PCR (polymerase chain reaction)

Polymerase chain reaction is a method of amplifying or generating many copies of a target region of DNA. PCR reactions are normally carried out in a thermocycler which can rapidly change the temperature of the reaction which take place in 3 steps repeated many times. High temperature is applied to double-stranded DNA to separate the two strands, a process called denaturation. Lowering the temperature allows primers to bind to the template which is called the annealing step. Primers are short nucleotide sequences, single-stranded DNA, that are complementary to a region on the template DNA. In DNA adenine pairs with thymine (A-T) and cytosine pairs with guanine (C-G). Next is the extension phase where the DNA polymerase adds nucleotides onto the 3’-OH at the end of the primer. From the primer the nucleotide sequence extends as DNA polymerase adds nucleotides are complementary to the template. PCR reactions are exponential and after 35 cycles 2³⁶ copies are produced. Reverse transcription PCR (RT-PCR) is a version of PCR that detects RNA by starting with a reverse transcription step to convert RNA into a DNA copy (cDNA), followed by PCR amplification of the cDNA.