Question

Which of the following is a correct sequence of steps in a PCR (Polymerase Chain Reaction)?

• A. Annealing, Denaturation, Extension
• B. Denaturation, Annealing, Extension
• C. Denaturation, Extension, Annealing
• D. Extension, Denaturation, Annealing

Answer 1

The Correct Option is B

Polymerase Chain Reaction (PCR) is a widely used technique in molecular biology to amplify DNA sequences. This process involves three main steps: Denaturation, Annealing, and Extension. Below is the correct sequence of steps involved in PCR along with explanations:

1. Denaturation: In the first step, the double-stranded DNA is heated to a high temperature of about 94-98°C. This high heat causes the hydrogen bonds between the DNA strands to break, resulting in the separation of the double helix into two individual strands. This separation is necessary for the primers to bind to the DNA strands in the next step.
2. Annealing: After denaturation, the temperature is lowered to about 50-65°C. During this step, short DNA sequences known as primers, which are complementary to the target DNA region, bind (or anneal) to their specific sites on the separated DNA strands. Annealing temperature is crucial as it ensures the specificity of primers binding to the template DNA.
3. Extension: In this step, the temperature is adjusted to approximately 72°C, which is optimal for the DNA polymerase enzyme to work. The enzyme extends the primers by adding nucleotides to the 3’ ends of each primer, effectively synthesizing new strands of DNA complementary to the original template strands.

Hence, the correct sequence that represents the steps of a PCR cycle is: Denaturation, Annealing, Extension. This sequence helps to exponentially amplify the desired DNA fragments.