A method in which recombinant DNAs are incubated with host cells on ice, followed by placing them briefly at `42^(@)`C and then putting them back on ice is called:

Options

A. Heat shock method
B. Tranfection
C. Biolistics
D. Gene gun

Answer

To solve the question, we need to identify the method described in the question. The method involves incubating recombinant DNAs with host cells on ice, then briefly heating them to 42°C, and finally returning them to ice. Let's break this down step by step:

### Step-by-Step Solution:

1. **Understanding the Process**: The method described involves a temperature shift that is crucial for introducing recombinant DNA into host cells. The initial incubation on ice helps stabilize the cells and the DNA.

2. **Heat Shock**: After the initial incubation, the cells are subjected to a brief heat shock at 42°C. This sudden increase in temperature creates a thermal imbalance, which induces the formation of pores in the cell membrane. This allows the recombinant DNA to enter the host cells.

3. **Return to Ice**: After the heat shock, the cells are returned to ice. This helps to stabilize the cells again and allows the pores to close, trapping the recombinant DNA inside the cells.

4. **Identifying the Method**: The entire process described is characteristic of the **heat shock method**, which is commonly used in molecular biology for transforming bacteria with plasmid DNA.

5. **Conclusion**: Therefore, the method in which recombinant DNAs are incubated with host cells on ice, followed by a brief exposure to 42°C, and then returned to ice is called the **heat shock method**.

### Final Answer:
The method is called the **heat shock method**.

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