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    Density Distribution of Viral DNA

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    A. Explain how the three density peaks in Figure 9-20 are related to replication of the injected DNA. Why is no 32P radioactivity associated with the light peak and why is no 3H radioactivity associated with the heavy peak?
    B. Does the injected DNA mimic the behaviour you would expect for the chromosomal DNA?
    C. Why do you think that cycloheximide prevents the appearance of the most dense peak of DNA?

    *(Please see attachment for further information and diagrams)

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    The lightest density peak is the original viral DNA labelled exclusively with 3H. After one round of DNA replication, (according to the semi-conservative mechanism of replication that we all know about), each new DNA duplex will be made up of one "old" 3H-labelled strand and one "new" 32P-labelled strand. Since all new DNA strands will be BrdU-containing strands, this "new" strand will have a heavier density. Therefore, in Panel A, the "light" peak are the original duplexes and the other peak represents duplexes that have undergone one replication event. As ...