The exponential phase of amplification occurs in the PCR cycles when reaction components are still in excess and the PCR products are accumulating at a constant rate.

To determine exponential range:
A PCR master mix was prepared including 10 µl [-³²P]dCTP in addition to the normal reaction components. The master mix was split into 10 aliquots which were then subjected to PCR. Aliquots were removed from the thermal cycler at the indicated cycle numbers and resolved by electrophoresis on a 5% polyacrylamide/ urea gel. The products were quantitated with a Bio-Rad Molecular Imager. Cycle number is plotted against the log of the signal and a straight line is obtained for samples in the exponential phase of amplification.