In the nearly two decades since its discovery, the polymerase chain reaction (PCR) and its derivative techniques have revolutionized molecular biology. The exponential nature of the technique allows one, in theory, to calculate the amount of starting material from the amount of product at any point in the reaction. In practice, however, reaction conditions can interfere with the exponential amplification and affect product concentration. Standard PCR works best, therefore, as a qualitative technique. But many researchers want to use PCR quantitatively (Q-PCR), to measure gene expression or copy number, or to calculate viral titer, for example.
Early attempts at quantitation involved stopping PCRs at various points to generate standard curves--a laborious, low-throughput process. Modern techniques are much friendlier, adding real-time reaction monitoring with dedicated instruments and fluorescent probes. Because these techniques combine amplification, detection, and quantitation in a single step, they are considerably faster than endpoint-driven analyses. And, scientists can ...
