Viruses Questions and Answers

Answer: D

Structural proteins are those found in the finished virion, such as capsid subunits and envelope glycoproteins. Non-structural proteins, including polymerases and proteases, are made in the infected cell to run the cycle but are mostly not packaged.

Answer: A

Enzymes such as the RNA-dependent RNA polymerase of a minus-sense virus do no structural work, yet they must travel in the particle because the cell cannot supply them. That is why the structural and non-structural distinction describes function rather than location.

Answer: B

A defective particle lacks a function it needs, usually because part of the genome is missing, so it can only replicate where another virus supplies what is absent. It is still a virus, unlike a viroid or a prion, and it still has a genome.

Answer: C

Correct answer: helper virus

A defective virus lacks a function it needs to complete its cycle, and the helper virus supplies that function when the two co-infect the same cell - hepatitis B acting as helper to hepatitis D is the classic medical example. The satellite virus is the dependent partner, not the provider, and a provirus is integrated viral DNA, which is a different idea entirely.

Answer: D

Correct answer: persistent

In a persistent infection the cell continues to release virus while remaining alive and dividing, so the culture is chronically infected rather than destroyed. That is what separates it from the lytic infection, which kills the cell to release the progeny, and from latency, in which the genome is retained but little or no virus is made.

Answer: A

During latency the genome is maintained in the cell without new virions being made, so the infection is invisible to methods that look for particles. Reactivation restores production, which is why serology and nucleic acid detection matter for such agents.

Answer: B

Correct answer: the virus enters but the cycle is not completed

In an abortive infection attachment and entry succeed but some later step fails, so no infectious progeny are produced. The cause may lie with the cell - a non-permissive cell lacking a factor the virus needs - or with the virus, if the particle is defective. The cell may nonetheless be damaged or transformed, so an abortive infection is not necessarily a harmless one.

Answer: C

Antibiotics work by attacking structures and processes peculiar to bacterial cells, and a virus has none of them, using the host's machinery instead. That is also why antiviral drugs are harder to design without harming the host.

Answer: D

Most virions are smaller than the resolving limit of visible light, so the electron microscope is the instrument that shows them. Their effects on cells, such as inclusions, can be seen by light microscopy even when the particles cannot.

Answer: A

Correct answer: indicate viral growth without resolving particles

A virion is far below the resolving power of a light microscope, but the aggregates of viral protein and nucleic acid that accumulate at sites of replication are large enough to be seen and stained. The inclusion is therefore evidence of infection rather than a view of the virus, and its position and staining can be characteristic - Negri bodies in rabies being the classic example.