Which of the following is the first artificial cloning vector that has two selectable markers—tetracycline (tetR) and antibiotic restriction enzymes (ampR)?

Options

A. YAC
B. BAC
C. pBR322
D. Cosmid vectors

Answer

To answer the question regarding the first artificial cloning vector that has two selectable markers—tetracycline (tetR) and antibiotic resistance enzymes (ampR)—we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Concept of Cloning Vectors**:
- Cloning vectors are DNA molecules used to transport foreign DNA into a host cell. They serve as vehicles for gene cloning.

2. **Identify the Purpose of Selectable Markers**:
- Selectable markers are genes that confer resistance to antibiotics or other agents, allowing for the identification of cells that have successfully taken up the vector containing foreign DNA.

3. **Review the Options Given**:
- The question provides several options, including various types of cloning vectors such as YAC (Yeast Artificial Chromosome), BAC (Bacterial Artificial Chromosome), PBR322, and cosmid vectors.

4. **Analyze Each Vector**:
- **YAC**: A yeast artificial chromosome that does not have the specified selectable markers.
- **BAC**: A bacterial artificial chromosome based on the F plasmid of E. coli, but it does not specifically mention having both selectable markers.
- **Cosmid Vectors**: These are hybrids of plasmids and phage DNA but do not have the two selectable markers mentioned.
- **PBR322**: This is a well-known plasmid cloning vector that contains two selectable markers: the tetracycline resistance gene (tetR) and the ampicillin resistance gene (ampR).

5. **Conclude the Correct Answer**:
- Based on the analysis, PBR322 is the first artificial cloning vector that has both selectable markers: tetracycline (tetR) and ampicillin (ampR). Therefore, the correct answer is **PBR322**.

### Final Answer:
The first artificial cloning vector that has two selectable markers—tetracycline (tetR) and antibiotic resistance enzymes (ampR)—is **PBR322**.

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