The Polymerase Chain Reaction (PCR) is simply the most used and the most powerful technique in modern molecular biology. Invented in 1983 by Kary Mullis*, PCR has completely revolutionised biology. PCR enables you to amplify billionfold specific regions of DNA from any given DNA "template". This may seem unimpressive at first, but the sheer number of uses that PCR has been adapted to means that it is an invaluable piece of modern technology.

Ingredients

PCR is in many ways like cooking, albeit very precise cooking with an "oven" that changes temperatures all the time according to a computer programme. Once all of the ingredients have been added, the reaction is only fifty microlitres in volume.

• Taq DNA Polymerase - DNA polymerases are enzymes that make copies of DNA, they are present in all living things. The polymerise DNA from it's building blocks, nucleotides. Normal DNA polymerase denatures and loses function at high temperatures, which was a problem because that is what is required in PCR to "open up" thr DNA double helix. Fortunately a thermostable polymerase was discovered in the deep-sea vent-inhabiting bacteria called Thermus aquaticus. Hence the name Taq polymerase.
  
• Nucleotides - Equal amounts of the four bases; Adenine, Cytosine, Guanine and Thymine are supplied to the reaction so that the polymerase can synthesise copies of the DNA.
  
• DNA Template- Perhaps the most crucial part, the DNA that you want amplified. It has to be DNA from the species that you are looking at, or DNA that contains the gene or sequence that you are interested in. The specific section of DNA is amplified by targetting it with short stretches of DNA known as "primers". PCR is a very sensitive reaction; it is said that it only needs a single molecule of DNA to be able to work.
  
• Primers - Primers are single-stranded, short* and they get the copying started. They are designed on computer programmes to flank the section of DNA that you are want to amplify. Primers must be duplicates of nucleotide sequences on either side of the piece of DNA of interest. After the DNA double helix is separated with high temperature, the primers are encouraged to stick, or "anneal" at the desired place by lowering the temperature. Taq DNA polymerase then binds these primers. The temperature is raised to the Taq's operating temperature and the DNA template is copied.
  
• Buffer - This has the unglamorous job of keeping everything in a stable and happy condition, especially the enzyme.
  
• Magnesium Chloride - Magnesium Chloride is a chelating agent, that means it mops up all the free ions floating about in the reaction that might otherwise inhibit the polymerase.
  
• Water - Water is added at just the right amount so that all of the other ingredients are at the correct concentrations.
  

Thermal Cycling

As meantioned before, PCRs have to go through temperature changes for different, crucial things to occur.

• Denaturation - The template DNA is denatured by high temperature; the double helix is split apart at 95 degrees Celsius.
  
• Primer Annealing - The primers anneal on the specific bases for which they were designed at anywhere between 50 to 70 degree Celsius.
  
• Extension - Taq polymerase binds the DNA where the primers have annealed and synthesises a new DNA strand on each of the two template strands at 72 degrees Celsius.
  

This accounts for one "cycle" of a PCR and it is repeated up to thirty-five times. PCR amplifies exponentially; If you have started of with a single molecule of DNA after one cycle you will have copied two. After the second cycle you will have four; after the third, eight; the fourth, sixteen. After just twenty-five rounds of thermal cycling you will have 33,554,432 copies of your desired DNA fragment from that single double-stranded DNA molecule.

Applications

The practical applications of PCR are far, far too numerous to mention in one brief guide entry. Below are just a few things one can do with PCR

That's the necessary technical stuff over with. So what on Earth can one do with PCR? PCR makes it possible to get a lot of DNA copies from a source that has very little DNA. This is useful for biologists who want to clone genes, or just want to study a certain area of DNA and would otherwise have very little to work with. PCR has enabled forensic scientists to identify an individual from small amounts of very old, decomposed tissue.

population studies

genome walking

systematics

sequencing

paternity lawsuits