Question

Which of the following chemical is used to make E. coli cells competent to take external DNA?

• A. Sodium acetate
• B. SDS detergent
• C. $\text{CaCl}_2$
• D. $\text{NaOH}$

Hint:

The divalent $\text{Ca}^{2+}$ ions from cold $\text{CaCl}_2$ act as electrostatic bridges, neutralizing negative charges on both the DNA and the bacterial cell wall so they can bind together before heat-shock.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Bacterial transformation is the process by which bacterial cells take up foreign DNA from their surroundings.
Under normal conditions, *Escherichia coli* cells are not naturally competent to take up DNA because their cell walls and membranes have a net negative charge that repels negatively charged DNA molecules.
Therefore, they must be treated chemically to become "competent."

Step 2: Detailed Explanation:
The standard chemical method to induce competence in *E. coli* involves treatment with a cold divalent cation solution, typically calcium chloride ($\text{CaCl}_2$):
- The divalent calcium ions ($\text{Ca}^{2+}$) neutralize the electrostatic repulsion between the negatively charged phosphate groups of the foreign DNA and the lipopolysaccharides of the bacterial cell membrane.
- Keeping the cells in a cold $\text{CaCl}_2$ solution stabilizes the membrane structure and promotes DNA binding to the cell surface.
- Subsequent rapid heat-shock treatment (at $42^\circ\text{C}$ for 45–90 seconds) temporarily alters the membrane permeability, creating thermal pores that allow the bound DNA to enter the bacterial cell.
Let us review the incorrect options:
- Sodium acetate: Commonly used in DNA precipitation protocols, not for inducing competence.
- SDS (Sodium Dodecyl Sulfate): An anionic detergent that lyses bacterial cells, which would kill them.
- NaOH: A strong base used to denature DNA during plasmid isolation, not for transformation.
Therefore, $\text{CaCl}_2$ is the correct chemical.

Step 3: Final Answer:
The chemical used to make *E. coli* cells competent is $\text{CaCl}_2$.