Bacteriophages are often studied by the following approach. First, cells are mixed with the…

Bacteriophages are often studied by
the following approach. First, cells are mixed with the bacteriophages; after a
period of time to allow the virus to bind the cells, the cells are mixed with a
small amount of liquid agar, and the mixture is poured onto the surface of a
plate containing solid agar. The top agar quickly hardens. Eventually, the
cells grow and form a dense layer of cells in the top agar layer. Because the
viruses multiply much more rapidly than the cells, however, they eat a hole, or
“plaque,” in the bacterial layer. Each plaque develops from a single infected
cell, so this
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Bacteriophages are often studied by
the following approach. First, cells are mixed with the bacteriophages; after a
period of time to allow the virus to bind the cells, the cells are mixed with a
small amount of liquid agar, and the mixture is poured onto the surface of a
plate containing solid agar. The top agar quickly hardens. Eventually, the
cells grow and form a dense layer of cells in the top agar layer. Because the
viruses multiply much more rapidly than the cells, however, they eat a hole, or
“plaque,” in the bacterial layer. Each plaque develops from a single infected
cell, so this is a way to count the number of viruses. It is also a way to do
genetics, as was done in the early days of studying bacteriophage _.
Plaques of bacteriophage _ differ from those of most viruses. Most viruses
kill all the cells in the plaque, so there are no surviving cells and the
plaques are “clear”; you can see through them. In contrast, plaques of
bacteriophage _ are cloudy or “turbid” because lysogens (bacterial cells
in which the virus is in the lysogenic stage) arise in the plaque and can grow.

a. In the plaque, lysogens are
often infected by other _ viruses present in the plaque. Why don’t
these newly infecting viruses grow lytically and kill the cells?

b. Mutants of _ can arise that
form clear plaques. Which virus genes are likely to be mutated?

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