Various hosts are required to grow different viruses. These include whole animals, embryonated chicken eggs, and cells taken from vertebrates. In constrast to normal cells which divide 50-100 times, cells taken from tumors divide indefinitely. The animal viruses that lyse their host cells can be assayed by counting plaques.
■ Which of the methods used in quantitating viruses requires an electron microscope?
■ Why is it necessary to continually make new primary cultures of normal cells?
■ Would you expect the number of virions to be the same if you measured them by the plaque assay or by counting using the electron microscope? Explain your answer.
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