Sigmoidal modeling of PCR amplification reveals symmetry between amplicon accumulation and loss in amplification efficiency. (A) Plots of the sigmoid functions describing amplicon accumulation (equation 1, solid line) and cycle efficiency (equation 2, dashed line) illustrate a symmetrical relationship, where C1/2 defines the fractional cycle when both reach half of their respective maxima. F0 is target quantity expressed in fluorescence units, Emax is the maximal amplification efficiency and Fmax is the maximal reaction fluorescence. (B) Plotting cycle efficiency against reaction fluorescence produces a line, predicting that the progressive loss in cycle efficiency is linearly coupled to amplicon accumulation. This generates a linear representation of PCR amplification (referred to as the "LRE plot"), where the Y intercept defines the maximal amplification efficiency (Emax) when FC = 0 and the X intercept defines the end-point or plateau phase (Fmax) when EC = 0, with the slope defining the rate of loss in cycle efficiency (ΔE) in relation to amplicon DNA quantity as reflected by reaction fluorescence.
Rutledge and Stewart BMC Biotechnology 2008 8:47 doi:10.1186/1472-6750-8-47