Question

What is the correct sequence of steps in a PCR (Polymerase Chain Reaction) cycle?

• A. Annealing $\rightarrow$ Denaturation $\rightarrow$ Extension
• B. Denaturation $\rightarrow$ Annealing $\rightarrow$ Extension
• C. Extension $\rightarrow$ Denaturation $\rightarrow$ Annealing
• D. Denaturation $\rightarrow$ Extension $\rightarrow$ Annealing

Hint:

Remember the PCR sequence using the mnemonic: \textbf{D-A-E}

• \textbf{D}enaturation
• \textbf{A}nnealing
• \textbf{E}xtension

Typical PCR temperatures:

• Denaturation: \(94^\circ C\)–\(98^\circ C\)
• Annealing: \(50^\circ C\)–\(65^\circ C\)
• Extension: \(72^\circ C\)

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
Polymerase Chain Reaction (PCR) is an in vitro technique used to rapidly amplify specific DNA sequences.
The process relies on thermal cycling, consisting of repeated cycles of heating and cooling to facilitate DNA replication.
Step 2: Key Approach:
Recall the three fundamental temperature-dependent phases that make up a single cycle of PCR.
Step 3: Detailed Explanation:
The first step is Denaturation, where the reaction mixture is heated to around \(94^{\circ}\text{C} - 98^{\circ}\text{C}\).
This high heat breaks the hydrogen bonds between bases, separating the double-stranded DNA into single strands.
The second step is Annealing, where the temperature is lowered to \(50^{\circ}\text{C} - 65^{\circ}\text{C}\).
This cooling allows the short, specific DNA primers to bind (anneal) to their complementary sequences on the single-stranded DNA template.
The third step is Extension (or Elongation), where the temperature is raised to \(72^{\circ}\text{C}\).
At this optimal temperature, the Taq polymerase enzyme synthesizes a new DNA strand by adding nucleotides to the primers.
This sequence is repeated to exponentially amplify the DNA.
Step 4: Final Answer:
The correct sequential order is Denaturation, followed by Annealing, and then Extension.