PCR or the polymerase chain reaction is a common technique in genetic engineering. This method is used to amplify a gene of interest in vectors such as plasmids using a series of reactions. The first step is denaturation which is around 90°C to make the DNA single-stranded. Once this happens, the primers anneal to the complementary sequences as the temperature drops to approximately 55°C. 55°C is a near-universal temperature but the annealing temperature typically varies between primers. The primers bind the appropriate ends of complementary strands that will be amplified. At approximately 72°C, DNA polymerase then generates the complementary strand of the primed DNA. This creates a new double stranded DNA molecule. The most commonly used polymerase is Taq-polymerase. A single cycle of PCR results in twice as much DNA as the original DNA concentration. When researchers run PCR, they typically use a thermocycler to automate the temperature changes and run around 30 cycles to achieve the desired amount of amplification. There are many applications for PCR including creating site-directed mutagenesis or simply amplifying a sample for analysis.