Host bacterium is made competent to accept DNA by treating it with:

The correct answer is Option (3) → Divalent cation

DNA is a hydrophilic (water-loving) molecule, so it cannot pass through cell membranes by itself. The bacterial cells must first be made ‘competent’ to take up DNA, by treating them with a specific concentration of a divalent cation such as calcium.

Option (1) Prolonged heat shock — Incorrect. The heat shock is brief: cells are kept on ice with the DNA, placed at 42°C for a short time, then put back on ice. A long heat shock would kill them.

Option (2) Short exposure to cold temperature — Incorrect. Cold alone does not make the cells competent. The ice steps only go with the calcium treatment and the heat shock.

Option (3) Divalent cation — Correct. Calcium ions (Ca²⁺) increase the entry of DNA through pores in the cell wall.

Option (4) Monovalent anion — Incorrect. A negatively charged ion with one charge is not what is used; the treatment is with a doubly charged positive ion.

NCERT Extract:
Cloning Vectors – …