The cells of E. coli may be made competent to take up external DNA in the process of transformation by expos- ing the cell to:

Options

A. cobalt chloride
B. calcium chloride
C. radiation
D. sulphuric acid

Answer

To solve the question regarding how the cells of E. coli can be made competent to take up external DNA during the process of transformation, we will analyze the options provided and identify the correct one.

### Step-by-Step Solution:

1. **Understanding Transformation**:
- Transformation is a process by which bacteria can take up external DNA from their environment. For E. coli to take up this DNA, it must first be made competent.

2. **Evaluating the Options**:
- **Cobalt Chloride**: This is not commonly used in the transformation process for E. coli. It does not facilitate the uptake of DNA effectively.
- **Calcium Chloride (CaCl2)**: This is a well-known chemical used to make E. coli competent. It helps to increase the permeability of the bacterial cell membrane, allowing external DNA to enter the cell.
- **Radiation**: This is not a method used to make E. coli competent for transformation. It may cause damage to the cells rather than facilitate DNA uptake.
- **Sulfuric Acid**: This is also not used in the transformation process and would likely harm the cells.

3. **Conclusion**:
- The correct answer is **Calcium Chloride (CaCl2)**, as it is specifically used to enhance the efficiency of E. coli cells in taking up external DNA during transformation.

### Final Answer:
The cells of E. coli may be made competent to take up external DNA in the process of transformation by exposing the cell to **Calcium Chloride (CaCl2)**.

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