Restriction endonucleases were discovered by

Options

A. Arber et al
B. Monod et al
C. Cech et al
D. Altman et al

Answer

### Step-by-Step Solution:

1. **Understanding Restriction Endonucleases**:
- Restriction endonucleases, also known as restriction enzymes, are proteins that cut DNA at specific sequences. They are essential tools in genetic engineering and molecular biology for manipulating DNA.

2. **Identifying the Discoverers**:
- The question asks who discovered restriction endonucleases. The options given are:
- A) Arbor et al.
- B) Monod et al.
- C) Ketch et al.
- D) Altman et al.

3. **Reviewing Historical Context**:
- It is important to recognize that restriction endonucleases were first discovered in the 1970s. They were identified as a part of the bacterial defense mechanism against viruses.

4. **Analyzing the Options**:
- **Arbor et al.**: They are credited with the discovery of the first restriction enzyme, EcoRI, from E. coli.
- **Monod et al.**: Known for work on the lac operon and gene regulation, not for restriction enzymes.
- **Ketch et al.**: Not widely recognized in the context of restriction enzymes.
- **Altman et al.**: Known for their work on ribozymes, not for restriction endonucleases.

5. **Selecting the Correct Answer**:
- Based on the historical context and the contributions of the researchers, the correct answer is **A) Arbor et al.** They were the ones who discovered restriction endonucleases.

### Final Answer:
Restriction endonucleases were discovered by **Arbor et al.** (Option A).