Semiconservative DNA replication was proved by Messelson and Stahl, in which DNA was made

Options

A. Radioactive using `N^(15)`
B. Heavy using `N^(14)`
C. Heavy using `""^(15)NH_(4)Cl`
D. Radioactive using `""^(14)NH_(4)Cl`

Answer

### Step-by-Step Solution:

1. **Understanding Semiconservative DNA Replication**:
- Semiconservative DNA replication means that when DNA replicates, each new DNA molecule consists of one original (parental) strand and one newly synthesized strand. This was a hypothesis proposed to explain how DNA is copied.

2. **The Experiment by Meselson and Stahl**:
- Meselson and Stahl conducted an experiment to prove the semiconservative model of DNA replication. They used isotopes of nitrogen to label the DNA.

3. **Use of Nitrogen Isotopes**:
- They cultured bacteria in a medium containing N15, which is a heavy isotope of nitrogen. This allowed the DNA synthesized in the bacteria to incorporate the heavy nitrogen, resulting in DNA of heavy density.

4. **Centrifugation Observations**:
- After allowing the bacteria to replicate in the N15 medium, they centrifuged the DNA. The DNA settled at the bottom of the centrifuge tube, indicating that it was heavy due to the incorporation of N15.

5. **Transfer to N14 Medium**:
- They then transferred some of the bacteria from the N15 medium to a medium containing N14 (the normal nitrogen isotope). After allowing replication in the N14 medium, they centrifuged the DNA again.

6. **Results After Second Centrifugation**:
- The results showed two distinct bands after the second centrifugation: one band at an intermediate density (indicating the presence of both N15 and N14) and one band at a lighter density (indicating DNA that had only incorporated N14). This confirmed that DNA replication was semiconservative.

7. **Evaluating the Options**:
- Now, let's evaluate the options given in the question:
- **Option 1**: Radioactive using N15 - Incorrect, as N15 is not radioactive.
- **Option 2**: Heavy using N14 - Incorrect, as N14 is not heavy.
- **Option 3**: Heavy using N15 NH4Cl - Correct, as N15 is the heavy isotope used in the experiment.
- **Option 4**: Radioactive using N14 NH4Cl - Incorrect, as N14 is not radioactive.

8. **Conclusion**:
- The correct answer is **Option 3: Heavy using N15 NH4Cl**.