A plus-sense single-stranded RNA genome is distinguished by the fact that it
Answer: C
Plus-sense RNA reads in the same direction as mRNA, so a ribosome can translate the genome as soon as it reaches the cytoplasm and the naked nucleic acid alone is infectious. Minus-sense RNA reads the opposite way and must first be copied, which is why such viruses carry their own polymerase.
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A minus-sense single-stranded RNA virus must carry into the cell its own
Answer: D
The host has no enzyme able to copy RNA into RNA, and a minus-sense genome cannot be translated as it stands, so the polymerase must arrive packaged in the virion. Reverse transcriptase is what retroviruses carry instead, and no virus carries a ribosome.
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Retroviruses are distinguished among plus-sense RNA viruses by carrying
Answer: A
A retrovirus has a plus-sense RNA genome but does not translate it directly; reverse transcriptase copies it into DNA, which then integrates into the host chromosome. That enzyme is the defining feature, and it travels inside the virion because no cell provides one.
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Which of these is NOT one of the recognised classes of viral genome?
Answer: B
A genome is always nucleic acid, so there is no protein class; the six classes are double- and single-stranded DNA, double-stranded RNA, plus- and minus-sense single-stranded RNA, and the retroviruses. Prions are proteinaceous but are not viruses and have no genome.
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The first stage of the viral multiplication cycle is
Answer: C
Attachment is the specific binding of a virion protein, the anti-receptor, to a constituent of the cell surface, and it decides which cells a virus can enter at all. Penetration, uncoating, synthesis, maturation and egress follow in that order.
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Specific binding of a virion protein to a molecule on the cell surface is
Answer: D
Attachment is a specific recognition event between a viral anti-receptor and a host cell constituent, which is why a virus infects only cells carrying the right molecule. Penetration is the entry that follows, and uncoating the release of the genome after that.
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Penetration of a virus into a host cell is described as
Answer: A
Penetration requires energy and proceeds by translocation across the membrane, by endocytosis, or by fusion of a viral envelope with a cell membrane. Attachment by contrast can occur on a cell that is metabolically inactive, because it is only binding.
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During uncoating,
Answer: B
Uncoating is the disaggregation of the particle, alone or with help from cellular enzymes, which sets the stage for the genome to be read. Assembly and envelopment happen much later, and egress is the final step of the cycle.
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The interval shortly after infection when only small amounts of parental infectious virus can be detected is the
Answer: C
During the eclipse the genome has been uncoated and exposed to the machinery that will express it, so no intact infectious particle exists to be recovered. Detectable virus reappears once maturation begins and progeny accumulate.
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The phase in which progeny virions accumulate at exponential rates is
Answer: D
Maturation is the interval in which finished particles build up inside the cell or outside it, following the eclipse during which none could be detected. The rise is exponential because many genomes and coat proteins are being assembled at once.
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