In PCR, which step involves the DNA polymerase extending the primers to synthesize new DNA strands?

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

Answer: (C)

The step where DNA polymerase extends the primers to synthesize new DNA strands is the extension phase. After the primers have bound to their complementary sequences during the annealing step, the polymerase adds nucleotides to the 3’ end of each primer, building a new strand in the 5’ to 3’ direction using the original strand as a template. This is where the actual synthesis of new DNA occurs, producing a longer double-stranded molecule that contains the target region.

Denaturation is the heating step that separates the DNA strands, making them single-stranded for primer binding. Annealing is when the primers attach to their complementary sequences on the template. Transcription is not part of PCR; it’s the process of synthesizing RNA from a DNA template, which PCR does not perform.