Which one of the following is not a stage of the viral replication cycle?
Answer: A
Binary fission is how a bacterium divides; a virus does not divide at all. Its progeny are assembled from separately made parts, which is one of the deepest differences between viral multiplication and cellular reproduction.
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The number of infectious particles in a preparation is its
Answer: B
Titre is the concentration of infectious virus, commonly expressed in plaque-forming units per millilitre. It counts only particles able to start an infection, which is usually well below the total number of particles present.
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Tissue tropism of a virus refers to
Answer: C
Correct answer: the range of tissues it can productively infect
Tropism is determined first by which cells carry the receptor the virus binds and then by whether those cells can support the rest of the replication cycle. It is what makes rabies neurotropic, hepatitis viruses hepatotropic and rhinoviruses confined to the upper airway, and it explains the pattern of disease each produces far better than any property of the particle itself.
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Persistent infection differs from latent infection in that virus is
Answer: D
In a persistent infection the cell survives and sheds virus steadily, which non-cytolytic budding makes possible. Latency is the other pattern: the genome is present and quiet, with production resuming only on reactivation.
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Which one of the following is the smallest?
Answer: A
Correct answer: A virion
The order of size is virion, bacterium, yeast, protozoan, spanning about four orders of magnitude - tens to hundreds of nanometres for a virus, one to a few micrometres for a bacterium, five to ten for a yeast, and tens for a protozoan. It is why viruses pass filters that retain bacteria and why they cannot be seen in a light microscope.
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Host range of a virus means the
Answer: B
Host range is the set of hosts and cells a virus can productively enter, and it follows from receptor distribution together with the intracellular factors replication requires. A narrow range is why many animal viruses pose no threat to people.
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