Restriction enzymes are used in genetic engineering be- cause they :

Options

A. can join DNA fragments
B. cut DNA at specific base sequence
C. cut DNA at variable sites
D. are proteolytic enzymes which degrade harmful pro- teins

Answer

### Step-by-Step Text Solution:

1. **Understanding Restriction Enzymes**:
- Restriction enzymes, also known as restriction endonucleases, are enzymes that cut DNA at specific sequences. The term "restriction" indicates that they cut at particular sites, while "endonuclease" refers to their ability to cut within the DNA strand.

2. **Analyzing the Options**:
- **Option 1**: "Can join DNA fragments" - This is incorrect. Restriction enzymes do not join DNA fragments; they cut them.
- **Option 2**: "Cut DNA at specific base sequence" - This is correct. Restriction enzymes recognize specific sequences in the DNA and cut at those sites.
- **Option 3**: "Cut DNA at variable sites" - This is incorrect. Restriction enzymes cut DNA at specific, predetermined sites, not at variable locations.
- **Option 4**: "Are proteolytic enzymes which degrade harmful proteins" - This is also incorrect. Restriction enzymes are not proteolytic enzymes; they specifically target DNA, not proteins.

3. **Conclusion**:
- The correct answer is that restriction enzymes are used in genetic engineering because they **cut DNA at specific base sequences**.