Figure 1.

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 C_1/2 defines the fractional cycle when both reach half of their respective maxima. F₀ is target quantity expressed in fluorescence units, E_max is the maximal amplification efficiency and F_max 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 (E_max) when F_C = 0 and the X intercept defines the end-point or plateau phase (F_max) when E_C = 0, with the slope defining the rate of loss in cycle efficiency (ΔE) in relation to amplicon DNA quantity as reflected by reaction fluorescence.