Question

Arrange steps of PCR:
A. Extension
B. Denaturation
C. Annealing

• A. B $\rightarrow$ C $\rightarrow$ A
• B. C $\rightarrow$ B $\rightarrow$ A
• C. B $\rightarrow$ A $\rightarrow$ C
• D. A $\rightarrow$ C $\rightarrow$ B

Hint:

Think of the process logically: you must first separate the strands (Denature), then attach the starting blocks (Anneal primers), and finally build the rest of the wall (Extend).

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
The Polymerase Chain Reaction (PCR) is an in vitro method used to exponentially amplify a specific DNA sequence.
It relies on thermal cycling, which consists of repeated cycles of heating and cooling to facilitate different temperature-dependent enzymatic reactions.

Step 2: Detailed Explanation:
A single cycle of PCR always proceeds through three main temperature steps in a specific order:
1. Denaturation (B): The reaction mixture is heated to a high temperature (usually 94-98$^\circ$C).
This high heat breaks the hydrogen bonds between the two strands of the template DNA, yielding single-stranded DNA required for the next step.
2. Annealing (C): The temperature is then lowered (usually to 50-65$^\circ$C).
This allows short, single-stranded DNA primers to naturally bind (anneal) to their complementary sequences on the single-stranded template DNA.
3. Extension/Elongation (A): Finally, the temperature is raised to the optimal working temperature of the thermostable DNA polymerase (typically 72$^\circ$C for Taq polymerase).
The enzyme binds to the primer-template junction and synthesizes a new DNA strand complementary to the template.

Step 3: Final Answer:
The correct chronological sequence is Denaturation $\rightarrow$ Annealing $\rightarrow$ Extension.
This translates to the sequence B $\rightarrow$ C $\rightarrow$ A, which corresponds to option (A).