PCR amplifies DNA through thermal cycling, but understanding the exact sequence of steps in the first cycle is crucial for NEET molecular biology success.

The Question

In a PCR, the reaction tubes are incubated first at 94 °C for 3 minutes, at 58 °C for 1 minute and at 72 °C for 2 minutes. What is the sequence of processes happening inside the tubes containing DNA template, dNTPs and Taq DNA polymerase?

A. Denaturation, elongation, annealing
B. Annealing, denaturation, elongation
C. Denaturation, annealing, elongation
D. Annealing, elongation, denaturation

Correct Answer: Option (C) – Denaturation, annealing, elongation

The temperature progression 94°C → 58°C → 72°C directly matches PCR’s standard cycle: high heat first separates DNA strands (denaturation), moderate temperature allows primer binding (annealing), then optimal polymerase temperature builds new strands (elongation).

Detailed Explanation of All Options

• A. Denaturation, elongation, annealing
  Incorrect. Elongation at 72°C follows annealing—Taq polymerase needs primers bound first. This skips primer annealing entirely.

• B. Annealing, denaturation, elongation
  Incorrect. Annealing (58°C) can’t precede denaturation (94°C)—double-stranded DNA must separate before primers can bind to single strands.

• C. Denaturation, annealing, elongation
  Correct. 94°C/3min = denaturation (dsDNA → ssDNA); 58°C/1min = annealing (primers bind templates); 72°C/2min = elongation (Taq adds ~1000 nucleotides/min). Initial 3-min denaturation ensures complete strand separation.

• D. Annealing, elongation, denaturation
  Incorrect. Denaturation must be first; starting with annealing on intact dsDNA fails amplification.

PCR First Cycle Breakdown

This cycle repeats 25-35 times, yielding billions of copies. Master PCR sequencing for NEET biotech section!