Identify the step involved in polymerase chain reaction from the give…

The correct answer is Option (4) → Annealing

The figure shows the two separated DNA strands, each with a short primer bound to its complementary region. PCR repeats three steps in every cycle: denaturation, annealing and extension.

Option (1) Primer extension — Incorrect. In extension, Taq polymerase lengthens the primers by adding nucleotides along the template. The figure shows only the short primers, not new strands.

Option (2) Denaturation — Incorrect. Denaturation is the first step: heat separates the two strands of the DNA. No primers are bound yet.

Option (3) Extension — Incorrect. This is the same step as primer extension, the third step of the cycle.

Option (4) Annealing — Correct. On cooling, the two primers pair with their complementary sequences on the single strands. This is what the figure shows.

NCERT Extract:
Amplification of Gene of Interest Using PCR – “… PCR stands for Polymerase Chain Reaction. In this reaction, multiple copies of the gene (or DNA) of interest is synthesised in vitro using two sets of primers (small chemically synthesised oligonucleotides that are complementary to the regions of DNA) and the enzyme DNA polymerase.”