During performing the Meselson and stahl's experiments, `.^(15)N` was distinguished by `.^(14)N` by

Options

A. Centrifugation in CsCl density gradient
B. Paper chromatography
C. Radioactivity counters
D. ELISA.

Answer

**Step-by-Step Solution:**

1. **Understanding the Isotopes**:
- Nitrogen has two isotopes relevant to the experiment: Nitrogen-14 (N14) and Nitrogen-15 (N15).
- N14 has 7 protons and 7 neutrons, while N15 has 7 protons and 8 neutrons, making N15 heavier.

2. **Incorporation into DNA**:
- In the Meselson and Stahl experiment, bacteria were grown in a medium containing N15.
- The N15 is incorporated into the DNA of the bacteria, making the DNA denser compared to DNA grown in a medium with N14.

3. **Density Difference**:
- The DNA synthesized in the presence of N15 is heavier (more dense) than the DNA synthesized in the presence of N14 due to the difference in the number of neutrons.

4. **Separation Method**:
- To distinguish between the two types of DNA (N14 and N15), Meselson and Stahl used a technique that exploits the difference in density.
- They employed centrifugation in a cesium chloride (CsCl) density gradient. This method allows for the separation of molecules based on their density.

5. **Conclusion**:
- The final conclusion is that during the Meselson and Stahl experiment, N15 was distinguished from N14 by centrifugation in a cesium chloride density gradient.

**Final Answer**: During performing the Meselson and Stahl's experiments, N15 was distinguished by N14 by centrifugation in CsCl density gradient.

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