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Polymerase Chain Reaction (PCR)

Polymerase Chain Reaction
Polymerase Chain Reaction

Polymerase Chain Reaction (PCR) in simple words. PCR is a technique used to make several copies of a small fragment of DNA or RNA. If you understand the meaning of the words in PCR, we can better tell you about this process.

PCR is made up of two words: Polymerase and Chain Reaction. Polymerase means an enzyme that makes polymers of any other molecule, which in this technique is the DNA chain. A chain reaction is a type of chemical reaction that progresses in an exponential way. In simple terms, if the first reaction produces two molecules, the second reaction will make four, and the third will make sixteen copies, and so on. So, in a matter of just a few hundred reactions, we can produce billions of copies of a single fragment of DNA.

Polymerase Chain Reaction
Polymerase Chain Reaction

Before proceeding further, make sure to watch the video on the structure of DNA and DNA replication on our channel for a better understanding of this topic.

Polymerase Chain Reaction

Now, let’s first talk about what are the things that we need to perform PCR, and then we will discuss how this process happens. The most commonly used equipment in PCR is the thermal cycler, also known as the PCR machine. Inside the PCR machine, we have these small tubes in which all the chemicals are inserted, and the reaction takes place.

The key ingredients of a PCR reaction are:

  • Taq polymerase
  • Primers
  • DNA templates
  • Nucleotides

Taq polymerase is a type of DNA polymerase and like DNA replication in any organism, PCR requires a DNA polymerase enzyme that makes new strands of DNA using existing strands as templates. The DNA polymerase typically used in PCR is called Taq polymerase, which is isolated from the heat-tolerant bacteria named Thermus aquaticus. This material lives in hot springs, and its DNA polymerase is very heat stable and is most active around 70 degrees. This heat stability is ideal as high temperatures are needed for this reaction.

The second important thing that we need to perform PCR are the primers. Primers provide a starting point for DNA synthesis, and they help to select the exact portion of DNA that will be amplified. We use two primers in each PCR reaction, and they are designed to anneal to the target region of the DNA to be copied.

The DNA template is the segment of original DNA which we want to amplify, and nucleotides are the basic building blocks used for DNA synthesis.

PCR involves three simple steps:

  1. Denaturation
  2. Annealing
  3. Extension

In the first step of the polymerase chain reaction, we raise the temperature of the machine to 96 degrees Celsius. This is done to denature or separate the two strands of DNA.

The next step of PCR is known as annealing. In this step, we cool the temperature to 55 degrees so that the primers in the PCR tube can bind to their target sequence on the single-stranded DNA.

The third step is known as extension. In this step, we increase the temperature again to 72 degrees so that the Taq polymerase extends the primers synthesizing new strands of DNA.

These steps are repeated again and again, making the PCR a chain reaction. The products of the first reaction are used as the substrates for the next reaction, allowing new DNA to be synthesized. By a few hundred reactions, we are able to produce billions of copies of the same fragment of DNA.

In conclusion, PCR is an amazing technique that has revolutionized the field of diagnosis and research. It is used in diagnosing infections and infectious diseases, genetic research, molecular biology, and crime investigations.


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