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 Experiment**: The Meselson and Stahl experiment was designed to demonstrate the semi-conservative nature of DNA replication. In this experiment, they used two isotopes of nitrogen: heavy nitrogen (N-15) and light nitrogen (N-14).

2. **Growth of E. coli**: E. coli bacteria were initially grown in a medium containing ammonium chloride with heavy nitrogen (N-15). This allowed the bacteria to incorporate the heavy nitrogen into their DNA.

3. **Transfer to Light Nitrogen**: After several generations, the E. coli were transferred to a medium containing only light nitrogen (N-14). This step was crucial for observing the incorporation of the light nitrogen into the newly synthesized DNA strands.

4. **Isolation of DNA**: At various time intervals after the transfer to the light nitrogen medium, DNA was extracted from the bacteria. This DNA would contain a mix of the heavy (N-15) and light (N-14) nitrogen depending on the replication cycles.

5. **Centrifugation in Cesium Chloride**: To distinguish between the DNA containing N-15 and that containing N-14, the extracted DNA was subjected to density gradient centrifugation using cesium chloride. This method exploits the difference in density between the heavy and light DNA.

6. **Observation of Bands**: During centrifugation, the DNA molecules settled into distinct bands based on their density. The heavy DNA (containing N-15) formed a band at a different position than the hybrid DNA (containing one strand of N-15 and one strand of N-14) and the light DNA (containing N-14).

7. **Conclusion**: The results showed that after one round of replication in the light nitrogen medium, the DNA consisted of hybrid molecules, confirming the semi-conservative model of DNA replication. The distinction between N-15 and N-14 was effectively achieved through density gradient centrifugation.

**Final Answer**: A. Centrifugation in cesium chloride density gradient.

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