Polymerase chain reaction

Polymerase chain reaction (PCR) is a way to make many copies of a sequence of DNA (this is sometimes called 'amplifying' the DNA). It is done in a lab, using an enzyme called DNA polymerase. It is called chain reaction because the result of one cycle is used immediately for the next cycle. This allows exponential growth to happen.

PCR has many uses in a biological or biochemical setting. Because DNA is unique for every living thing, experimenters can often extract only small amounts of the DNA they are interested in from a specimen. These amounts are usually too little to be useful, and so a scientist will use PCR to make enough copies to start experimenting with. For this reason, it is one of the most common techniques used in genetics labs around the world, making it useful in experiments on many things, including gene therapy, infectious diseases, and even forensics.

This process was developed in 1983 by Kary Mullis. He was not the first person to develop the PCR process, but he was the first to make the process usable. He was awarded the Nobel Prize in Chemistry for his work in the PCR process. It was a joint award, the other scientist being Michael Smith, who worked on a related project.

PCR procedure

The method consists of repeated heating and cooling, causing "melting" (separation of the two strands) and replication of the original DNA, also called a template. Short DNA fragments consisting of DNA sequences complementary to the ends of the template, called primers, and a DNA polymerase are key materials for selective and repetitive steps.

The technique proceeds in three steps:

1. denaturation — The template strands that are bound together cannot be replicated, so the first step of PCR is to separate them by heating up the sample, breaking the hydrogen bonds between them.
2. annealing — The sample is cooled just enough to allow the primers to bind to the ends of each of the two template strands
3. extension — DNA polymerase attaches to the primers and makes a copy of each template strand.

After the first cycle, there are 4 DNA strands. The process repeats with the 4 DNA strands, which will go on to make 8 strands, then repeat itself again to make 16 strands. In this way, PCR doubles the amount of DNA in a sample after each cycle, making it possible to obtain millions of copies of a DNA strand overnight.

Polymerase Chain Reaction Media

• PCR tubes.png

  A strip of eight PCR tubes, each containing a 100 μL reaction mixture

• PCR masina kasutamine.jpg

  Placing a strip of eight PCR tubes into a thermal cycler

• Primitive PCR machine for scrap.JPG

  An older, three-temperature thermal cycler for PCR

• Polymerase chain reaction-en.svg

  Schematic mechanism of PCR.

• Tucker PCR.png

  Mechanistic Color Coded Diagram of PCR

• Exponential Amplification.svg

  This is a visualization of Exponential Amplification to give a better understanding of what it is and why it is so important in the PCR process. Showing how it makes copies of the sample collected at the scene.

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  Electrophoresis of PCR-amplified DNA fragments:

  1. Father
  2. Child
  3. Mother

  The child has inherited some, but not all, of the fingerprints of each of its parents, giving it a new, unique fingerprint.
• DNA samples are often taken at crime scenes and analyzed by PCR.

• Primers RevComp.svg

  Diagrammatic representation of an example primer pair. The use of primers in an in vitro assay to allow DNA synthesis was a major innovation that allowed the development of PCR.