Question

The figure below shows three steps $(A,B,C)$ of Polymerase Chain Reaction (PCR). Select the option giving correct identification together with what it represents?

• A. B - denaturation at a temperature of about ${98}^0$C separating the two DNA strands.
• B. A - denaturation at a temperature of about ${50}^0$C
• C. C - extension in the presence of heat stable DNA polymerase.
• D. A - annealing with two sets of primers.

Answer 1

The correct option is C C - extension in the presence of heat stable DNA polymerase.

The key ingredients of a PCR reaction are Taq polymerase, primers, template DNA, and nucleotides (DNA building blocks). The ingredients are assembled in a tube, along with cofactors needed by the enzyme, and are put through repeated cycles of heating and cooling that allow DNA to be synthesized.

The basic steps are:-

1. Denaturation (96°C): Heat the reaction strongly to separate, or denature, the DNA strands. This provides single-stranded template for the next step.
   
2. Annealing (55 - 65°C): Cool the reaction so the primers can bind to their complementary sequences on the single-stranded template DNA.
   
3. Extension (72°C): Raise the reaction temperatures so Taq polymerase extends the primers, synthesizing new strands of DNA.
   

This cycle repeats 25- 35 times in a typical PCR reaction, which generally takes 22 - 44 hours, depending on the length of the DNA region being copied. If the reaction is efficient (works well), the target region can go from just one or a few copies to billions.

So the correct option is 'C - extension in the presence of heat stable DNA polymerase'.