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What is PCR?

In 2020 with the COVID 19 outbreak we got familiar with the term “PCR” which is used to identify the infected patients. But what is this PCR?

PCR or Polymerase Chain Reaction is a method used to make copies of DNA. This method allows scientists to convert a small amount of DNA sample into millions of copies. It was invented in 1983 by Kary Mullis an American biochemist.

How Does PCR Work?

There are 5 ingredients required. They are

  1. DNA template – Sample DNA which shoild be amplified
  2. DNA Polymerase Enzyme – To add new DNA bases
  3. DNA nucleotide bases – The building blocks of DNA
  4. DNA Primers – Short streches of DNA that initiate PCR rection
  5. Buffer – To ensure the right conditions for the reaction

There are 3 main stages and they are

  1. Denaturing
  2. Annealing
  3. Extending


The template DNA is heated to 94-95oC. Then the hydrogen bonds between the bases of the strand break which cause DNA separates into two single strands. These separated DNA acts as templates for the production of new strands. This step usually takes 15-30 seconds. The temperature should be maintained for long enough to ensure that the DNA strands have separated completely.


Temperature is lowered to 50-650C enabling DNA primers to attach to the template DNA. These primers are around 20-30 bases in length. They serve as the starting point for DNA synthesis. Only if the primers are attached the polymerase enzyme can attach new loose DNA bases. This step usually takes about 10-30 seconds.


Again temperature is raised up to 720C and the DNA bases are attached to the primer by DNA Polymerase enzyme. This enzyme is harvested from the Thurmus aquaticus bacteria which lives in hot springs and can tolerate temperature above 800C. Therefore the enzyme is stable at high temperatures and 720C is optimum for the enzyme to work.

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These 3 stages are repeated over and over again which doubles the number of DNA copies at each round.

After the PCR is finished a method called electrophoresis can be used to check the quantity and size of the DNA fragments produced.

How does PCR is used to detect COVID 19?

A sample is collected from a place where the virus would be (in this case nose and throat). Then the sample is treated with several chemical solutions to remove proteins and fats. After that the sample would contain the patient’s RNA and if infected the virus’s. The RNA is used to make the target DNA (DNA of the virus). If the RNA of the virus is present only then the DNA of the virus will be made if not only the DNA that related to the patient will be made. After PCR a huge amount of DNA is present which makes them easier to recognize through Electrophoresis.

Thank you for reading…. ❤ 🙂 🙂 🙂

52 replies on “What is PCR?”

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