CDS (I) 2022 General Knowledge · Question 28 of 120

Correct — B, genetic material. A mutation is by definition a change in the sequence of an organism's genetic material, and for a virus that material is either DNA or RNA, packed inside a protein coat. When the virus is copied inside a host cell, mistakes creep into the sequence; those altered sequences are mutations, and they are what produce new variants. Anything else that changes about the virus — the shape of the coat, the proteins on its surface, how easily it spreads — is a consequence of a change in the sequence, not the mutation itself. This is also why RNA viruses generate variants so much faster than DNA ones: measured mutation rates for RNA viruses run at about 10⁻³ to 10⁻⁵ per base per generation against 10⁻⁶ to 10⁻⁸ for DNA viruses, roughly a hundred to a thousand times higher.

• (a)cell size. — A virus is not a cell. It has no cytoplasm, no organelles and no membrane of its own making — only genetic material inside a protein coat — so it has no cell size to change.
• (c)shape. — The shape of the protein coat can indeed change, but only because the genes coding for the coat proteins changed first. Shape is a downstream effect, and calling it the mutation confuses the result with the cause.
• (d)colour. — Viruses have no colour. They are far smaller than the wavelength of visible light, which is why they cannot be seen in an ordinary light microscope and are imaged with electron microscopes instead.

A virus sits at the boundary of the living and the non-living. Outside a host it is inert, behaving like a large chemical particle; inside a host cell it takes over the machinery and is copied. Its genome may be DNA or RNA, single- or double-stranded, and it is that genome which mutates. Because the copying happens at enormous scale and with limited accuracy, changes accumulate quickly. Most are neutral or harmful to the virus and disappear; a few make it spread more easily or evade immunity, and those spread — which is the ordinary process of natural selection working on a very short generation time.

Three of the four options describe features a virus either does not have or has only as a consequence of its genes, so naming what a mutation actually is settles the question immediately. The subject was unavoidably topical when this paper was set, since the coronavirus pandemic had made variant a household word, and it explains why an influenza vaccine has to be reformulated for each season while a measles vaccine does not — influenza is an RNA virus and drifts continuously, measles far less so. The general lesson outlasts any particular outbreak: for a fast-mutating pathogen, surveillance and sequencing are the tools that tell public-health systems when the target has moved.

• A mutation is a change in the sequence of genetic material; for a virus that material is DNA or RNA.
• A virus consists of genetic material inside a protein coat and is not a cell.
• RNA viruses mutate at about 10⁻³ to 10⁻⁵ per base per generation, against 10⁻⁶ to 10⁻⁸ for DNA viruses.
• Changes in a virus's shape or surface proteins follow from changes in its genes, not the reverse.
• Viruses are too small to be seen in a light microscope and are imaged with electron microscopes.

• Treating a virus as a cell and reasoning about its cytoplasm or organelles.
• Confusing a visible change in the virus with the genetic change that caused it.
• Assuming every mutation is harmful or advantageous, when most are neither and simply disappear.

As a definition question like this one, or as a statements item on what viruses are and are not — the standard trap being whether they can make their own food or reproduce outside a host.

• practice — not a real PYQ

  The genetic material of a virus may be

  • (a)DNA only
  • (b)RNA only
  • (c)either DNA or RNA
  • (d)neither DNA nor RNA

  Answer(c) either DNA or RNA — viruses are unusual in this respect, since cellular organisms all carry DNA as their genetic material.

• practice — not a real PYQ

  The influenza vaccine has to be reformulated for each season, while the measles vaccine does not. The main reason is that

  • (a)influenza vaccine loses potency in storage
  • (b)the influenza virus mutates rapidly and its surface proteins change
  • (c)measles affects only children
  • (d)influenza is caused by a bacterium rather than a virus

  Answer(b) the influenza virus mutates rapidly and its surface proteins change — the immune system's target moves, so the vaccine has to be aimed afresh.