Assertion:In a DNA molecules the total quantity of purines equals the total quantity of pyrimidines
Reason : Adenine pairs with thymine and cytosine pairs with guanine

Options

A. Both Assertion and Reason are true and Reason is the correct explanation of Assertion
B. Both Assertion and Reason are true but Reason is not a correct explanation of Assertion
C. Assertion is true but Reason is false
D. Assertion is false but Reason is true

Explanation

To solve the question, we need to analyze both the assertion and the reason provided.

### Step 1: Understand the Assertion
The assertion states that in a DNA molecule, the total quantity of purines equals the total quantity of pyrimidines.

- **Purines** are nitrogenous bases that include Adenine (A) and Guanine (G).
- **Pyrimidines** are nitrogenous bases that include Cytosine (C) and Thymine (T).

### Step 2: Analyze the Reason
The reason given is that Adenine pairs with Thymine and Cytosine pairs with Guanine.

- In DNA, the base pairing occurs as follows:
- Adenine (A) pairs with Thymine (T) through two hydrogen bonds.
- Cytosine (C) pairs with Guanine (G) through three hydrogen bonds.

### Step 3: Establish the Relationship
From the base pairing rules:
- The amount of Adenine (A) will always equal the amount of Thymine (T).
- The amount of Guanine (G) will always equal the amount of Cytosine (C).

### Step 4: Conclude the Assertion
Since A = T and G = C, we can deduce that:
- Total Purines (A + G) = Total Pyrimidines (C + T).

Thus, the assertion is true.

### Step 5: Verify the Reason
The reason correctly explains why the assertion is true, as it describes the base pairing that leads to the equality of purines and pyrimidines.

### Final Conclusion
Both the assertion and the reason are true, and the reason is the correct explanation of the assertion.

### Answer
The correct option is: **Both assertion and reason are true and reason is the correct explanation of the assertion.**

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