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

Concept: Polymerase Chain Reaction (PCR) is a molecular biology technique used to amplify a specific segment of DNA. It allows scientists to produce millions of copies of a particular DNA sequence in a short period of time. PCR is widely used in:

• Genetic research
• Disease diagnosis
• Forensic science
• DNA fingerprinting

The PCR process consists of a repeating cycle of three main steps that occur at different temperatures. These steps are Denaturation, Annealing, and Extension.
Step 1: Denaturation} During denaturation, the double-stranded DNA is heated to about \(94^\circ C\)–\(98^\circ C\). At this high temperature, the hydrogen bonds between the complementary DNA strands break, causing the DNA to separate into two single strands. \[ \text{Double-stranded DNA} \rightarrow \text{Two single strands} \]
Step 2: Annealing} In this step, the temperature is lowered to about \(50^\circ C\)–\(65^\circ C\). Short DNA sequences called primers bind (anneal) to their complementary sequences on the single-stranded DNA templates. These primers provide a starting point for DNA synthesis.
Step 3: Extension (Elongation)} During extension, the temperature is raised to about \(72^\circ C\). At this temperature, the enzyme Taq DNA polymerase adds nucleotides to the primers and synthesizes a new complementary DNA strand. This results in the formation of new double-stranded DNA molecules.
Step 4: Correct order of PCR steps} The PCR cycle always follows this sequence: \[ \text{Denaturation} \rightarrow \text{Annealing} \rightarrow \text{Extension} \] This cycle is repeated many times (usually 25–35 cycles) to amplify the target DNA.
Step 5: Evaluating the options}

• Annealing $\rightarrow$ Denaturation $\rightarrow$ Extension – Incorrect sequence
• Denaturation $\rightarrow$ Annealing $\rightarrow$ Extension – Correct sequence
• Extension $\rightarrow$ Denaturation $\rightarrow$ Annealing – Incorrect
• Denaturation $\rightarrow$ Extension $\rightarrow$ Annealing – Incorrect

Thus, the correct sequence is: \[ \text{Denaturation} \rightarrow \text{Annealing} \rightarrow \text{Extension} \]