Name the heavy isotope used by Meselson and Stahl for proving the semiconservative mode of DNA.

Options

A. `""^(15)NH_(4)CI`
B. `""^(14)NH_(3)CI_(2)`
C. `""^(13)NH_(2)CI_(2)`
D. All of these

Answer

To answer the question regarding the heavy isotope used by Meselson and Stahl for proving the semiconservative mode of DNA replication, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding the Concept of Semiconservative Replication**:
- Semiconservative replication means that during DNA replication, each new DNA molecule consists of one old (template) strand and one newly synthesized strand. This was a key concept that Meselson and Stahl aimed to prove through their experiments.

2. **Identifying the Isotope Used**:
- Meselson and Stahl used a heavy isotope of nitrogen to trace the DNA strands during replication. The specific isotope they used is Nitrogen-15 (N-15).

3. **Experimental Setup**:
- They grew bacteria in a medium containing ammonium chloride (NH4Cl) that was labeled with N-15. This allowed the bacteria to incorporate the heavy nitrogen into their DNA.

4. **Transition to Normal Isotope**:
- After several generations, the bacteria were transferred to a medium containing the normal isotope of nitrogen, Nitrogen-14 (N-14). This transition was crucial to observe the changes in the DNA.

5. **Results of the Experiment**:
- After the transfer, the DNA was analyzed, and it was found that after one round of replication, the DNA contained one strand with N-15 and one strand with N-14. This confirmed the semiconservative model of replication, as each new DNA molecule had one old strand and one new strand.

6. **Conclusion**:
- The heavy isotope used by Meselson and Stahl to prove the semiconservative mode of DNA replication is Nitrogen-15 (N-15), which was present in the ammonium chloride (NH4Cl) medium.

### Final Answer:
The heavy isotope used by Meselson and Stahl for proving the semiconservative mode of DNA replication is Nitrogen-15 (N-15).

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