What are the typical primer length and the main steps of a PCR cycle?

• A. Primers ~18–25 nt; steps: denaturation (~95C), annealing (temperature ~Tm of primers), extension (~72C).
• B. Primers ~30–40 nt; steps: denaturation 60C, annealing 50C, extension 72C.
• C. Primers ~10–12 nt; steps: denaturation 100C, annealing 80C, extension 60C.
• D. Primers ~18–25 nt; steps: denaturation ~95C, annealing (temperature ~Tm of primers), extension (around 72C). Repeats amplify target.

Answer: (D)

PCR uses primers that are typically 18–25 nucleotides long. This length hits a balance between being long enough to bind specifically to the target sequence and short enough to bind efficiently under practical conditions. The cycle has three main steps: denaturation around 95°C to separate the DNA strands, annealing at a temperature near the primers’ melting temperature so the primers can bind to their complementary sequences, and extension around 72°C where the polymerase extends from the primers to synthesize new DNA. These steps are repeated many times, so the target region is amplified exponentially. The described combination reflects the standard primer length and the classic cycle temperatures, and it also explicitly acknowledges that the repeated cycles drive amplification.