Question

In order to force bacteria to take up the recombinant DNA, they must be made competent. Explain how it can be achieved.

Hint:

To make bacteria competent: 1. Neutralize charges using $Ca^{2+}$ ions on ice. 2. Force the DNA through membrane pores by shocking them at $42^\circ\text{C}$ for a few seconds. 3. Lock the DNA inside by putting them back on ice.

Answer 1

Concept: DNA is a hydrophilic, macromolecular polymer. Because its phosphate backbone contains numerous negatively charged oxygen groups, the DNA molecule carries a net negative electrical charge. Conversely, the bacterial cell wall and outer plasma membrane are composed of lipopolysaccharides and phospholipids, which also present a net negative charge facing the external aqueous environment. Due to electro-static repulsion, the negatively charged DNA cannot naturally traverse the hydrophobic core of the bacterial plasma membrane. To overcome this barrier, bacteria must be rendered biologically "competent," meaning their membrane permeability must be transiently altered to facilitate the uptake of foreign genetic material.

Step 1: Chemical Treatment with Divalent Cations ($Ca^{2+}$).
The first operational step involves treating exponentially growing bacterial cells (typically Escherichia coli) with a chilled solution of divalent cations, most commonly calcium chloride ($\text{CaCl}_2$):
• The positively charged divalent calcium ions ($Ca^{2+}$) interact in the aqueous environment to neutralize the negative charges.
• They act as an electrostatic bridge, binding simultaneously to the negatively charged phosphate groups of the recombinant DNA molecule and the negatively charged phosphate heads of the lipopolysaccharides in the outer bacterial membrane.
• This neutralization brings the plasmid DNA into close physical proximity with the bacterial cell envelope. Chilling the mixture on ice ($0^\circ\text{C}$) stabilizes the lipids within the cell membrane, making them more rigid and setting up the system for thermal disruption.

Step 2: The Thermal Heat-Shock Protocol.
Once the recombinant DNA has bound to the bacterial cell surface on ice, the mixture is subjected to a rapid, sudden thermal shift:
• Heat Shock application: The tube containing the cells and DNA is transferred directly from the ice bath into a water bath regulated precisely at $42^\circ\text{C}$ for a brief, timed duration of exactly $42$ seconds.
• Mechanism of Uptake: This sudden increase in temperature creates a thermal gradient across the bacterial cell wall, altering the fluidity of the membrane and generating transient, microscopic pores or structural fractures in the cellular envelope. The convective fluid movement dynamically sweeps the surface-bound recombinant DNA through these temporary pores into the bacterial cytoplasm.
• Recovery Phase: Immediately following the $42$-second heat window, the bacterial cells are placed back into the ice bath ($0^\circ\text{C}$) for $2$ minutes. This rapid cooling re-stabilizes the lipid bilayer, closes the temporary membrane pores, and traps the internalized recombinant DNA safely inside the cell. Following this recovery, the bacteria are incubated in a nutrient-rich liquid medium (like SOC or LB broth) at $37^\circ\text{C}$ for an hour to allow expression of antibiotic resistance marker genes before selection on selective agar plates.