is right about the small size and right about the infection of plant cells, but it includes the false statement about a protein coat and it omits the true statement about circular RNA. The circular form matters more than it may appear: the genome of a viroid is a covalently closed single-stranded circle, which folds on itself by internal base pairing into a compact rod-shaped structure, and this self-pairing is what allows a naked RNA to survive and to be replicated in a host cell without any protein protecting it. A candidate who has learnt viroids only as very small plant pathogens will accept statements (a) and (d) readily and then be uncertain between statements (b) and (c); the way to settle that is to recall the phrase used in every textbook description, free RNA or naked RNA, which asserts (b) and denies (c) in the same breath.  (d) This option keeps the false statement about the protein coat and drops the true statement about size, which is a poor exchange. That a viroid is much smaller than a virus is not incidental to its identity but part of the definition of the class: viroid genomes are only a few hundred nucleotides long, an order of magnitude below even the smallest viral genomes, precisely because they encode nothing. It is worth adding that the option is internally uncomfortable as well: an agent said to consist of RNA within a protein coat and to infect plant cells is simply a plant virus, and if statements (b), (c) and (d) were the identifying features of a viroid there would be no reason to have coined a separate name for it. Whenever an option set defines a category in terms that also define the neighbouring category, the definition has gone wrong somewhere, and here the fault is statement (c).

Concept: Viroids sit within a small family of infectious agents that are simpler than cells and simpler even than viruses. A virus consists of a nucleic acid genome, DNA or RNA, enclosed in a protein coat called a capsid, sometimes with an outer lipid envelope taken from the host membrane; it carries genes and can direct the synthesis of its own structural proteins once inside a cell. A viroid is stripped further: it is a single molecule of single-stranded RNA, closed into a circle, a few hundred nucleotides long, with no capsid, no envelope and no protein-coding capacity whatsoever. It replicates entirely by commandeering the host plant's own enzymes, and the disease it causes appears to arise from interference with the regulation of the host's genes rather than from anything the viroid itself makes. Its stability without a coat comes from extensive internal base pairing, which folds the circle into a rod. At the far end of the series stands the prion, which reverses the arrangement altogether — an infectious protein carrying no nucleic acid at all, which propagates by inducing the normal form of the same protein to misfold, and which is responsible for scrapie in sheep, bovine spongiform encephalopathy in cattle and Creutzfeldt-Jakob disease in human beings. Holding the three together as a descending series, nucleic acid plus protein, nucleic acid alone, protein alone, makes each of them memorable through its contrast with the others.

State PCS 2022 Q44 — Science & Technology PYQ with Answer

State PCS 2022 Q44 — Science & Technology PYQ with Answer