In a PCR reaction, the primers are chosen such that they frame the desired sequence. This also determines the length of the resulting fragment, and the primers are included in the final PCR product. The figure to the right shows how a single strand is shortened to the desired PCR product length through several rounds of replication directed by the site specific primers.

In the figure below we follow a single strand and observe how the primer placement shortens the fragment length through multiple rounds of replication (three rounds are required to get the first double-stranded fragment where both strands are the length of the primers).

The length of a PCR product is determined by the placement of the primers.