Polymerase Chain Reaction (PCR)
The PCR is an enzymatic method that can generate a billion identical molecules (amplification) of a desired segment of the DNA within about two hours. A typical PCR-reaction comprises 35 to 50 amplification cycles, whereby each cycle consists of three successive steps (see animation) .
Denaturation
The reaction mixture with all the components necessary to synthesise new DNA is heated to about 95°C. This denatures the double-stranded DNA, or in other words dismantles it into two individual strands.
Annealing
Primers (short, single-stranded DNA-sequences) bind highly specifically at defined sections of the denatured DNA – provided they are contained in the sample to start with. A fundamental condition for producing specific primers ist he availibility of DNA-sequence information. In other words it must be known what sequences are typical for certain species, orders, classes etc. In many cases such information can be called up in international data bases.
Elongation
In this step the enzyme – the polymerase – recognises the double stranded DNA-segment formed by the denatured target-DNA and the primer binding to it. The new DNA strand is synthesised from this starting point, whereby the complementary individual strand serves as a template.