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 E. coli cells can be made competent to take up external DNA through transformation, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding Transformation**:
- Transformation is a process where bacterial cells, such as E. coli, are made competent to take up foreign DNA from their environment.

2. **Identifying the Role of Calcium Chloride**:
- One of the methods to induce competence in E. coli is by using calcium chloride (CaCl2). This salt is known to enhance the ability of the cells to uptake DNA.

3. **Mechanism of Action**:
- Calcium chloride is believed to either create pores in the bacterial cell membrane or precipitate DNA on the surface of the cells, facilitating the uptake of the DNA.

4. **Evaluating the Options**:
- The question provides several options to choose from. We need to evaluate each one to determine which is correct.
- Option 1: Cobalt chloride - Incorrect, as it is not used for making E. coli competent.
- Option 2: Calcium chloride - Correct, as it is the standard method for inducing competence in E. coli.
- Option 3: Radiation - Incorrect, as radiation is not a method used for this purpose.
- Option 4: (Not specified in the transcript) - Likely incorrect based on the context.

5. **Conclusion**:
- The correct answer is that E. coli cells may be made competent to take up external DNA in the process of transformation by exposing them to calcium chloride.

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

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