Screening Colonies by Polymerase Chain Reaction (PCR).

Polymerase chain reaction (PCR), rather than the more labor-intensive minipreps, is often used to screen colonies of Escherichia coli for recombinant plasmids containing sequences of interest. When screening colonies for a DNA fragment amplified by PCR, the same primers used in the amplification reaction can be used for screening. To discover the orientation of the cloned fragment, a third primer can be used that binds to the plasmid sequences immediately adjacent to the insertion site and points toward the insert.