Restriction enzymes are :

Options

A. Not always required in genetic engineering
B. Essential tool in genetic engineering
C. Nucleases that cleave DNA at specific sites
D. Both (2) and (3)

Answer

### Step-by-Step Solution:

1. **Understanding Restriction Enzymes**:
- Restriction enzymes, also known as restriction endonucleases, are proteins that cut DNA at specific sequences.

2. **Definition of Endonucleases**:
- The term "endonuclease" can be broken down into "endo," meaning inside, and "nuclease," which refers to enzymes that cleave nucleic acids. Thus, endonucleases cut nucleic acids at internal sites, as opposed to exonucleases, which cut nucleic acids at the ends.

3. **Role in Genetic Engineering**:
- In genetic engineering, these enzymes are essential because they allow scientists to cut DNA at precise locations. This is crucial for manipulating genes, such as inserting or removing DNA segments.

4. **Analyzing the Options**:
- The first option states that restriction enzymes are not always required in genetic engineering. This is incorrect because they are fundamental to the process.
- The second option claims that they are an essential tool in genetic engineering, which is true.
- The third option states that they are nucleases that cleave DNA at specific sites, which is also true.
- The fourth option combines the second and third statements, which are both correct.

5. **Conclusion**:
- Since both the second and third options are correct, the best answer is the fourth option: "both two and three."