The Polymerase Chain Reaction (PCR)

Sequence of the PCR

The polymerase chain reaction (PCR) is an artificial method of amplifying DNA.

It finds practical application in paternity tests, genetic fingerprinting for criminal crimes or for the detection of diseases (genetic diseases as well as viral infections)

In a so-called thermocycler, the DNA to be amplified is combined with free nucleotides, DNA polymerases and special primers. The thermocycler then allows the PCR to run automatically, as several cycles are necessary until sufficient DNA has been replicated. Already a DNA double strand is enough to apply the procedure. The number of DNA duplexes then increases exponentially per cycle (1-2-4-8-16 etc.), so that enough genetic material is available after 30-50 cycles. However, in the polymerase chain reaction, no complete DNA duplexes are amplified, but only predetermined sections. These sections can be defined very precisely by artificially synthesized primers.

* Denaturation: The DNA is heated to about 90 ° C, which dissolves the hydrogen bonds between the complementary bases. This separates the DNA into its single strands.

* Hybridization: At approximately 60 ° C, the specific primers anneal to the 3 'ends of the DNA single strands. The principle of complementarity applies: According to their bases, the primers can only attach to a complementary counterpart (adenine with thymine and cytosine with guanine).

* Polymerization: The temperature is raised to about 70 ° C. DNA polymerases start at the primers from 3 'to 5' with the attachment of complementary bases (elongation). In the end, two DNA single strands, two DNA double strands, have been created.

Now, as already mentioned, there is a repetition of these three steps.

Summary

With the polymerase chain reaction, special DNA sections can be amplified indefinitely

The PCR runs in 30-50 cycles. One cycle is divided into denaturation, hybridization and polymerization