Genetic Engineering Toolbox

Polymerase Chain Reaction


The three-step PCR process was developed in the late 1980s by the Cetus Corporation of California.

Description and Procedure:

PCR is a method for making millions of copies of a specific segment of DNA in only a few hours rather than in days as with gene cloning by plasmid or viral DNA.

A solution of double stranded DNA containing the target sequence; thermally stable DNA polymerase; a supply of all 4 nucleotides; and primers complementary to the ends of the target DNA, is heated and cooled repeatedly causing the strands of the double helix to periodically separate. The polymerase remains active throughout.

  1. The desired sample--a tiny fragment of the nucleic acid DNA--is heated to separate the two strands of the molecule.
  2. Segments of DNA called primers are then attached to each strand to identify them for copying.
  3. Copying is accomplished by adding the natural enzyme called DNA polymerase.

A given sample can be increased a millionfold within hours by repeating these steps about 20 times. An animated illustration of PRC may help you understand how it works.

Uses or Function:

DNA is easily replicated in the test tube rather than in vivo (the cell). There is no need to isolate the desired section of DNA because of the specificity of the primases. Only a tiny amount of starting material is sufficient.

PCR is useful in many areas of medical research. The investigation of genetic disorders, for example, must often make use of tiny samples of DNA. Mutations of viruses such as the AIDS virus can also be studied more readily through PCR. The technique can also enhance investigations of remaining genetic samples from earlier life forms.