Bidirectional synthesis was demonstrated by which of the following methods?
A. Bacteria were grown on 15NH4 until the entire DNA contained heavy nitrogen and then the cells were transferred to normal, light media for one round of replication. The resulting DNA molecules were separated in density gradients and compared to the expected results for each proposed model.
B. Cells were labeled with low specific activity [3H] thymidine followed by a pulse of high specific activity [3H] thymidine. This experiment revealed that a new strand was growing away from the midpoint toward one fork.
C. Cells infected with T4 bacteriophage were grown in the presence of radioactive nucleotides. When DNA was analyzed in alkaline CsCl gradients, the presence of short DNA fragments indicated that bidirectional synthesis had occurred.
D. Cells were labeled with low specific activity [3H] thymidine followed by a pulse of high specific activity [3H] thymidine. This experiment revealed a gradient of radioactive density that is heaviest at both forked junctions.
E. None of these answers is correct.
D
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