Reaction cycle lengths, temperatures, and number of cycles, all have a critial role in determining how well a PCR will work. The initial heating step must be long enough to completely denature the template and cycles must be long enough to prevent melted DNA from reannealing to itself. Increased yield can be achieved by increasing the extension time about every 20 cycles, to compensate for less enzyme to amplify more template. Usually less than 40 cycles is enough to amplify less than 10 template molecules to a concentration large enough to view on an ethidium bromide-stained agarose gel.