Identify the statement which is not true for restriction enzymes ?

Options

A. Some 900 restriction enzymes have been isolated till date
B. They cut each of the two strands of DNA at spe- cific points through their sugar phosphate back- bone
C. The naming of restriction enzymes follows the se- quence strain name followed by species name followed by genus name
D. The restriction enzymes can be both exonucleases as well as endonucleases

Answer

To identify the statement that is not true for restriction enzymes, we will analyze each of the provided options step by step.

### Step-by-Step Solution:

1. **Option Analysis**:
- **Option 1**: "Some 900 restriction enzymes have been isolated till date."
- This statement is true. There are indeed around 900 known restriction enzymes that have been isolated from various organisms.

2. **Option 2**: "They cut each of the two strands of DNA at specific points through their sugar phosphate backbone."
- This statement is also true. Restriction enzymes cleave the DNA at specific sequences, cutting both strands at the sugar-phosphate backbone.

3. **Option 3**: "The naming of restriction enzymes follows the sequence strain name followed by species name followed by genus name."
- This statement is incorrect. The correct naming convention for restriction enzymes is that the genus name comes first, followed by the species name, and then the strain name (if applicable). Therefore, this statement is not true.

4. **Option 4**: "The restriction enzymes can be both exonucleases as well as endonucleases."
- This statement is true. Restriction enzymes can function as both exonucleases (which cut at the ends of the DNA) and endonucleases (which cut within the DNA strand).

### Conclusion:
The statement that is not true for restriction enzymes is: "The naming of restriction enzymes follows the sequence strain name followed by species name followed by genus name."