Question

Why and how can bacteria be made 'competent' ?

Hint:

Bacteria are made competent so they can take up recombinant DNA. Treat them with calcium ions, chill them with the DNA and give a heat shock at 42 degrees Celsius.

Answer 1

Step 1: Why bacteria must be made competent:
DNA is a large, charged, hydrophilic molecule. The bacterial cell membrane is hydrophobic, so DNA cannot pass through it on its own.
In recombinant DNA technology, the recombinant plasmid has to enter the host bacterium. So the host cell must first be made able to take up DNA. Such a cell is called competent.

Step 2: How competence is created, chemical treatment:
The bacterial cells are treated with a specific concentration of a divalent cation, such as calcium (\( \mathrm{Ca^{2+}} \)).
This treatment increases the efficiency of DNA entry through pores in the cell wall and membrane.

Step 3: How competence is created, heat shock:
The cells are first incubated with the recombinant DNA on ice.
They are then given a brief heat shock at \( 42^{\circ}\mathrm{C} \) for about 90 seconds, and put back on ice.
During the heat shock the bacterium takes up the recombinant DNA.

Final Answer:
Bacteria are made competent so that they can take up recombinant DNA. This is done with divalent cations like \( \mathrm{Ca^{2+}} \) followed by a heat shock.