Assertion: Restriction endonucleases are also called 'molecular scissors'.
Reason: When fragments generated by restriction endonucleases are mixed, they are joined due to their sticky ends.

Options

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

Explanation

**Step-by-Step Text Solution:**

1. **Understanding the Assertion:**
- The assertion states that "Restriction endonucleases are also called 'molecular scissors'." This is true because restriction endonucleases are enzymes that cut DNA at specific sequences, similar to how scissors cut paper.

2. **Understanding the Reason:**
- The reason provided states that "When fragments generated by restriction endonucleases are mixed, they are joined due to their sticky ends." This is also true. When restriction endonucleases cut DNA, they can create fragments with sticky ends (overhanging sequences), which can anneal or join together.

3. **Analyzing the Relationship:**
- While both the assertion and reason are true, the reason does not adequately explain the assertion. The term "molecular scissors" refers to the cutting action of restriction endonucleases, while the joining of fragments due to sticky ends is a separate process involving DNA ligase.

4. **Conclusion:**
- Therefore, the correct evaluation is that both the assertion and reason are true, but the reason is not a correct explanation of the assertion. The answer is: Assertion is true, Reason is true, but Reason is not the correct explanation of Assertion.

**Final Answer:**
- Both A and R are true, but R is not a correct explanation of A.

---