Compared with large enveloped viruses, small naked viruses such as poliovirus are generally
Answer: C
A naked virion is a protein shell around nucleic acid with no fragile lipid membrane, so it tolerates drying, detergents and solvents orders of magnitude better than an enveloped virus does. That is why lipid solvents inactivate herpesviruses readily but not polioviruses.
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Which one of the following virus families has a DNA genome?
Answer: D
Herpesviruses carry double-stranded DNA, as do the adenoviruses, poxviruses, papillomaviruses, parvoviruses and hepadnaviruses. The other three families listed are all RNA viruses, which is the larger group among human pathogens.
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The genome of the Coronaviridae consists of
Answer: A
Coronaviruses carry positive-sense single-stranded RNA, one of the largest genomes among RNA viruses, and being plus-strand it can be translated directly on arrival. Adenoviruses and poxviruses are the double-stranded DNA families, and double-stranded RNA belongs to the reoviruses.
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A positive-sense RNA genome is one that
Answer: B
Plus-strand RNA reads as messenger RNA, so the naked genome introduced into a cell is infectious on its own: ribosomes translate it and the viral proteins follow. Poliovirus is the classic example, and its purified RNA can start an infection without any protein.
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A negative-sense RNA virus must carry in its virion
Answer: C
Antisense RNA has no translational function and cannot produce viral components by itself, and the cell has no enzyme able to copy RNA into RNA. The virus therefore packages its own transcriptase so that mRNA can be made as soon as the genome is uncoated.
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Reverse transcriptase catalyses the synthesis of
Answer: D
The enzyme copies the retroviral RNA genome into a complementary DNA strand and then into double-stranded DNA, reversing the usual direction of information flow. That DNA is carried into the nucleus and integrated into the host genome by viral proteins.
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Retroviral DNA made by reverse transcriptase is
Answer: A
The double-stranded DNA copy is translocated into the nucleus and inserted into host chromosomal DNA, where it is called a provirus. Because it becomes part of the cell's own genome it is copied at every division, which is why retroviral infection is lifelong.
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Which one of the following viruses has a double-stranded RNA genome?
Answer: B
Rotavirus belongs to the Reoviridae, the family defined by segmented double-stranded RNA. Poliovirus is positive-sense single-stranded RNA, rabies virus is negative-sense single-stranded RNA, and hepatitis B virus is a DNA virus that uses reverse transcription.
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A segmented genome is characteristic of
Answer: C
The orthomyxovirus genome is divided into separate RNA segments, and so are those of the bunyaviruses, arenaviruses and reoviruses. Segmentation allows reassortment when two strains infect one cell, which is the mechanism behind the abrupt antigenic shifts of influenza.
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Which one of the following DNA viruses replicates in the cytoplasm rather than the nucleus?
Answer: D
Poxviruses are the exception among DNA viruses: they replicate entirely in the cytoplasm and therefore carry the enzymes they need, including their own transcriptase, rather than borrowing nuclear machinery. All the other families listed replicate in the nucleus.
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