The first round of PCR in step (c) generates two long fragments that run off the 5' ends of the target region. In the second round in step (e), extension again generate variable length strands. Thus, the first two rounds of PCR do not generate short DNA products corresponding to the interval between the primers. Only in the third round in step (g) are  'unit length' products generated uniformly for the first time.

    Compare the process illustrated in this diagram with that in the simplified diagram., which shows the process as it works beginning in the fourth round of amplification.