Restriction enzymes were first discovered with observation that-

Options

A. DNA is restricted to the nucleus
B. Phage DNA is destroyed in host cell
C. Foreign DNA is kept out of a cell
D. Foreign DNA is restricted to cytoplasm

Answer

To answer the question about the discovery of restriction enzymes, we can analyze the options provided and understand the context of their discovery.

### Step-by-Step Solution:

1. **Understanding Restriction Enzymes**:
- Restriction enzymes, also known as restriction endonucleases, are proteins that cut DNA at specific sequences. They were discovered in bacteria, where they serve as a defense mechanism against foreign DNA, such as that from viruses (bacteriophages).

2. **Analyzing the Options**:
- **Option A**: "DNA is restricted to the nucleus."
- This statement is not relevant to the discovery of restriction enzymes as it pertains to the location of DNA in eukaryotic cells rather than the function of restriction enzymes.
- **Option B**: "Bacteriophage DNA is destroyed in host cell."
- This option is relevant because the primary function of restriction enzymes is to cut and destroy foreign DNA, like that from bacteriophages, which attempts to invade bacterial cells.
- **Option C**: "Foreign DNA is kept out of a cell."
- While this statement hints at a protective mechanism, it does not accurately describe the function of restriction enzymes, which act on DNA that has already entered the cell.
- **Option D**: "Foreign DNA is restricted to cytoplasm."
- This option is misleading as it suggests a location rather than a function. Restriction enzymes do not restrict foreign DNA to the cytoplasm; they cut it.

3. **Conclusion**:
- The most accurate observation leading to the discovery of restriction enzymes is **Option B**: "Bacteriophage DNA is destroyed in host cell." This reflects the role of restriction enzymes in protecting bacteria from viral infections by cutting foreign DNA.

### Final Answer:
Restriction enzymes were first discovered with the observation that bacteriophage DNA is destroyed in host cells.