Restriction enzymes are used in genetic engineering because they
(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 proteins

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 proteins.

Answer

**Step-by-Step Solution:**

1. **Understanding the Role of Restriction Enzymes:**
- Restriction enzymes, also known as restriction endonucleases, are crucial tools in genetic engineering. They function as molecular scissors that cut DNA at specific sequences.

2. **Analyzing the Options:**
- **Option (a)**: "Can join DNA fragments" - This is incorrect. Restriction enzymes do not join DNA fragments; this function is performed by DNA ligase.
- **Option (b)**: "Cut DNA at specific base sequence" - This is correct. Restriction enzymes recognize specific sequences in the DNA (known as restriction sites) and cut at these sites.
- **Option (c)**: "Cut DNA at variable sites" - This is incorrect. Restriction enzymes cut DNA at specific sequences, not at variable sites.
- **Option (d)**: "Are proteolytic enzymes which degrade harmful proteins" - This is incorrect. Restriction enzymes are not proteolytic enzymes; they do not degrade proteins.

3. **Conclusion:**
- Based on the analysis, the correct answer is **(b) Cut DNA at specific base sequence**. This is the primary reason why restriction enzymes are used in genetic engineering.