MPSC 2022 Prelims Paper-I · Question 44 of 100

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.

• (2)(a), (b), (c) — 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.
• (3)(a), (c), (d) — 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.
• (4)(b), (c), (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).

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.

This is a multi-statement question whose four options differ in only one place that matters, and recognising that structure is worth as much as knowing the biology. Statement (c) appears in three of the four options and is absent from only one; a candidate who is certain that a viroid has no protein coat can therefore mark option (1) without evaluating any of the other statements at all, and can be done with the question in fifteen seconds. Looking for the statement that discriminates, rather than checking all four statements in order, is the general technique for this family, and MPSC uses the family heavily — thirty-six questions in this paper turn on a list of statements. The subject matter is standard higher secondary biology, drawn from the chapter that introduces viruses, viroids, prions and lichens, and the Commission tends to test that chapter through exactly these definitional contrasts because they can be verified against a textbook line for line. The wider lesson for preparation is that the categories which are examined are the ones defined by a single distinguishing feature, and that feature is what a question will be built on.

• A viroid is an infectious agent consisting of a single molecule of free RNA with no protein coat, which is the feature that distinguishes it from a virus and the reason statement (c) in this question is false.
• The RNA of a viroid is single-stranded and covalently closed into a circle, and it folds on itself by internal base pairing into a rod-like structure that gives it stability without a capsid.
• Viroid genomes are only a few hundred nucleotides long and encode no protein at all, which makes a viroid much smaller than even a small virus and leaves it wholly dependent on the enzymes of the host cell.
• Viroids are pathogens of higher plants, causing potato spindle tuber disease, citrus exocortis, cadang-cadang of coconut, avocado sunblotch and chrysanthemum stunt; none is known to cause disease in animals or human beings.
• The first viroid was identified by T. O. Diener in 1971 as the agent of potato spindle tuber disease, which he showed to be a free RNA of low molecular weight rather than a conventional virus.

• Attributing a protein coat to a viroid, which is the one thing a viroid does not have and the single statement this question is built on
• Confusing viroids with prions, which are the opposite case — infectious protein with no nucleic acid — rather than a smaller kind of virus
• Checking all four statements in a multi-statement question when only one of them discriminates between the options, which costs time that the paper does not allow
• Assuming viroids infect animals as well as plants, when every viroid disease so far described is a disease of higher plants

The acellular infectious agents reach MPSC papers as definitional contrasts: which agent lacks a protein coat, which consists of protein alone, which disease is caused by a viroid, who discovered the first one. The Commission also builds statement-list questions from the same material, as here, in which three true features and one false one are offered and the false one is the entire question. Because the material is small and well bounded, the productive preparation is a single comparison table with rows for virus, viroid and prion and columns for composition, size, host range, replication and an example disease, from which any of the shapes the question can take is answerable. Statement-list questions of this kind also reward the technique of reading the options first, since the statement that appears in three options and is missing from one is usually the statement the examiner has built the item around.

No directly related past PYQ was found.

• practice — not a real PYQ

  Which of the following infectious agents consists of protein alone, with no nucleic acid whatsoever ?

  • (a)Viroid
  • (b)Bacteriophage
  • (c)Prion
  • (d)Mycoplasma

  Answer(c) Prion — a prion is an abnormally folded form of a protein which propagates by inducing normal molecules of the same protein to adopt its misfolded shape, and it carries no genetic material at all. It is responsible for scrapie in sheep, bovine spongiform encephalopathy in cattle and Creutzfeldt-Jakob disease in human beings. A viroid, by contrast, is naked RNA without protein, a bacteriophage is a virus of bacteria with a nucleic acid genome in a protein coat, and a mycoplasma is a cellular organism that merely lacks a cell wall.

• practice — not a real PYQ

  The first viroid to be identified was the causal agent of which of the following plant diseases ?

  • (a)Potato spindle tuber disease
  • (b)Tobacco mosaic disease
  • (c)Citrus canker
  • (d)Late blight of potato

  Answer(a) Potato spindle tuber disease — T. O. Diener showed in 1971 that its agent was a small circle of free RNA rather than a conventional virus, and gave the new class the name viroid. Tobacco mosaic disease is caused by a virus, the one on which much early virology was done; citrus canker is a bacterial disease; and late blight of potato, the disease behind the Irish famine of the eighteen-forties, is caused by a fungus-like oomycete.