Restriction endonuclease was isolated for the first time by W. Aber in 1962 in bacteria. Restriction endonucleases cut the DNA duplex at specific points therefore they are also called as molecular scissors or biological scissors. Three types of restriction endemicleases are Type 1. Type II and Type III but only Type II restriction endonucleases are used in recombinant DNA technology. Restriction endonuclease EcoR I recognises the base sequence GAATTC In DNA duplex and cut strands between G and A.
Only type II restriction enzymes are used in gene manipulation because:

Options

A. ATP is not required for cleaving
B. it consists of three different subunits
C. it makes cleavage or cut in both the strands of DNA molecule
D. both (A) and (C)

Explanation

### Step-by-Step Solution:

1. **Understanding Restriction Endonucleases**:
- Restriction endonucleases are enzymes that cut DNA at specific sequences. They are often referred to as molecular scissors due to their ability to cleave DNA strands.

2. **Types of Restriction Endonucleases**:
- There are three types of restriction endonucleases: Type I, Type II, and Type III. Each type has different mechanisms of action and requirements for cutting DNA.

3. **Focus on Type II Restriction Endonucleases**:
- Only Type II restriction endonucleases are used in recombinant DNA technology. This is because they cut DNA at specific sites without the need for additional energy sources.

4. **Mechanism of Action**:
- Type II restriction enzymes recognize specific palindromic sequences in the DNA. For example, EcoR I recognizes the sequence GAATTC and cuts between the G and A.

5. **Advantages of Type II Enzymes**:
- Type II restriction endonucleases are preferred in genetic engineering because:
- They produce clean cuts at specific sites.
- They do not require ATP or other energy sources for cleavage.
- They typically require only magnesium ions as a cofactor for their activity.

6. **Conclusion**:
- The specific action of Type II restriction endonucleases makes them ideal for gene manipulation, allowing for precise cutting and joining of DNA fragments.