Which of the following is a correct sequence of steps in a PCR (Polymerase Chain Reaction) ?

The correct sequence of steps in a Polymerase Chain Reaction (PCR) is Denaturation, Annealing, Extension.

PCR is a widely used laboratory technique designed to make millions of copies of a specific DNA segment. The process consists of a series of temperature-controlled cycles, each involving three fundamental steps:

1. Denaturation: This first step involves heating the reaction mixture to a high temperature, typically around 94-98 °C. The high heat breaks the hydrogen bonds between the complementary base pairs of the double-stranded DNA template, separating it into two single strands. These single strands will serve as templates for the next steps.

2. Annealing: In the second step, the temperature is lowered, usually to about 50-65 °C. This lower temperature allows short DNA sequences called primers to bind (anneal) to their complementary targets on the single-stranded DNA templates. Two primers are used (one for each strand) to define the specific region of DNA to be amplified.

3. Extension (or Elongation): In the final step of the cycle, the temperature is raised to an optimal level for the DNA polymerase enzyme, typically around 72 °C. The DNA polymerase (most commonly Taq polymerase, which is heat-stable) synthesizes a new DNA strand complementary to the template strand by adding free nucleotides (dNTPs) starting from the primers.

By repeating these three steps (denaturation, annealing, and extension) for multiple cycles (often 25 to 35 times), the target DNA sequence is amplified exponentially.