[A]: The base ratio (A+T )/(G+C) is constant in a particular organism.
[R]: The ratio is, however, different in the dif- ferent DNA molecules

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

To solve the question regarding the assertion and reason related to the base ratio in DNA, we can break it down into clear steps:

### Step-by-Step Solution:

1. **Understanding the Assertion**:
- The assertion states that the base ratio (A+T)/(G+C) is constant in a particular organism. This is based on Chargaff's rule, which indicates that in any DNA molecule, the amount of adenine (A) is equal to thymine (T), and the amount of guanine (G) is equal to cytosine (C).
- Therefore, in a specific organism, the ratio of A+T to G+C remains constant.

2. **Understanding the Reason**:
- The reason states that this ratio is different in different DNA molecules. This means that while the ratio may be constant within a single organism, it can vary between different organisms or different DNA molecules within the same organism.
- For example, one DNA molecule may have a different number of A and T compared to another DNA molecule, leading to a different (A+T)/(G+C) ratio.

3. **Analyzing Both Statements**:
- Both the assertion and reason are true. The assertion is supported by Chargaff's rule, which holds for any given organism, while the reason acknowledges the variability of this ratio across different DNA molecules.

4. **Conclusion**:
- Since both the assertion and reason are true and the reason correctly explains the assertion, we conclude that the correct answer is that both A and R are true, and R is a correct explanation for A.

### Final Answer:
Both A and R are true, and R is a correct explanation for A.

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