Movement of a plant virus from cell to cell within a host occurs mainly through
Answer: C
Plant cells are connected by plasmodesmata, and viruses encode movement proteins that widen these channels enough for the genome to pass. Long-distance spread then uses the phloem, which is why systemic symptoms often follow the vascular pattern of the leaf.
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Tobacco mosaic virus is a virus of
Answer: D
It is the classic rigid helical rod, built of identical subunits spiralling around a single positive-sense RNA strand. Its stability and simple construction are why it was the first virus to be crystallised and a mainstay of early structural virology.
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Viruses that infect fungi are known as
Answer: A
Mycoviruses infect fungi, frequently carry double-stranded RNA, and are often transmitted only by hyphal fusion or through spores rather than by an extracellular route. Some reduce the virulence of plant-pathogenic fungi, which has been exploited for biological control.
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Viroids differ from viruses in that they
Answer: B
A viroid is a small circular RNA with no capsid and no protein-coding capacity of its own, relying entirely on host enzymes. They are agents of plant disease, so they belong to the eukaryotic infection story even though they are not viruses.
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Prions differ fundamentally from viruses because they
Answer: C
A prion is a misfolded protein that propagates its abnormal conformation to the normal form of the same protein, with no genome of any kind. Because there is no nucleic acid, treatments and sterilisation methods aimed at genomes have no effect.
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A plaque in a virus-infected cell monolayer represents
Answer: D
One infectious particle infects a cell, its progeny infect neighbours, and the resulting cleared zone becomes visible against the intact monolayer. Because each plaque traces back to a single particle, counting them gives the titre in plaque-forming units.
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Animal viruses are most commonly propagated in the laboratory using
Answer: A
A virus can only multiply inside a living cell, so a monolayer or suspension of cultured cells is the standard host; embryonated eggs and laboratory animals are the older alternatives. No purely chemical medium will support viral replication, however rich.
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Filtration was historically used to distinguish viruses from bacteria because viruses
Answer: B
Viruses are small enough to cross filters with pores that hold back bacteria, so an infectious agent in the filtrate was originally called a filterable agent. That property, rather than any direct observation, was the first evidence that such agents existed.
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The electron microscope was necessary to visualise viruses because they are
Answer: C
Even the largest virion is around a thousand nanometres and most are far smaller, well below what visible light can resolve. Electrons have a much shorter wavelength, so the electron microscope revealed both the size and the symmetry of virions for the first time.
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Viruses are described as obligate intracellular parasites because they
Answer: D
A virion carries no ribosomes, no metabolic pathways and no means of making adenosine triphosphate, so every step of multiplication borrows the cell's machinery. That dependence, not the killing of cells, is what the phrase describes.
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