During PCR, which statement best describes the extension step?

• A. DNA polymerase adds nucleotides to extend the new strand.
• B. Primers anneal.
• C. DNA is ligated.
• D. DNA strands separate.

Answer: (A)

During the extension step, DNA polymerase uses the primers as starting points and adds nucleotides to synthesize the new DNA strand, extending from the 3' end of each primer. This happens at an temperature that optimizes the enzyme’s activity (often around 72°C for Taq polymerase). The enzyme reads the template strand and incorporates complementary nucleotides, building the new strand in the 5' to 3' direction. As a result, the region between the two primers is copied, producing a double-stranded product that includes the newly synthesized strand. The other steps—denaturation (separating strands) and primer annealing (binding primers)—and ligation (joining fragments) do not describe extension.