Restriction endonucleases

Options

A. Are used in genetic engineering
B. Are used for in vitro DNA synthesis
C. Are synthesised by bacteria as part of their defense mechanism
D. Are present in mammalian cells for degradation of DNA when the cell dies

Explanation

### 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 primarily found in bacteria.

2. **Function of Restriction Endonucleases**:
- These enzymes recognize specific nucleotide sequences in the DNA and cleave the DNA at these sites. This ability is crucial for genetic engineering and molecular biology.

3. **Role in Bacterial Defense**:
- In bacteria, restriction endonucleases serve as a defense mechanism against invading viruses (bacteriophages). When a virus infects a bacterium, the restriction enzymes can cut the viral DNA, thereby neutralizing the threat.

4. **Options Analysis**:
- a) Used in genetic engineering: While true, this is not the primary role of restriction endonucleases.
- b) Used in in vitro DNA synthesis: This is incorrect as restriction enzymes are not involved in DNA synthesis.
- c) Synthesized by bacteria as part of their defense mechanism: This is correct.
- d) Present in mammalian cells for degradation of DNA when the cell dies: This is incorrect as mammalian cells do not use restriction endonucleases in this manner.

5. **Conclusion**:
- Based on the analysis, the correct answer is **C**: Restriction endonucleases are synthesized by bacteria as part of their defense mechanism.