The PCR reaction exploits the 5' nuclease activity of DNA Polymerase to cleave a TaqMan probe which anneals within a DNA region amplified by a specific set of primers during PCR. The TaqMan probe contains a reporter dye at the 5' end of the probe and a quencher dye at the 3' end. The quencher dye quenches the fluorescence emitted by reporter dye via FRET when the probe is intact. During the PCR, as the DNA polymerase extends the primer and synthesizes the nascent strand, the 5' to 3' exonuclease activity of the DNA polymerase degrades the probe that has annealed to the template. Degradation of the probe releases the fluorophore from the reporter and breaks the quenching effect of the quencher. Hence, fluorescence detected in the quantitative PCR thermal cycler is directly proportional to the fluorophore released and the amount of DNA template present in the PCR.