Polymerase chain reaction, or PCR, is a method used to increase the number of copies of a single DNA sequence. Four main components are needed to carry out a PCR reaction: the target DNA, two short DNA sequences called primers, an enzyme called a DNA polymerase, and a machine called a thermocycler. The first of these components, the target DNA, can be any number of DNA sequences, including genomic DNA or complementary DNA (cDNA), which is DNA generated from messanger RNA (mRNA) through Reverse Transcription, depending on what the final product is needed for (Cloning, SNP analysis, quantitative PCR). The primers are short DNA sequences, around 20 bases long, that are chemically synthesized to be complementary to each end of the target DNA sequence. The DNA polymerase added to the PCR reaction is capable of using a single stranded DNA sequence as a template on which it builds a complimentary DNA sequence, generating a double stranded DNA molecule. The polymerases used in PCR are stable at high temperatures, the importance of which will be discussed shortly. The final requirement for a PCR reaction is the PCR machine itself, known as a thermocycler. This machine is designed to rapidly change the temperature of the PCR reaction mixture. These changes carried out by the thermocycler make up the PCR cycle (Figure), with different events happening at each step. The first step, DNA melting, occurs at ~90°C, which results in the double stranded DNA splitting into two single stranded molecules. The temperature is then lowered to ~60°C to allow for complementary DNA molecules to anneal together and form a double stranded molecule. In the PCR reaction, however, the target DNA strands to not come together, but rather a target DNA strand and the complementary primer; this is because the primers are present in excess and are more available during the annealing step. After annealing, the thermocycler raises the temperature to ~70°C, where Taq polymerase works optimally. The polymerase binds to the double stranded DNA segment generated by the DNA target and primer, and generates a new DNA strand using the single stranded DNA target as a template. The result of this extension step is a new double stranded target sequence. The cycle is then reinitiated by raising the temperature to 90°C to melt the new double stranded DNA, and the subsequent steps are repeated. This results in a doubling of the target DNA molecule with each PCR cycle, exponentially increasing its amount.