Question

During recombinant DNA technology, why are bacterial cells kept on ice after calcium chloride treatment?

• A. To destroy plasmid DNA
• B. To reduce membrane permeability permanently
• C. To facilitate binding of DNA to cell membrane
• D. To stop bacterial respiration completely

Hint:

In biotechnology: \[ \text{Ice} \rightarrow \text{DNA attachment} \] \[ \text{Heat shock} \rightarrow \text{DNA entry} \]

Answer 1

The Correct Option is C

Concept: In recombinant DNA technology, bacterial cells are made competent so they can take up foreign DNA molecules such as plasmids.

Step 1: Role of calcium chloride treatment.
Bacterial cell membrane and DNA are both negatively charged, so DNA cannot easily enter the cell naturally. Treatment with chilled calcium chloride:

• Neutralizes negative charges
• Increases competency of cells
• Helps DNA approach the membrane

Step 2: Importance of ice-cold conditions.
Keeping cells on ice stabilizes the membrane and allows plasmid DNA to attach properly to the bacterial surface. Cold temperature also prepares cells for the next step known as: \[ \text{Heat Shock} \]

Step 3: Effect of heat shock.
A sudden increase in temperature creates pores in the membrane through which plasmid DNA enters the bacterial cell.

Step 4: Selecting the correct option.
Therefore, cells are kept on ice mainly to facilitate binding of DNA to the cell membrane.