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 can analyze each option based on the characteristics and functions of these enzymes.

### Step-by-Step Solution:

1. **Understanding Restriction Endonucleases**:
- Restriction endonucleases are enzymes that cut DNA at specific sequences. They are often referred to as "molecular scissors."

2. **Identifying the First Statement**:
- The first statement claims that restriction endonucleases attach to both strands of DNA.
- This statement is valid because these enzymes recognize specific sequences on both strands and cut the DNA.

3. **Identifying the Second Statement**:
- The second statement mentions that restriction endonucleases serve as defense enzymes for bacteria.
- This is also valid, as these enzymes help bacteria defend against foreign DNA, such as that from viruses.

4. **Identifying the Third Statement**:
- The third statement states that the site of attachment for restriction endonucleases is the sugar-phosphate backbone of DNA.
- This statement is correct because the enzymes act on the sugar-phosphate backbone to cleave the DNA.

5. **Identifying the Fourth Statement**:
- The fourth statement claims that the action of restriction endonucleases always creates sticky ends in the DNA molecule.
- This statement is not valid because restriction endonucleases can produce both sticky ends and blunt ends, depending on the specific enzyme and the DNA sequence it recognizes.

### Conclusion:
The statement that is not valid with respect to restriction endonuclease enzymes is: "The action of the restriction endonuclease enzyme always creates sticky ends in the DNA molecule."

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