Assertion: Three main types of restriction endonucleases, i.e., Type I, Type IT and Type ill are known with slightly different modes of action.
Reason: In palindromes with rotational symmetry, second half of complementary strand in a DNA double helix is the mirror image of base sequence in the first half of another strand.

Options

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

Answer

### Step-by-Step Solution:

1. **Understanding the Assertion**:
The assertion states that there are three main types of restriction endonucleases: Type I, Type II, and Type III, each with slightly different modes of action.
- **Type I** cleaves at a site far from its recognition site and requires ATP and S-adenosylmethionine.
- **Type II** cleaves within a short distance from its recognition site and requires magnesium.
- **Type III** cleaves at a short distance from its recognition site and requires ATP.

**Conclusion**: The assertion is true.

2. **Understanding the Reason**:
The reason discusses palindromic sequences in DNA, stating that the second half of the complementary strand in a DNA double helix is the mirror image of the base sequence in the first half of another strand.
- A palindromic sequence is a sequence that reads the same in both directions when considering complementary strands. For example, in a sequence like GAATTC, the complementary strand will be CTTAAG, which is the mirror image of the first strand.

**Conclusion**: The reason is also true.

3. **Analyzing the Relationship**:
While both the assertion and reason are true, they are not directly related. The assertion discusses the types of restriction endonucleases and their modes of action, while the reason explains the concept of palindromic sequences, which are relevant for the recognition of these enzymes but does not explain the assertion itself.

4. **Final Conclusion**:
Since both the assertion and reason are true but the reason does not correctly explain the assertion, the correct option is:
- **Assertion true, Reason true, but Reason is not the correct explanation of Assertion**.