A simple mathematical model based on step growth processes to describe the polymerase chain reaction (PCR) is introduced. It has been used to analyze different reaction conditions and to predict the distribution of the chain lengths of the PCR products. The model suggests that choosing the initial concentrations in such a way that the primers are the limiting reagent in the reaction will produce narrow chain length distributions around the target length, whereas conditions in which the yield of the reaction is limited through the concentration of the nucleotides produce shorter side products, and thus, a wider distribution. Furthermore, the model proposes that the time that should be spent in each cycle is mainly determined by the enzyme concentration. Experiments were carried out to confirm the predictions made by the model.