Viruses in Eukaryotes Questions and Answers

71.

A defective virus is one that

A.carries two complete genomes
B.replicates faster than the wild type
C.cannot complete replication without a helper virus
D.has lost the ability to attach to any cell

Answer: C

Correct answer: cannot complete replication without a helper virus

A defective virus lacks a function it needs - commonly through a deletion in its genome - and can replicate only in a cell co-infected by a helper that supplies it. Hepatitis D is the clinical example, being unable to make its own envelope and dependent on hepatitis B for it, which is why hepatitis D infection occurs only alongside hepatitis B.

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72.

An abortive infection is one in which

A.the cell lyses before the virus attaches
B.the genome integrates permanently
C.progeny are produced but cannot leave the cell
D.the virus enters but no progeny are produced

Answer: D

The virus gets in but the cell cannot support some necessary step, so the cycle stops and nothing infectious is made. Such a cell may still be damaged or altered, which is one route by which a non-permissive infection contributes to disease.

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73.

The number of different proteins in a small virus is limited chiefly by

A.the small size of its genome
B.the thickness of the envelope
C.the temperature of the host
D.the shape of the capsid

Answer: A

A genome only a few thousand bases long can encode very few proteins, which is why small viruses reuse one coat protein many times over and often cut a single polyprotein into several products. Economy of coding is the constraint that shapes their whole strategy.

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74.

Virions are described as metabolically inert because they

A.are unable to bind to any cell
B.carry out no chemical reactions outside a host cell
C.cannot be inactivated by heat, drying or disinfectant
D.contain no protein at all

Answer: B

Outside a cell a virion neither respires nor synthesises anything; it is a stable package awaiting delivery. That inertness is why viruses were long argued not to be alive, and why they can persist on surfaces without any nutrient supply.

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75.

The nucleic acid of a virus determines

A.the shape of the host cell nucleus
B.the temperature of the host
C.which proteins the virus can specify
D.the lipid composition of the envelope

Answer: C

The genome encodes the capsid proteins, the glycoproteins and any enzymes the virus needs. The envelope's lipid, by contrast, comes from the host membrane the particle budded through, so it is not specified by the virus at all.

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76.

Viral capsids are built from many copies of a few proteins because this

A.makes the particle heavier
B.allows the capsid to metabolise
C.prevents the genome from being copied
D.needs only a small amount of genetic information

Answer: D

Correct answer: needs only a small amount of genetic information

A virus has very little genome to spare, and building a shell from many copies of one or a few proteins means encoding only those few. The subunits then assemble by self-assembly into a symmetrical structure - helical or icosahedral - without any instructions beyond their own shape. It is genetic economy, and it is why viral capsids show such regular geometry.

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77.

Attachment of a virion to its receptor is described as specific because

A.a particular virion protein binds a particular cell constituent
B.only one virion is able to bind to each susceptible cell
C.binding occurs only at a particular temperature
D.the bond formed is covalent

Answer: A

The anti-receptor on the virion recognises one constituent of the cell surface, and the fit between them is what decides whether a cell can be infected at all. It is that molecular specificity, not any limit on numbers, that the word describes.

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78.

Viral genomes may be

A.always branched
B.linear or circular
C.linear only
D.circular only

Answer: B

Both arrangements occur, and the distinction is one of the criteria used to define families: papillomavirus DNA is circular while adenovirus DNA is linear. Genomes may equally be monopartite or divided into several segments.

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79.

A virus with a monopartite genome is one whose genome is

A.made of both DNA and RNA
B.present in two identical copies
C.contained in a single molecule of nucleic acid
D.divided among several separate virus particles

Answer: C

Correct answer: contained in a single molecule of nucleic acid

A monopartite genome is one molecule; a segmented or multipartite one is divided among several. The distinction matters because segmented viruses can reassort - two strains infecting the same cell exchange whole segments - which is how influenza produces the abrupt antigenic shifts that cause pandemics, and which a monopartite virus cannot do.

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80.

Assembly of progeny virions takes place

A.outside the host cell, in the surrounding fluid
B.always in the mitochondria
C.only after the cell has died
D.at a site characteristic of the virus family

Answer: D

Where the capsid is put together, in the nucleus, in the cytoplasm or at a membrane, is constant for a given family and is one of the features used to classify viruses. Inclusion bodies often mark those assembly sites under the microscope.

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