Polymerase Chain Reaction - PCR

Polymerase Chain Reaction - PCR

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PCR: important genetic technique

What is (definition)

The PCR technique is based on amplification - in vitro - of a single DNA sequence millions of times.

Basically she makes several copies of the DNA fragment of interest that will be studied.


If we looked only at DNA extracted from a cell, it would be extremely difficult to find a fragment of interest; In other words, it would be almost impossible to identify that particular part of the DNA.

Simply put, you can state that through the PCR technique it is possible to "hum" and see only that part that matters.


The applications of the PCR technique are many and in the most different areas. Here are some examples:

- Diagnosis of infectious and genetic diseases;

- Determination of genetic variability;

- Sexing of embryos;

- Study of gene expression;

- forensic investigation;

- Determination of animal lineage;

- Identification of pathogens, paternity, etc.

How it happens

PCR occurs in three steps and all of them depend on a temperature gradient.

Step 1: Denaturation (at 95 ° C) of the double strand of DNA, which occurs due to temperature rise.

Step 2: Priming (57ºC to 63ºC) of the primers, which can occur in a temperature range between 57ºC to 63ºC.

Step 3: Extension (at 72 ° C) with the operation of Taq Polymerase, which assists in the introduction of nucleotides that will complement the chain being synthesized.


A normal PCR has about 36 cycles, each of which encompasses the three steps described above. And with each cycle there is the exponential multiplication or amplification of the amount of DNA molecules.

For example, if in the first cycle we start PCR with only one DNA molecule, in the end we will have two molecules. At the end of the second cycle we will have four. At the end of the third we will have eight molecules and so exponentially.

By the end of the 36 cycles we will have billions of copies of DNA that can be analyzed in the lab. This analysis will take place through the use of Electrophoresis.