E. coli cells grown on 15N medium are transferred to 14N medium and allowed to grow for two more generations (two rounds of DNA replication). DNA extracted from these cells is centrifuged. What density distribution of DNA would you expect in this experiment? a. one high-density and one low-density band b. one intermediate-density band c. one high-density and one intermediate-density band d. one low-density and one intermediate-density band

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1. Understand the experiment setup: E. coli cells initially grown in a medium containing the heavy isotope of nitrogen (15N) are transferred to a medium containing the lighter isotope (14N). This change in nitrogen source will affect the density of newly synthesized DNA.
2. Recall the semi-conservative model of DNA replication: Each new DNA molecule consists of one old (template) strand and one newly synthesized strand. After the first round of replication in 14N medium, each DNA molecule will have one strand with 15N (heavier) and one strand with 14N (lighter).
3. Predict the outcome after the first round of replication: The DNA molecules will consist of hybrid DNA (one old 15N strand and one new 14N strand), resulting in an intermediate density compared to the original high-density 15N DNA and the potential low-density 14N DNA.
4. Consider the second round of replication: Each of the hybrid DNA molecules will undergo replication. This will produce some molecules with two 14N strands (low-density) and more hybrid molecules (one 15N strand and one 14N strand, intermediate-density).
5. Conclude the expected result of the centrifugation: After two rounds of replication in 14N medium, the DNA extracted and centrifuged will show two bands - one of intermediate-density (hybrid DNA) and one of low-density (fully 14N DNA).