Viruses Questions and Answers

Q: Compared with a bacterial cell, a typical virion is

Correct answer: very much smallerVirions run from about twenty to a few hundred nanometres, against one to several micrometres for a bacterium - roughly one to two orders of magnitude smaller in each dimension. That is why viruses pass filters that retain bacteria and why they cannot be seen in the light microscope, both facts being how they were first recognised.

Q: Which event marks the end of the multiplication cycle?

Correct answer: EgressEgress is the release of the progeny from the cell, by lysis for a naked virus or by budding for an enveloped one, and it completes the cycle by making the particles available to infect further cells. Attachment, penetration and uncoating are the opening steps, so the cycle runs from one attachment to the next.

Q: A virus that infects only one species is described as having

Correct answer: a narrow host rangeHost range is set chiefly by whether a cell carries a receptor the virus can bind, and secondarily by whether it can support the later stages of the cycle. A virus restricted to one species has a narrow range - and the narrowness matters for eradication, since a pathogen with no animal reservoir, as smallpox had none, can in principle be eliminated entirely.

81.

Compared with a bacterial cell, a typical virion is

A.very much smaller
B.very much larger
C.the same size
D.larger only when enveloped

Answer: A

Correct answer: very much smaller

Virions run from about twenty to a few hundred nanometres, against one to several micrometres for a bacterium - roughly one to two orders of magnitude smaller in each dimension. That is why viruses pass filters that retain bacteria and why they cannot be seen in the light microscope, both facts being how they were first recognised.

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82.

Historically, viruses were first distinguished from bacteria by being

A.sensitive to penicillin
B.able to pass through filters that retain bacteria
C.visible under a light microscope
D.culturable on solid media

Answer: B

Correct answer: able to pass through filters that retain bacteria

The filterable agents were defined by exclusion: the sap of a diseased tobacco plant remained infectious after passing a filter fine enough to hold back every known bacterium. That single observation, made in the 1890s, established that something smaller than a bacterium could cause disease, and the word virus came to mean it. It also explains the older name filterable agent.

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83.

A virus genome may encode all of the following EXCEPT

A.envelope glycoproteins
B.a polymerase
C.ribosomal RNA for its own ribosomes
D.capsid proteins

Answer: C

No virus encodes a ribosome; protein synthesis is always borrowed from the host, and that dependence is part of what makes a virus an obligate intracellular parasite. Capsid proteins, glycoproteins and polymerases are all virus-coded.

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84.

Which is true of the eclipse phase?

A.Progeny virus is at its peak
B.Attachment has not yet occurred
C.The cell has already lysed
D.Infectious parental virus cannot be recovered

Answer: D

The genome has been uncoated and exposed to the machinery that will express it, so the intact parental particle no longer exists and progeny have not yet been assembled. Recoverable virus returns once maturation begins.

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85.

Which event marks the end of the multiplication cycle?

A.Egress
B.Uncoating
C.Attachment
D.Penetration

Answer: A

Correct answer: Egress

Egress is the release of the progeny from the cell, by lysis for a naked virus or by budding for an enveloped one, and it completes the cycle by making the particles available to infect further cells. Attachment, penetration and uncoating are the opening steps, so the cycle runs from one attachment to the next.

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86.

The receptor a virus binds is located on the

A.host cell nucleus
B.host cell surface
C.viral envelope
D.viral capsid

Answer: B

The receptor belongs to the cell and the anti-receptor to the virus, so binding brings the two surfaces together. Because the receptor is a normal cell constituent, its distribution across tissues shapes which organs a virus can affect.

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87.

Which of these is NOT a route by which a virus penetrates a cell?

A.Membrane fusion
B.Translocation
C.Binary fission
D.Endocytosis

Answer: C

Binary fission is how a bacterium divides, not how anything enters a cell. Penetration proceeds by endocytosis, by fusion of a viral envelope with a cell membrane, or by direct translocation, and all three require energy.

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88.

A virus that infects only one species is described as having

A.a broad host range
B.no host range
C.an unlimited host range
D.a narrow host range

Answer: D

Correct answer: a narrow host range

Host range is set chiefly by whether a cell carries a receptor the virus can bind, and secondarily by whether it can support the later stages of the cycle. A virus restricted to one species has a narrow range - and the narrowness matters for eradication, since a pathogen with no animal reservoir, as smallpox had none, can in principle be eliminated entirely.

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89.

Which best describes viral tropism?

A.The tissues a virus can infect
B.The speed at which a virus replicates
C.The size of the viral genome
D.The stability of the virion

Answer: A

Tropism is the preference of a virus for particular cells or tissues, and it usually traces back to receptor distribution together with whether the cell supports the rest of the cycle. It explains why one virus causes hepatitis and another a respiratory illness.

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90.

Which is a valid reason a cell may resist infection despite carrying the receptor?

A.It has no nucleic acid of its own
B.It may not support a later stage of the cycle
C.It cannot be attached to
D.It has no cytoplasm

Answer: B

Correct answer: It may not support a later stage of the cycle

Carrying the receptor makes a cell susceptible, but permissiveness is a separate matter: the cell must also supply whatever polymerases, transcription factors and processing enzymes the virus needs. A susceptible but non-permissive cell can be entered and yet yield no progeny, which is precisely the abortive infection described elsewhere in this topic.

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