Restriction endonucleases are used in genetic engineering because :

Options

A. They can degrade harmful proteins
B. They can join DNA fragments
C. They can cut DNA at Variable sites
D. They can cut DNA at specific base sequences

Answer

### Step-by-Step Solution:

1. **Understanding Restriction Endonucleases**:
- Restriction endonucleases are enzymes that cut DNA at specific sites. The term "endonuclease" indicates that these enzymes cut within the DNA molecule, as opposed to "exonucleases," which remove nucleotides from the ends.

2. **Analyzing the Options**:
- **Option A**: "They can degrade harmful proteins."
- This option is incorrect because restriction endonucleases do not target proteins; they specifically act on nucleic acids (DNA or RNA).

- **Option B**: "They can join DNA fragments."
- This option is also incorrect. Restriction endonucleases are not responsible for joining DNA fragments; that function is performed by ligases.

- **Option C**: "They can cut DNA at variable sites."
- This option is misleading. While endonucleases can cut DNA, restriction endonucleases are characterized by their ability to cut at specific sequences, not variable sites.

- **Option D**: "They can cut DNA at specific base sequences."
- This option is correct. Restriction endonucleases recognize specific sequences of nucleotides in the DNA and cut at those sites, which is essential for genetic engineering.

3. **Conclusion**:
- Based on the analysis of the options, the correct answer is that restriction endonucleases are used in genetic engineering because they can cut DNA at specific base sequences.

### Final Answer:
Restriction endonucleases are used in genetic engineering because they can cut DNA at specific base sequences.

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