Polymerase Chain Reaction
Polymerase chain reaction (PCR) is a laboratory technique that is common in gene technology. It is a way to amplify, or copy, a short sequence of DNA.

PCR consists of three phases. The three phases are cycled, usually about 30 times, in order to generate enough copies of the target DNA.

In order to do PCR, you need a template strand of DNA, primers, nucleotides and a heat-tolerant polymerase such as Taq Polymerase, and a thermocycler, a programmable machine that can automatically raise or lower the temperature of the reaction.

The primers are designed to flank the region of DNA a scientist wants to amplify. A primer is a sequence of nucleotides, usually 8-12 base pairs long.

The first phase of the cycle is called the denaturing phase. It occurs around 94-96C. In this phase, the DNA is heated to the point that the two strands fall apart. Thus, two strands are now available for base pairing with the primers.

In the second phase, the machine cools down to 50-65C. At this point, the DNA primers can anneal to the DNA, so each strand of DNA has a primer bound to it.

In the third phase, occurring at 72C, the Taq polymerase binds to the DNA and primer, and copies the strand using the nucleotides in the mixture. The result is two double-strands of DNA, but one side of each strand is only copied from the primer to one end.

This cycle is repeated a number of times. During the third cycle, the first piece of DNA containing just the target sequence appears. After this cycle, the number of strands containing only the target strand increases exponentially.

Related Links