Figure 1. Polymerase chain reaction involves repeated cycles of incubation at three temperatures. The reaction is initially heated to above 90°C. At this temperature, the double-stranded DNA template is denatured and becomes single-stranded. The temperature is next reduced to 45–65°C. At this temperature, the oligonucleotide primers bind to their target sequences on the single-stranded template. The temperature is then raised to 72°C and the Taq DNA polymerase begins to synthesize a new DNA strand complementary to the template strand beginning at the primers. Further cycles of incubation result in the synthesis of increasing amounts of the target sequence.