Given below are two statements: one is labelled as Assertion (A) and the other is labelled as Reason (R).
Assertion (A): In order to force bacteria to take up the plasmid, the bacterial cells must first be lysed.
Reason (R): DNA is a hydrophilic molecule, so it cannot pass through cell membranes.
In the light of the above statements, choose the most appropriate answer from the options given below:
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → (A) is not correct but (R) is correct.
To clone a gene, the recombinant plasmid has to enter a living bacterial cell, where it can multiply. A bacterium that can take up such DNA is called a competent cell. Cells are made competent by treating them with a specific concentration of a divalent cation such as calcium.
Assertion (A): In order to force bacteria to take up the plasmid, the bacterial cells must first be lysed. — False. The bacterial cells must first be made competent, not lysed. Lysis breaks the cell open; this is done (for example with lysozyme) when DNA is to be taken out of a cell. A lysed cell is dead, so it cannot take up the plasmid or multiply it. Changing “made competent” to “lysed” makes A false.
Reason (R): DNA is a hydrophilic molecule, so it cannot pass through cell membranes. — True. DNA is a hydrophilic molecule, so it cannot pass through cell membranes on its own. This is why the cells have to be made competent before they take up DNA.
NCERT Extract:
Competent Host (For Transformation with Recombinant DNA) – “Since DNA is a hydrophilic molecule, it cannot pass through cell membranes. … In order to force bacteria to take up the plasmid, the bacterial cells must first be made ‘competent’ to take up DNA. This is done by treating them with a specific concentration of a divalent cation, such as calcium, which increases the efficiency with which DNA enters the bacterium through pores in its cell wall.”