Question

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

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

Answer 1

The Correct Option is A

The Polymerase Chain Reaction (PCR) is a molecular biology technique used to amplify DNA sequences. It involves several steps to achieve this amplification. The correct sequence of steps in a PCR is crucial for its success:

1. Denaturation: This step involves heating the reaction mixture to around 94-98°C. The high temperature causes the double-stranded DNA to denature, breaking the hydrogen bonds between the strands and resulting in the separation of these two strands.
2. Annealing: After denaturation, the temperature is lowered to 50-65°C. This allows the primers to bind or anneal to their complementary sequences on the template DNA. The exact temperature can vary depending on the primers used.
3. Extension: In this final step, the reaction temperature is raised to 72°C. This is the optimal temperature for the enzyme Taq polymerase, which synthesizes the new strand of DNA by adding nucleotides to the primer, thus extending the DNA sequence.

Given the options provided, the correct sequence of steps in a PCR is Denaturation, Annealing, Extension. This sequence ensures that DNA is amplified correctly and efficiently.

Let us analyze why the other sequences are incorrect:

• Denaturation, Extension, Annealing: Extension cannot occur before annealing, as there would be no primers bound to the DNA for extension to occur.
• Extension, Denaturation, Annealing: Extension cannot start before denaturation because the DNA needs to be separated into single strands, and primers need to anneal before any replication.
• Annealing, Denaturation, Extension: Annealing must follow denaturation, otherwise, the primers will not have access to the single-stranded DNA templates.

Understanding the sequential flow of PCR is essential for the amplification of the target DNA regions. Selecting the correct sequence ensures the method's efficiency and accuracy.