Restriction endonucleases are

Options

A. Synthesized by bacteria as part of their defence mechanism
B. Used in vitro DNA synthesis
C. Present in mammalian cells to degrade DNA when the cells die
D. Used in genetic engineering for ligation of two DNA molecules.

Answer

**Step-by-Step Solution:**

1. **Definition of Restriction Endonucleases**:
- Restriction endonucleases, also known as restriction enzymes, are specialized proteins that can cut DNA at specific sequences. They are essential tools in molecular biology.

2. **Functionality**:
- These enzymes recognize specific nucleotide sequences in DNA and cleave the DNA at or near these sites. This process can be either targeted (specific sites) or random.

3. **Role in Bacteria**:
- Restriction endonucleases are primarily synthesized by bacteria as a defense mechanism. They help protect the bacterial cell from foreign DNA, such as that from viruses, by recognizing and cutting it.

4. **Incorrect Options**:
- **In vitro DNA synthesis**: This process typically uses DNA polymerases (like Taq polymerase), not restriction endonucleases.
- **Presence in Mammalian Cells**: Restriction endonucleases are not found in mammalian cells for degrading DNA. Instead, mammalian cells utilize caspase-activated DNases for this purpose.
- **Genetic Engineering**: In genetic engineering, DNA ligase is used to join two DNA molecules, not restriction endonucleases.

5. **Conclusion**:
- The correct statement regarding restriction endonucleases is that they are enzymes synthesized by bacteria as a part of their defense system against foreign DNA.

**Final Answer**: Restriction endonucleases are enzymes synthesized by bacteria as a part of their defense system.

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