Which of the following is used to make the bacterial cell as 'Competent cell' to take up DNA?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Calcium ion
DNA is hydrophilic and cannot pass through the cell membrane on its own. So the bacterial cells are first made ‘competent’ to take up DNA.
Option (1) Carbonate ion — Incorrect. It is not a divalent cation; it is not used for this.
Option (2) Calcium ion — Correct. Then the cells are heat-shocked (ice, 42°C, ice) so they take up the recombinant DNA.
Option (3) Sodium ion — Incorrect. Sodium is a monovalent ion; NCERT names a divalent cation such as calcium.
Option (4) Sulphate ion — Incorrect. Sulphate is a negative ion (anion), not a cation.
NCERT Extract:
Cloning Vectors – “… Recombinant DNA) Since DNA is a hydrophilic molecule, it cannot pass through cell membranes. Why? 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.”