Correct option is A
Experiment Summary:
- The experiment involved growing E. coli cells in 14N (normal nitrogen) and then transferring them to a medium containing 15N (heavy nitrogen) at time 0.
- The proportion of hybrid DNA (composed of both 14N-15N strands) was measured at different time points: 0, 10, 20, 30, and 40 minutes.
Observations:
- At time 0 minutes: No hybrid DNA was observed, indicating that no DNA replication had occurred.
- At time 10 minutes: 40% hybrid DNA was observed, suggesting that DNA replication started and incorporated the newly available 15N isotope.
- At 20, 30, and 40 minutes: The percentage of hybrid DNA remained at 40%, indicating that no further DNA replication was occurring beyond the first 10 minutes.
Analysis of the Data:
- The key finding is that 40% hybrid DNA was formed after 10 minutes and then remained constant from 20 minutes onward. This pattern implies that the chemical C stopped further DNA replication after the initial synthesis.
Understanding the Potential Actions of Chemical C:
Option 1: Inhibits the initiation of replication: If chemical C blocks the initiation of DNA replication, it would stop the formation of new replication forks. The initial hybrid DNA seen at 10 minutes would come from the DNA that was replicated before the chemical could take effect. After this point, no additional DNA replication could occur, and the hybrid DNA level would plateau at 40%, which is consistent with the data observed.
Option 2: Inhibits elongation phase of replication: If chemical C halted the elongation of DNA, we would expect stalled replication forks, leading to incomplete replication and varying levels of hybrid DNA. However, the data shows a consistent 40% hybrid DNA across all time points after the first 10 minutes, which suggests that elongation is not the issue.
Option 3: Inhibits termination of replication: Termination inhibitors typically result in incomplete replication products, which would lead to variable hybrid DNA levels. Since the hybrid DNA remains constant, this option is unlikely.
Option 4: Competes with dNTPs for incorporation: If chemical C were competing with dNTPs for incorporation, we would expect to see irregular hybrid DNA levels, and possibly a decrease in the hybrid DNA percentage over time. The observed stable hybrid DNA at 40% suggests that this is not the cause.
Conclusion:
The most plausible explanation is that chemical C inhibits the initiation of DNA replication, preventing any new rounds of DNA replication after the initial synthesis at 10 minutes.
