General

The polymerase chain reaction (PCR)


Sequence of the PCR

The polymerase chain reaction (PCR) is an artificial process for the amplification of 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 brought together with free nucleotides, DNA polymerases and special primers. The thermocycler then allows an automatic sequence of the PCR, because it takes several cycles until enough DNA has been amplified. Already one 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.
hybridizationAt about 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 begin 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.