First discovered restriction endonuclease that always cuts DNA molecule at a particular point by recognising a specific sequence of six base pairs is

Options

A. EcoRI
B. Adenosine deaminase
C. Thermostable DNA polymerase
D. Hind II

Answer

To solve the question regarding the first discovered restriction endonuclease that cuts DNA at a specific sequence of six base pairs, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Concept of Restriction Enzymes**:
- Restriction enzymes are proteins that cut DNA at specific sequences. They are crucial in biotechnology for manipulating DNA.

2. **Differentiate Between Types of Restriction Enzymes**:
- There are two main types:
- **Exonucleases**: These cut DNA from the ends.
- **Endonucleases**: These cut DNA at specific internal sites, which is what we are interested in for this question.

3. **Identify the First Restriction Endonuclease**:
- The first restriction enzyme was discovered in 1963, but it was not an endonuclease. The first true restriction endonuclease that could cut DNA at a specific internal location was discovered later.

4. **Recognize the Importance of Specificity**:
- The restriction endonuclease we are looking for recognizes a specific sequence of six base pairs in the DNA. This specificity is crucial for genetic engineering applications.

5. **Determine the Correct Answer**:
- Based on the historical context provided, the first restriction endonuclease that fits the criteria of cutting DNA at a specific six-base pair sequence is **HindII**, which was discovered in 1968.

### Final Answer:
The first discovered restriction endonuclease that always cuts DNA molecules at a particular point by recognizing a specific sequence of six base pairs is **HindII**.

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