The PCR detector uses temperature to denature DNA, and uses a controlled endonuclease to break the double stranded DNA. Under the action of polymerase, the single stranded DNA is transformed into a double stranded DNA, thereby achieving the purpose of gene replication. The PCR reaction consists of three main steps;
They are Denaturation, Annealing of primers，Extension of primers。  The so-called denaturation is the process of heating (to 90-95 ℃) DNA to denature it, converting double stranded DNA into single stranded DNA as a template for replication. And Annealing allows Primers to adhere to both ends of the template DNA at certain temperatures (cooled to 55-60 ℃). Afterwards, the extension of primers and the synthesis of another strand were carried out under the action of DNA polymerase e.g. Taq polymerase (heated to 70-75 ℃).
The early concept of PCR in nucleic acid research has a history of more than 100 years. In the late 1960s and early 1970s, people devoted themselves to studying the in vitro isolation skills of genes. In 1971, Korana proposed the idea of nucleic acid amplification in vitro: "After DNA denaturation, hybridization with suitable primers, extension of primers with DNA polymerase, and continuous repetition of the process, tRNA genes can be cloned.
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