Interferons act principally by
Answer: A
An infected cell secretes interferon, and cells that receive it switch on genes whose products block steps of viral replication, so the tissue around the infection becomes resistant. It is the host's own first-line response and it acts on cells rather than on the virion.
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Neutralising antibody prevents infection mainly by
Answer: B
Antibody binds the surface protein that would otherwise engage the cellular receptor, so the particle never gains entry. It therefore works on virus outside cells, which is why humoral immunity limits spread while cell-mediated immunity is needed to clear cells already infected.
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A live attenuated viral vaccine consists of
Answer: C
Attenuation leaves the virus able to multiply enough to provoke a full immune response while no longer producing the illness. Because it replicates, the response is usually broader and longer lasting than an inactivated preparation gives, at the cost of being unsuitable for the severely immunocompromised.
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Antiviral drugs are harder to design than antibacterial drugs mainly because
Answer: D
Much of what a virus does is done by the cell's own polymerases, ribosomes and membranes, so a drug that stops replication often harms the cell as well. Useful agents therefore aim at the few virus-specific enzymes, such as a viral polymerase, protease or neuraminidase.
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Amantadine interferes with influenza virus at the stage of
Answer: A
The drug blocks the viral ion channel needed to acidify the interior of the endosome and release the genome, so the particle is taken up but never uncoats. It is a good illustration of a step that looks minor being an effective point of attack.
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Antigenic shift in influenza virus arises from
Answer: B
Because the genome is segmented, two strains infecting one cell can exchange whole segments and produce a virus carrying a surface protein the population has never met. That abrupt change is what produces pandemics, in contrast to the slow drift of point mutations.
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Antigenic drift in influenza virus is caused by
Answer: C
Drift is the steady accumulation of point mutations in the haemagglutinin and neuraminidase genes, which slowly erodes the fit of existing antibody. It is why the seasonal vaccine is reformulated each year, whereas shift is the sudden segment-swapping event.
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Haemagglutinin of influenza virus is responsible for
Answer: D
Haemagglutinin is the attachment protein, and its name comes from its ability to clump red cells in the laboratory by binding their surface sialic acid. Neuraminidase performs the opposite office, cleaving sialic acid so that new particles do not stick to the cell they came from.
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The great majority of plant viruses have genomes made of
Answer: A
Almost all plant viruses carry RNA, most of it positive-sense single-stranded, tobacco mosaic virus being the type example. The DNA plant viruses, such as the geminiviruses and caulimoviruses, are a small minority by comparison.
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Plant viruses commonly spread from plant to plant by
Answer: B
A plant cell is protected by a wall and a cuticle that a virion cannot breach unaided, so most plant viruses rely on a vector, typically a sap-feeding insect such as an aphid or a leafhopper, to deliver them past that barrier. Mechanical wounding serves the same purpose experimentally.
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