Which divalent cations are usually used to make competent cells?

Options

A. carbon
B. Nitrogen
C. Copper
D. Calcium

Explanation

To answer the question about which divalent cations are usually used to make competent cells, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding Competent Cells**: Competent cells are bacterial cells that have been treated to allow them to take up foreign DNA. This is a crucial step in molecular biology for cloning and genetic engineering.

2. **Role of Divalent Cations**: Divalent cations are positively charged ions with a charge of +2. They play a significant role in making the bacterial cell membrane more permeable to DNA.

3. **Common Divalent Cation**: The most commonly used divalent cation for making competent cells is **Calcium (Ca²⁺)**.

4. **Calcium Chloride**: In practice, Calcium is often used in the form of **Calcium Chloride (CaCl₂)**. This salt is used in the preparation of competent cells.

5. **Mechanism of Action**: The presence of calcium ions helps to stabilize the negatively charged DNA and the bacterial cell membrane, facilitating the uptake of foreign DNA into the cells.

6. **Conclusion**: Therefore, the divalent cation typically used to make competent cells is Calcium, specifically in the form of Calcium Chloride.

### Final Answer:
The divalent cation usually used to make competent cells is Calcium (Ca²⁺), commonly in the form of Calcium Chloride (CaCl₂).