In PCR, which step involves DNA polymerase synthesizing a new strand?

In PCR, which step involves DNA polymerase synthesizing a new strand?

• A. the extension step at elevated temperature where DNA polymerase adds nucleotides to primers
• B. the denaturation step at high temperature to separate strands
• C. the annealing step at moderate temperature when primers bind
• D. gel electrophoresis step to separate DNA fragments

Answer: (A)

During the extension (elongation) step, DNA polymerase synthesizes a new strand. After the strands are separated in denaturation and primers have annealed in the annealing step, the polymerase extends from the 3' ends of the primers, adding nucleotides to build the new strand in the 5'→3' direction (typically at around 72°C with a thermostable enzyme like Taq polymerase). Gel electrophoresis is used afterward to check the amplified products, not during synthesis.

During the extension (elongation) step, DNA polymerase synthesizes a new strand. After the strands are separated in denaturation and primers have annealed in the annealing step, the polymerase extends from the 3' ends of the primers, adding nucleotides to build the new strand in the 5'→3' direction (typically at around 72°C with a thermostable enzyme like Taq polymerase). Gel electrophoresis is used afterward to check the amplified products, not during synthesis.