PCR stands for Polymerase Chain Reaction. Polymerase chain reaction is a molecular biology technique used to amplify specific DNA (or RNA) fragments. It is mainly used in medicine for bacterial detection, identification of viral diseases, tumor diagnosis, and at the forensic forensics level. The polymerase chain reaction can be roughly composed of three basic reaction steps: denaturation-annealing-extension: 1. Denaturation: refers to the denaturation of the biological template DNA template, DNA in the heating to 94 ℃ about a certain period of time, the DNA double-stranded or PCR increase in the formation of double-stranded DNA will be dissociated, so that it becomes a single-stranded, so that it combines with the primer, in order to prepare for the next round of reaction. 2. Reactivation is also called annealing: that is, after the template DNA is denatured into a single strand by heating, the temperature is lowered to about 55 ℃, and the primer and the template DNA single-stranded complementary sequences are paired and combined. 3. Primer extension: Under the action of DNA template-primer conjugate enzyme, according to the principle of base pairing and half-conserved replication, synthesize a new half-conserved replicated strand which is complementary to the template DNA strand. After repeated cycles of denaturation-annealing-extension process, more semi-conserved replicated strands can be obtained, and this new strand can become the template for the next cycle. 2-3 hours can amplify the target genes to be amplified by millions of times.