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. 2 & 3 both

Answer

### Step-by-Step Solution:

1. **Definition of Restriction Enzymes**:
- Restriction enzymes, also known as restriction endonucleases, are proteins that cut DNA at specific sequences. They act like "molecular scissors" that can cleave DNA into fragments.

2. **Function in Genetic Engineering**:
- These enzymes are crucial in genetic engineering because they allow scientists to cut DNA at precise locations. This cutting is essential for manipulating genes, such as inserting, deleting, or modifying DNA sequences.

3. **Specificity of Action**:
- Restriction enzymes recognize specific nucleotide sequences in the DNA. Each enzyme has a unique recognition site, which is a specific sequence of nucleotides where it will cut the DNA.

4. **Importance in Biotechnology**:
- Because of their ability to create fragments of DNA, restriction enzymes are an essential tool in molecular cloning, gene splicing, and various other biotechnological applications. They enable the creation of recombinant DNA, which is fundamental in genetic engineering.

5. **Conclusion**:
- Based on the above points, we can conclude that restriction enzymes are both an essential tool in genetic engineering and nucleases that cleave DNA at specific sites. Therefore, the correct answer to the question is option D: both 2 and 3.

### Final Answer:
**Restriction enzymes are essential tools in genetic engineering and nucleases that cleave DNA at specific sites. The correct option is D (both 2 and 3).**

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