PCR in

The Polymerase Chain Reaction (PCR) in detail

    The first two rounds of PCR do not generate short DNA products corresponding to the interval between the primers.  Note that the first round of PCR in step (c) generates 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. 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.

Figure 2002 by Griffiths et al. ; text 2013 by Steven M. Carr