Question

Assertion: The bacterial cells must first be made ‘competent’ to take up foreign DNA.
Reason: DNA cannot pass through cell membranes since it is a hydrophilic molecule.
Select the correct option.

• A. Both the assertion and reason are true and the reason is the correct explanation of assertion
• B. Both the assertion and reason are true and reason is not the correct explanation of assertion
• C. Assertion is true but the reason is false
• D. Both the assertion and reason are false

Answer 1

The correct option is A Both the assertion and reason are true and the reason is the correct explanation of assertion

Cell competence refers to a cell's ability to take up foreign (extracellular) DNA from its surrounding environment. Bacterial cells are made competent in order to force them to take up foreign DNA. This is necessary because the cell membrane, being selectively permeable, allows only certain molecules to pass through it to maintain a consistent internal environment. The cell membrane is composed of a bilayer of phospholipids. The hydrophilic heads of the phospholipids are arranged on either side and interact with the external environment. The hydrophobic tails are arranged towards the inner side, to protect them from the surrounding environment. The DNA being a hydrophilic molecule finds it difficult to pass through the hydrophobic lipid bilayer of the host cell membrane.

In order to make the bacterial cells competent, they are first treated with a specific concentration of a divalent cation, such as calcium. It increases the efficiency with which DNA enters the bacterium through pores in its cell wall. This is followed by heat shock treatment. This involves incubation in ice followed by brief exposure to ${42}^o$C and transferring back into ice.