Question

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

• A. Annealing, Denaturation, Extension
• B. Denaturation, Annealing, Extension
• C. Denaturation, Extension, Annealing
• D. Extension, Denaturation, Annealing

Answer 1

The correct option is B Denaturation, Annealing, Extension

PCR stands for Polymerase Chain Reaction.
It is a technique used to amplify DNA segments in vitro in a short time duration.
The reaction requires;

• two sets of primers (small chemically synthesised oligonucleotides that are complementary to the regions of DNA)
• dNTPs
• The enzyme DNA polymerase which is thermostable, isolated from a bacterium, Thermus aquaticus. The enzyme extends the primers using the nucleotides provided in the reaction and the genomic DNA as template.

Each cycle of PCR has three steps:

1. Denaturation - The double-stranded template DNA is heated to separate it into two single strands.

2. Annealing - The temperature is lowered to enable attachment of DNA primeres to the 3' ends of the separated template strands of DNA.

3. Extension - The temperateure is raised again to allow polymerisation of the new DNA strands by Taq DNA polymerase.
Fig: Steps of PCR