Restriction endonucleases :

Options

A. Are synthesized by bacteria as part of their defense compound
B. Are present in mammalian cells for degradation of DNA when the cell dies
C. Are used in genetic engineering for ligating two DNA molecules
D. Are used for in vitro DNA sythesis

Answer

### Step-by-Step Solution for the Question on Restriction Endonucleases:

1. **Definition of Restriction Endonucleases**:
- Restriction endonucleases, also known as restriction enzymes, are proteins that can cut DNA at specific sequences. They play a crucial role in molecular biology, particularly in genetic engineering.

2. **Historical Context**:
- The discovery of restriction endonucleases dates back to 1963. Researchers found that certain enzymes in E. coli bacteria could restrict the growth of bacteriophages (viruses that infect bacteria).

3. **Mechanism of Action**:
- There are two types of enzymes involved in this process:
- One enzyme adds methyl groups to the DNA, which is a modification that protects the bacterial DNA from being cut.
- The other enzyme cuts the DNA of the invading bacteriophage, preventing its replication and thereby acting as a defense mechanism.

4. **Function in Bacteria**:
- Restriction endonucleases serve as a defense mechanism for bacteria against foreign DNA, such as that from bacteriophages. This is an important aspect of the bacterial immune system.

5. **Production**:
- It is important to note that restriction endonucleases are produced exclusively by bacterial cells. This specificity is crucial for their role in bacterial defense.

6. **First Discovery**:
- The first restriction endonuclease was discovered by a scientist named Albert. This historical context is important for understanding the development of biotechnology.

7. **Conclusion**:
- In summary, restriction endonucleases are synthesized by bacteria as part of their defense mechanisms against foreign DNA, particularly from bacteriophages.