The given schematic illustration shows three steps 'P', 'Q' and 'R' of the polymerase chain reaction. Which of the following statements are correct with reference to the illustration given above ?

(i) Step 'P' is showing denaturation at low temperature.

(ii) Step 'Q' is a denaturation of DNA strand at high temperature, followed by annealing.

(iii) Step 'R' is an extension of DNA in presence of thermostable DNA polymerase.

(iv) Step 'Q' is extension with two sets of primers.

The correct option is (ii) and (iii) only.

Polymerase Chain Reaction (PCR) is a laboratory technique used to amplify a specific segment of DNA through repeated cycles of heating and cooling. The process involves three primary steps: denaturation, annealing, and extension.
Let us analyze the illustration and evaluate each statement:

1. Step 'P' (Denaturation):
The schematic shows Step 'P' with a double-stranded DNA template containing complementary strands with 5' and 3' terminals. In PCR, denaturation is the first step where this double-stranded DNA is separated into two single strands by heating it to a high temperature (typically 94°C–96°C) to break the hydrogen bonds between base pairs. Therefore, statement (i), which states that denaturation occurs at a low temperature, is incorrect.

2. Step 'Q' (Annealing):
In Step 'Q', the two template strands are shown separated (running from 5' to 3' and 3' to 5' directions). Two short primer sequences are shown bound (annealed) to their complementary bases at the 3' ends of the template strands. This step occurs after the high-temperature denaturation, where the temperature is lowered (usually to 50°C–65°C) to allow primers to bind. Thus, statement (ii) is correct, while statement (iv) is incorrect because Step 'Q' represents annealing, not extension.

3. Step 'R' (Extension):
In Step 'R', the primers are extended along the template strands, shown by the longer shaded blocks representing the newly synthesized DNA. A thermostable DNA polymerase (such as Taq polymerase) adds nucleotides to the 3' end of each primer in the 5' to 3' direction. This step occurs at a temperature of around 72°C. Therefore, statement (iii) is correct.

By compiling these observations, statements (ii) and (iii) are correct, which matches the option (ii) and (iii) only.