An antibiotic resistance gene in a vector usually helps in the selection of

Options

A. Competent cells
B. Transformed cells
C. Recombinant cells
D. Both (2) & (3)

Answer

To solve the question about the role of an antibiotic resistance gene in a vector for selection, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Context**: The question is asking about the function of an antibiotic resistance gene within a vector. Antibiotic resistance genes are often used in genetic engineering to select for cells that have successfully taken up a plasmid (vector) containing the gene of interest.

2. **Identify the Options**: The options provided are:
- Competent cell
- Transformed cell
- Recombinant cell
- Both 2nd and 3rd (Transformed cell and Recombinant cell)

3. **Define Key Terms**:
- **Competent Cell**: A bacterial cell that is capable of taking up foreign DNA.
- **Transformed Cell**: A cell that has taken up foreign DNA (in this case, the plasmid with the antibiotic resistance gene).
- **Recombinant Cell**: A cell that contains a plasmid with a gene of interest inserted into it.

4. **Analyze the Role of Antibiotic Resistance Gene**: When a plasmid containing an antibiotic resistance gene (like tetracycline resistance) is introduced into bacteria, only those bacteria that successfully take up the plasmid (transformed cells) will survive when grown on a medium containing that antibiotic. The cells that do not take up the plasmid will die.

5. **Conclusion**: Since the antibiotic resistance gene helps in selecting the bacteria that have taken up the plasmid, the correct answer is that it helps in the selection of **transformed cells**. However, since transformed cells can also be considered recombinant (if they contain a gene of interest), the best choice from the options provided is **both 2nd and 3rd**.

### Final Answer:
The antibiotic resistance gene in a vector usually helps in the selection of **both transformed cells and recombinant cells** (Option 4).

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