Amongst the following statements which one is not valid w.r.t. restriction endonuclease enzymes:

Options

A. They attach to both the strands of DNA
B. Restriction endonucleases are actually defence enzymes of bacteria
C. Site of attachment is the sugar-phosphate back- bone
D. Action of the restriction endonuclease enzymes always creates sticky ends in a DNA molecule

Answer

To determine which statement is not valid with respect to restriction endonuclease enzymes, we will analyze each of the provided statements step by step.

### Step-by-Step Solution:

1. **Understanding Restriction Endonucleases**:
- Restriction endonucleases are enzymes that cut DNA at specific recognition sites. They are primarily found in bacteria and serve as a defense mechanism against foreign DNA, such as that from viruses.

2. **Analyzing the Statements**:
- **Statement 1**: "They attach to both the strands of DNA."
- This statement is valid. Restriction endonucleases indeed act on both strands of the DNA double helix at their specific recognition sites.

- **Statement 2**: "Restriction endonucleases are actually a defense mechanism of bacteria."
- This statement is also valid. These enzymes protect bacteria from invading viruses by cutting foreign DNA.

- **Statement 3**: "The site of attachment is the sugar-phosphate backbone."
- This statement is valid. Restriction endonucleases cleave the phosphodiester bonds in the sugar-phosphate backbone of DNA.

- **Statement 4**: "Action of the restriction endonucleases enzyme always creates sticky ends in the DNA molecules."
- This statement is not valid. While some restriction enzymes create sticky ends, others produce blunt ends. Therefore, it is incorrect to say that they always create sticky ends.

3. **Conclusion**:
- The statement that is not valid with respect to restriction endonuclease enzymes is **Statement 4**.

### Final Answer:
The statement that is not valid with respect to restriction endonuclease enzymes is: **"Action of the restriction endonucleases enzyme always creates sticky ends in the DNA molecules."**

---