[A]: Meselson and Stahl tested the Watson and Crick Theory of DNA replication.
[R]: They confirmed the mechanism of DNA replication by using the isotopic and Centrifu- gation technique

Options

A. If both A and R are true and R is the correct explanation of A
B. If both A and R are true but R is not the correct explanation of A
C. If A is true and R is false
D. If both A and R are false

Answer

**Step-by-Step Solution:**

1. **Understanding the Assertion (A):**
- The assertion states that Meselson and Stahl tested the Watson and Crick theory of DNA replication. This theory posits that DNA replication is semi-conservative, meaning that each strand of the original DNA serves as a template for the new strand.

2. **Understanding the Reason (R):**
- The reason provided states that Meselson and Stahl confirmed the mechanism of DNA replication using isotopic labeling and centrifugation techniques. They specifically used density gradient centrifugation to separate DNA based on density.

3. **Details of the Meselson-Stahl Experiment:**
- In 1958, Meselson and Stahl conducted their famous experiment using isotopes of nitrogen (N-14 and N-15). They grew bacteria in a medium containing heavy nitrogen (N-15) and then transferred them to a medium containing lighter nitrogen (N-14).
- They extracted the DNA and used density gradient centrifugation to separate the DNA based on its density. The results showed that after one round of replication, the DNA had an intermediate density, confirming that each DNA molecule consisted of one old (heavy) strand and one new (light) strand.

4. **Conclusion:**
- Both the assertion and the reason are true. The assertion correctly states that Meselson and Stahl tested the Watson and Crick theory, and the reason accurately describes the method they used to confirm this theory. Therefore, the correct answer is that both A and R are true, and R is a correct explanation of A.

**Final Answer:**
Both A and R are true; R is a correct explanation of A.

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