In bacteria, the restriction enzyme plays an important role in defence against viral infection by chopping foreign DNA into pieses, but it does not cut its own DNA because of

Options

A. difference in recognition sequence.
B. methylation of A or C at the recognition site.
C. restriction endonuclease leave the DNA out of the cells.
D. none of these

Answer

**Step-by-Step Solution:**

1. **Understanding Restriction Enzymes**:
- Restriction enzymes, also known as restriction endonucleases, are proteins produced by bacteria that can cut DNA at specific sequences known as recognition sites. This mechanism serves as a defense against viral infections by cleaving the foreign viral DNA.

2. **Function of Restriction Enzymes**:
- These enzymes identify and cut DNA at specific sequences, which are typically palindromic in nature. When a virus infects a bacterium, the restriction enzymes recognize the viral DNA and chop it into pieces, preventing the virus from replicating.

3. **Why Bacteria Don't Cut Their Own DNA**:
- Despite having the same recognition sequences in their own DNA, bacteria do not cut their own DNA. This is primarily due to a protective mechanism involving methylation.

4. **Role of Methylation**:
- Methylation is the addition of a methyl group (CH3) to specific bases in the DNA, such as adenine (A) and cytosine (C) at the recognition sites. When the bacterial DNA is methylated, the restriction enzymes cannot recognize these modified sites as their own.

5. **Conclusion**:
- Therefore, the reason why restriction enzymes do not cut the bacterial DNA is due to the methylation of the recognition sites. This modification prevents the enzymes from identifying and cleaving their own DNA, thus protecting the bacteria from self-damage.

**Final Answer**: The restriction enzyme does not cut its own DNA because of methylation of adenine and cytosine at the recognition sites.

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