Why is polymerase chain reaction done

PCR is also valuable in a number of laboratory and clinical techniques, including DNA fingerprinting, detection of bacteria or viruses particularly AIDS , and diagnosis of genetic disorders. Next, an enzyme called "Taq polymerase" synthesizes - builds - two new strands of DNA, using the original strands as templates. This process results in the duplication of the original DNA, with each of the new molecules containing one old and one new strand of DNA.

Then each of these strands can be used to create two new copies, and so on, and so on. The cycle of denaturing and synthesizing new DNA is repeated as many as 30 or 40 times, leading to more than one billion exact copies of the original DNA segment. DNA polymerase - a type of enzyme that synthesizes new strands of DNA complementary to the target sequence.

Although these enzymes are subtly different, they both have two capabilities that make them suitable for PCR: 1 they can generate new strands of DNA using a DNA template and primers, and 2 they are heat resistant. Primers - short pieces of single-stranded DNA that are complementary to the target sequence. The polymerase begins synthesizing new DNA from the end of the primer. Only during the exponential phase of the PCR reaction is it possible to extrapolate back to determine the starting quantity of the target sequence contained in the sample.

How does PCR work? We will explain exactly what each of these do as we go along. PCR involves a process of heating and cooling called thermal cycling which is carried out by machine.

There are three main stages: Denaturing — when the double-stranded template DNA is heated to separate it into two single strands.

Extending — when the temperature is raised and the new strand of DNA is made by the Taq polymerase enzyme. These three stages are repeated times, doubling the number of DNA copies each time. A complete PCR reaction can be performed in a few hours, or even less than an hour with certain high-speed machines. After PCR has been completed, a method called electrophoresis can be used to check the quantity and size of the DNA fragments produced. Related Content:. What is DNA replication? What is DNA?

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