The polymerase extends the primers in the 5' to 3' direction. Search for free COVID19 testing locations that are convenient for you and schedule an appointment.Efficient amplification of such short products does not need any extension time; it also needs no annealing time and no denaturing time. You make a mistake in programming the PCR machine and set the extension time to 30 seconds instead of 1 minute. A 1 min extension is typically sufficient to synthesize PCR fragments up to 2 kilobases (kb). Repeat steps 2-4 25-30 times. Final Extension for 5 minutes at 72°C: A final extension to fill-in any protruding ends of the newly synthesized strands.