Host cell RNA polymerase can be used to make viral mRNA only when
Answer: A
Cellular transcriptases sit in the nucleus and act on DNA, so only viruses whose DNA genome arrives there can use them. Every RNA virus, and every cytoplasmic DNA virus such as a poxvirus, has had to evolve or carry a transcriptase of its own.
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Parvoviruses are unusual among animal viruses in having a genome that is
Answer: B
Most DNA viruses of animals carry double-stranded genomes; the parvoviruses and circoviruses are the single-stranded exceptions. Their very small genome is also why parvoviruses depend heavily on host functions and replicate only in dividing cells.
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The classification of viruses rests principally on
Answer: C
Families are defined by whether the genome is DNA or RNA, single or double stranded, linear or circular, monopartite or segmented, together with how it is replicated and where the capsid is assembled. Clinical syndrome is a poor guide, since unrelated viruses cause similar illnesses.
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The first step in the multiplication of an animal virus is
Answer: D
Attachment is the specific binding of a virion protein, the anti-receptor, to a constituent of the cell surface that serves as receptor. Nothing else can follow until that contact is made, which is also why the step is such an attractive target for neutralising antibody.
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The range of cells a virus can infect is limited chiefly by
Answer: A
Susceptibility depends on the availability of an appropriate receptor, and not all cells in an otherwise susceptible organism carry one. That single requirement explains both host range and tissue tropism, and it is why a virus that binds a lymphocyte marker produces disease in the immune system.
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Penetration of a virus into a host cell
Answer: B
Penetration is an energy-dependent process that begins almost instantaneously after attachment, by direct translocation, by endocytosis, or by fusion of the envelope with a cell membrane. Because it needs cellular energy it does not occur on a dead cell.
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Uncoating refers to
Answer: C
After penetration the virion comes apart, on its own or with the help of cellular enzymes, so that the nucleic acid becomes available for transcription and replication. Until uncoating happens the genome is inert, which is the step amantadine blocks in influenza.
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The eclipse period of a viral growth cycle is the interval during which
Answer: D
Shortly after infection the parental virion has been taken apart and progeny particles have not yet been assembled, so assays find almost no infectious virus even though the genome is being expressed. The count rises again only once assembly begins.
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Naked viruses such as the picornaviruses are released mainly by
Answer: A
Having no envelope to acquire, a naked virion has no budding step, so egress depends largely on the infected cell breaking down. That coupling of release to cell death is one reason such viruses are typically strongly cytolytic.
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Release by budding is significant because it
Answer: B
Budding combines the last step of assembly with departure from the cell and does not depend on the cell disintegrating, so enveloped viruses released this way are frequently non-cytolytic. A cell can therefore shed virus for a long period while still alive.
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