A prion is best described as
Answer: B
The term means a proteinaceous infectious agent, and the defining claim is that no nucleic acid can be demonstrated in it. That sets it apart from viroids, which are RNA without protein, and from viruses, which always carry a genome.
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Which pair correctly contrasts a viroid with a prion?
Answer: C
The two are near mirror images. A viroid is a naked circular RNA with no coat, while a prion is proteinaceous with no demonstrable nucleic acid, so they represent fundamentally different infectious mechanisms and neither is a virus.
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The range of cells a particular virus can infect is determined largely by
Answer: D
Attachment is a specific binding event, so a cell without the matching surface constituent cannot be entered however permissive its interior might be. This is why tropism is often described first in terms of receptors, and why receptor distribution predicts which tissues a virus damages.
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The viral surface protein that binds the host cell receptor is called the
Answer: A
The virion protein that recognises the cell is the anti-receptor, and the cell surface molecule it binds is the receptor. The matrix protein lies beneath an envelope and the core protein packages the genome, so neither is exposed for binding.
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Which feature is present in bacteria but absent from viruses?
Answer: B
Bacteria are cells and carry ribosomes, so they synthesise their own proteins and generate their own energy. A virus has nucleic acid and protein but no ribosomes, which is precisely why it must borrow a cell's machinery to replicate.
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Unlike a bacterium, a virus cannot
Answer: C
The inability to generate energy is one of the two constraints that make a virus an obligate intracellular parasite, alongside having no protein-synthesising machinery. Viruses do contain both protein and nucleic acid and are readily inactivated by heat.
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Binary fission is a means of reproduction used by
Answer: D
Bacteria divide in two, growing steadily until they split. A virus does not divide at all: its parts are made separately inside the host cell and then assembled, so the number of particles rises in a burst rather than by doubling.
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The nucleocapsid of a virus is
Answer: A
Nucleocapsid names the genome and its protein coat as one structure. In a naked virus that is the whole particle; in an enveloped virus it is what lies inside the lipid layer, and it may be helical even when the particle looks roughly spherical.
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A Sendai virus nucleocapsid is described as about
Answer: B
The nucleocapsid is long and very thin, which is characteristic of helical symmetry: the length follows the length of the genome while the diameter is fixed by the coat protein. It is coiled inside the envelope, so the particle appears far more compact than the figure suggests.
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A circovirus isometric capsid measures about
Answer: C
Circoviruses are among the smallest animal viruses, with a capsid of roughly 17 nanometres. Isometric means the particle is equal in its dimensions, the appearance an icosahedral shell presents, in contrast with the elongated form of a helical nucleocapsid.
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