by Janet Grischy

Created on: June 21, 2008

Polymerase chain reaction, PCR, is a technique used to quickly increase the size of a DNA sample. It is used in medicine to help diagnose infections and hereditary diseases. In genetic research, it amplifies the part of the genome under investigation. In forensics it can be used to establish a criminal's presence from a fragment smaller than the root of one hair. It can also be used to establish paternity and other genetic relationships. It was developed in 1983 by Kary Mullis, who won a Nobel Prize in Chemistry for his work.

Polymerase chain reaction takes advantage of the doubled and complementary structure of DNA. DNA may be visualized as a coiled ladder with rungs built of four building blocks that fit together with their complements. These four building blocks (called nucleotides) may also be thought of as letters, whose sequence spells out instructions to the cell, in a language that is called the genetic code. Just as a jigsaw puzzle piece locks together only with its matching pieces, each nucleotide fits only with its specific partner, its complement. The DNA, by copying itself, constructs RNA that carries its directions for making certain proteins. This is how genetic instructions are passed in along in cells.

When a cell divides, its DNA must divide as well, so that each cell will have its full share. To do this, the DNA ladder splits into two strands. They can be thought of as the side poles of a ladder with the four molecules whose order contains the genetic code sticking out from them like half rungs. At the starting site of reproduction, primers place some complementary pieces to start reproduction going. A new partner to each half is formed, in fragments, prompted by an assembling enzyme called DNA polymerase. Each piece is matched jigsaw-fashion to its complement. Now there are two identical ladders of DNA, each produced as a complementary image of the template formed by half the original strand of DNA. The cell can divide. Dr. Mullis brilliantly exploited this mechanism when he invented PCR.

In polymerase chain reaction, the DNA strands to be investigated are placed in a solution of DNA polymerase, primers, buffers, and an assortment of the four building blocks of DNA. Then the mixture is heated to denature the DNA ladders, that is, to unzip them into single strands. This takes place at around 94-96 degrees Centigrade (near boiling). Then the mixture is cooled to 50-65 degrees, and the primers bond to the strands at the ends of the areas to be investigated. This step is called annealing. Next the mixture is reheated, to about 72 degrees, and the DNA polymerase builds the new strands of DNA, just as it would in a dividing cell. The cycle is repeated and repeated, the amount of DNA doubling each time, rapidly producing a large amount of DNA. It's automated.

The DNA polymerase used most often comes from an organism, thermus aquaticus, which lives in hot springs, and is stable at temperatures that denature DNA. The primers are bits of single strand DNA that match (are complimentary to) the beginning and end of the section of DNA that is under investigation. They show the replication where to start and where to stop. The buffers keep the solution within chemical boundaries where the reaction can thrive.

PCR has been a great boon to researchers, and to patients waiting for a diagnosis. Thanks to PCR, the guilty are more likely to be caught, and the innocent to go free.

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