The usual source of restriction endonucleases used in gene clonning is

Options

A. fungi
B. bacteria
C. plants
D. viruses

Answer

**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 essential tools in molecular biology, particularly in gene cloning.

2. **Historical Context**:
- The discovery of restriction endonucleases dates back to 1963 when two enzymes were isolated from E. coli bacteria. These enzymes were found to inhibit the growth of bacteriophages, which are viruses that infect bacteria.

3. **Source of Restriction Enzymes**:
- The primary source of restriction endonucleases is bacteria. Specifically, they are often derived from various strains of the bacterium Escherichia coli (E. coli).

4. **Examples of Restriction Enzymes**:
- One of the most well-known restriction enzymes is EcoRI, which is derived from E. coli. The naming convention includes the genus (Escherichia), species (coli), and strain (RY13).

5. **Functionality of Restriction Enzymes**:
- These enzymes recognize specific palindromic sequences in DNA and cut the DNA at those sites, which is crucial for gene cloning and other genetic engineering techniques.

6. **Conclusion**:
- Therefore, the usual source of restriction endonucleases used in gene cloning is bacteria, particularly strains of E. coli.

**Final Answer**: The usual source of restriction endonucleases used in gene cloning is bacteria (specifically E. coli).

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