The restriction enzymes are used in genetic engineering because

Options

A. They can cut DNA at specific base sequence
B. They are nucleases that cut DNA at variable sites
C. They can degrade harmful proteins
D. They can join different DNA fragment

Explanation

### Step-by-Step Solution

1. **Understanding Restriction Enzymes**:
- Restriction enzymes, also known as restriction endonucleases, are proteins that can cut DNA at specific sequences. They are crucial tools in genetic engineering.

2. **Identifying the Function**:
- The primary function of restriction enzymes is to recognize specific base sequences in DNA and cleave the DNA at those sites. This allows scientists to manipulate DNA for various applications, such as cloning, gene insertion, and DNA analysis.

3. **Analyzing the Options**:
- **Option A**: "They can cut DNA at specific base sequence."
- This is true. Restriction enzymes are known for their ability to cut DNA at specific sequences, which is essential for genetic engineering.
- **Option B**: "They are nucleases that cut DNA at variable sites."
- This is incorrect. Restriction enzymes cut at specific sites, not variable ones.
- **Option C**: "They can degrade harmful proteins."
- This is also incorrect. Restriction enzymes do not degrade proteins; they cut DNA.
- **Option D**: "They can join different DNA fragments."
- This is incorrect. Joining DNA fragments is the role of DNA ligase, not restriction enzymes.

4. **Conclusion**:
- Based on the analysis of the options, the correct answer is **Option A**: "They can cut DNA at specific base sequence."