pBR-322 which is frequently used as a vector for cloning gene is

Options

A. an original bacterial plasmid
B. a modified bacterial plasmid
C. a viral genome
D. a transposon

Answer

**Step-by-Step Solution:**

1. **Understanding PBR-322:**
- PBR-322 is a type of plasmid, which is a small, circular piece of DNA found in bacteria. It is commonly used as a vector for cloning genes.

2. **Identifying the Nature of PBR-322:**
- It is important to note that PBR-322 is not an original bacterial plasmid but rather a modified version. It was synthesized in a laboratory setting.

3. **Historical Context:**
- PBR-322 was developed in 1977 at the University of California by two postdoctoral researchers named Bolivar and Rodrigues. This modification allows it to serve as an effective cloning vector.

4. **Key Features of PBR-322:**
- The plasmid is approximately 4361 base pairs long.
- It contains an origin of replication, which is essential for the plasmid to replicate within a host cell.
- PBR-322 has selectable markers, specifically antibiotic resistance genes, such as ampicillin and tetracycline resistance genes. These markers allow for the identification of successfully transformed cells.

5. **Evaluating the Options:**
- **Original bacterial plasmid:** Incorrect, as PBR-322 is a modified plasmid.
- **Modified bacterial plasmid:** Correct, since it was synthesized and modified in a lab.
- **Viral genome:** Incorrect, as it is not derived from a virus.
- **Transposon:** Incorrect, as transposons are jumping genes that can move within the genome, which PBR-322 is not.

6. **Conclusion:**
- Therefore, the correct answer is that PBR-322, which is frequently used as a vector for cloning genes, is a **modified bacterial plasmid**.

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