What is PCR ?

What is PCR ?

The polymerase chain reaction (PCR) is one of the most important and powerful technique, that is being applied in virtually all fields of modern molecular biology. PCR was invented by Kary Mullis at the Cetus Corporation for which he received in 1993 the Nobel prize. The first description of this method on amplifying Beta-globin sequences in sickle cell anemia was published in 1985.

PCR is a method for the in vitro exponential amplification of a specific DNA region (called target region), that lies between two regions (called primers) of known DNA sequence, resulting in a large quantity of DNA (this means micrograms of DNA).

Most organisms copy their DNA in the same way: DNA polymerases carry out the synthesis of a complementary strand of DNA in the 5' to 3' direction using a single-stranded template. The PCR mimics this process, only it does it in a test tube and employs two primers, each complementary to opposite strands of the region of DNA. The PCR amplification occurs by repeated cycles of three temperature dependent steps:

*1*denaturation
*2*annealing
*3*elongation

The three parts of the PCR are carried out in the same vial, but at different temperatures. The vial contains all necessary components. In its native state, DNA exists as a double helix. The first step of the PCR (denaturation) separates the two DNA chains by heating the test tube to 90 - 95 degrees centigrade (Scheme - Denaturation).

Annealing of the primers is the second step of the PCR. The primers cannot bind (anneal) to the strands of DNA at temperature of the denaturation, so the vial is cooled to 45-60 degrees C (Scheme - Annealing of the primers) .

The final step of the reaction is elongation of the primers - the synthesis of new strands by making a complete copy of the templates. Since the Taq polymerase, which is usually added to the PCR, works the best at around 72 degrees centigrade, the temperature of the test tube is raised (Scheme - Elongation).

At the end of a cycle of these three steps, each target region of DNA in the vial has been duplicated. This cycle is usually repeated 30 times. Each new DNA piece can act in the next cycle as a new template, so after 30 cycles, 1 million copies of a single fragment of DNA can be produced (Scheme - Diagram of PCR).

The PCR solves two of the more universal problems in the chemistry of natural nucleic acids. It allows for the separation of any sequence of interest from its context, and then it provides an in vitro amplification of this sequence virtually without limit.


Go to Reagents


Back to How can the polymerase chain reaction be applied in structural molecular biology?


Dominika Borek borek@leucine.ibch.poznan.pl
Department of Crystallography
Faculty of Chemistry
Adam Mickiewicz University
Grunwaldzka 6
Poznan, Poland