Why is the polymerase chain reaction important and how does it work?

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The polymerase chain reaction (or PCR) is extremely useful at copying fragments of DNA so by the end of it you have a much bigger sample than what you started with. This is particularly useful for example, in forensics when collecting DNA samples from the scene of a crime.

There are essentially 3 steps to this process but each one follows the other automatically inside a piece of equipment called a thermocycler. Firstly, the 2 strands in your double stranded DNA are separated. This is done at temperatures of around 95 degrees Celsius which is enough to break the hydrogen bonds between the two strands.

Secondly, the mixture is cooled to 55 degrees C and primers are added. Primers are short sequences of nucleotides which are complimentary to a set of bases at one end of each DNA fragment. This process of "annealing" provides the starting point for the third stage in PCR...

Finally, the temperature is increased to 72 degrees C which is the optimum temperature for DNA polymerase to extend these primer sequences to make the complimentary strand of DNA by joining complimentary nucleotides (which are also in your reaction mixture). It will continue to do so until it reaches the end of the chain.

So the end result is double the amount of DNA you started with. You can do the reaction again to get quadruple the amount and so on until you have a large enough DNA sample to work with.

Benito W. A Level Biology tutor, GCSE Biology tutor

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