Question

Identify and explain steps ‘A’, ‘B’ and ‘C’ in the PCR diagram given below.

Answer 1

Steps involved in PCR:

Polymerase chain reaction (PCR) is a reaction carried in vitro to amplify the DNA or gene of interest and create its multiple copies. It involves three steps - denaturation (A), annealing (B) and extension (C).



Step A :

Step A is denaturation:
In denaturation, hydrogen bonds between the two strands of a double stranded DNA melts. This is done to separate both the DNA strands at higher temperatures such as$94$ - ${95}^{\circ }$C. Each separated strand acts as a template for the formation of new strands.

Step B

Step B is annealing:
In this step, synthetic oligonucleotides or primers bind to their complementary region, usually the gene of interest to be amplified. This process is carried out at $50$ - ${56}^{\circ }$C.

Step C

Step C is extension:
In this step, Taq DNA polymerase synthesises DNA by adding dNTPs to the free ends of the primers - forward and reverse. The extension of DNA is carried out at${72}^{\circ }$C. This helps in synthesising complementary strands.