Polymerase chain reaction and agarose gel electrophoresis of DNA

Electrophoresis run (blank) without PCR product, only control ('ladder') DNA.

Primer-dimers; note everything is less than 100 bp in size

Monitoring the progress of electrophoresis with bromophenol blue, which migrates at the same rate as a 150-300 bp DNA fragment

Seen on an UV transilluminator (blue band is bromophenol)

Often, one is not sure of the optimal temperature of a primer to anneal; in which case, trial and error at several temperatures is done. Modern thermocyclers allow different temperature for each row of wells (gradient PCR). The temperature with most product formation (thickest band) is selected.