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How Polymerase Chain Reaction (PCR) Works

What exactly is PCR and how does it work? PCR, also known as Polymerase Chain Reaction, is a method that many scientists use to replicate DNA. This is done in three steps. These steps are denaturing which is where the DNA double strand is heated and melted. The next step is annealing which is where primers and DNA polymerase attaches to the single strand DNA. The last step is elongation which is when the temperature is increased to optimal conditions for the DNA polymerase to begin synthesizing the DNA compliment strand.

1. What is Polymerase Chain Reaction?

Polymerase Chain Reaction (PCR) is a method used by many scientists to amplify a particular gene or section of DNA that is needed to be studied. It is a method used to replicate strands of DNA into large amounts thus allowing scientists to run multiple tests on them. PCR can be done simply by anyone. Anyone who has the proper materials can perform a polymerase chain reaction.  Previous methods used to replicate DNA took many weeks. However, with PCR, one can replicate DNA within hours.

2. How does it work?

PCR is broken down into 3 major steps. These steps are denaturing, annealing, and then extension/elongation. Denaturing is the process where you heat up the DNA to melt the double strand leaving you with a single strand template strand to copy. Annealing is when your primers and polymerase attaches to your template strand preparing for the elongation step. Extension/elongation is when the DNA polymerase begin building the complimentary strand to the template strand using the dNTPs in the solution. Firstly, you must have a sample of DNA that you want to replicate. This can be obtained by taking a sample of something and then extracting the DNA. To save time, there are labs that sell specific DNA strands for this purpose. Once you’ve obtained the DNA you wish to replicate, place it inside a test tube. To this test tube you will need to add primers which will attach to the ends of your DNA which tells the DNA polymerase to begin building the complimentary strand to your template strand. You will add a DNA polymerase or some heat-sensitive polymerase such as Taq Polymerase which will replicate the compliment strand of the template strand. Adding buffer solution and magnesium ions will help stabilize the taq polymerase as it binds dNTPs to the template strand. Finally you must add dNTPs which will be used by the DNA polymerase to form the compliment strand of the DNA. Once you’ve had these materials all inside a tube, you are ready to perform a Polymerase Chain Reaction. The first step is denaturing. This step is done by bringing the reaction to a temperature of roughly 95 degrees Celsius. Heating the reaction will “melt” the DNA double strand into single strand which allows the Taq polymerase to begin replicating the compliment strands. Once the double strand DNA is separated, the reaction is lowered to roughly 60 degrees Celsius to allow the primers to attach to the 3’ ends of the single stranded DNA. The primers tell the DNA polymerase or Taq polymerase to bind to this strand and to begin synthesis. The reaction is then increased to roughly 75 degrees Celsius depending on which DNA polymerase used to allow it to synthesis the DNA optimally. This step is called extension or elongation. For Taq polymerase, it synthesizes optimally at a temperature of 75 degrees Celsius. The Taq polymerase slides along the DNA template strand and picks up the dNTPs in the solution that is compliment to the template in the 5’ to 3’ direction.  Once the entire complimentary strand is synthesized, the primer and polymerase releases from the newly synthesized strand. This reaction is then repeated multiple times by increasing the temperature once again to denature the DNA, and then decreased again for the primers to anneal the DNA, and then increase the temperature for the optimal performance of synthesizing the rest of the compliment strand. Repeating these steps a couple of times would yield millions of copies of the initial DNA strand you started out with. With these massive amounts of DNA, you are free to do whatever tests you may please without having to worry about running out of DNA samples.

3. Why is it important?

The reason why PCR is so important to scientists is because before there was a concern of depleting the DNA sample before obtaining any results. However with PCR, scientists are able to make many copies of the DNA they want to experiment with without having to worry of losing the sample if an experiment failed.

PCR may seem like a very complicated reaction to pull off, but in reality, it is fairly simple. It is a very simple concept, but has proven itself to be a very useful in the science industry. Scientists use PCR every day to make replications of DNA to do experiments on them.

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