Restriction endonucleases,
A. Are found in bacteria
B. are very useful in genetic engineering
C. cut the DNA at particular sites
D. are used naturally in a bacterial cell to defend against foreign DNA

Options

A. A,B
B. B,C
C. C,D
D. A,B,C,D

Answer

To answer the question about restriction endonucleases, we will analyze each option provided:

### Step 1: Analyze Option A
**Statement**: Restriction endonucleases are found in bacteria.
**Explanation**: This statement is true. Restriction endonucleases are enzymes that are produced by bacteria as a defense mechanism against foreign DNA, such as that from viruses.

### Step 2: Analyze Option B
**Statement**: Restriction endonucleases are very useful in genetic engineering.
**Explanation**: This statement is also true. These enzymes are essential tools in genetic engineering because they can cut DNA at specific sequences, allowing scientists to manipulate genes and create recombinant DNA.

### Step 3: Analyze Option C
**Statement**: Restriction endonucleases cut the DNA at particular sites.
**Explanation**: This statement is true as well. Restriction endonucleases recognize specific nucleotide sequences in DNA and cleave the DNA at those sites, which is crucial for various molecular biology techniques.

### Step 4: Analyze Option D
**Statement**: Restriction endonucleases are used naturally in a bacterial cell to defend against foreign DNA.
**Explanation**: This statement is true. Bacteria use restriction endonucleases as a defense mechanism to cut and destroy foreign DNA, such as that from bacteriophages (viruses that infect bacteria).

### Conclusion
All options A, B, C, and D are true statements regarding restriction endonucleases.

### Final Answer
**All of the above statements (A, B, C, and D) are correct regarding restriction endonucleases.**

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