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Polymerase Chain Reaction: (PCR)


Created by George Rice, Montana State University


PCR is the supreme biotechnological invention -

"PCR has transformed molecular biology through vastly extending the capacity to identify, manipulate and reproduce DNA. It makes abundant what was once scarce -- the genetic material required for experimentations."

Paul Rabinow
(Making PCR, A Story of Biotechnology, University of Chicago Press, 1996)


What is PCR?

Sometimes called "molecular photocopying," the polymerase chain reaction (PCR) is a fast and inexpensive technique used to "amplify" - copy - small segments of DNA. Because significant amounts of a sample of DNA are necessary for molecular and genetic analyses, studies of isolated pieces of DNA are nearly impossible without PCR amplification. Often heralded as one of the most important scientific advances in molecular biology, PCR revolutionized the study of DNA to such an extent that its creator, Kary B. Mullis, was awarded the Nobel Prize for Chemistry in 1993.
(from the National Human Genome Research Institute (more info) )

What is it used for?

Once amplified, the DNA produced by PCR can be used in many different laboratory procedures such as -
  • Most mapping techniques in the Human Genome Project rely on PCR.
  • PCR is intigral in a number of new laboratory and clinical techniques, including DNA fingerprinting (think CSI and catching criminals).
  • Diagnosing disease and genetic disorders.
  • Detection of bacteria and viruses in the environment.
  • Analysis of microbial communities.



How does it work?

To amplify a segment of DNA using PCR, the sample is first heated so the DNA denatures, or separates into two pieces of single-stranded DNA. 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. The entire cycling process of PCR is automated and can be completed in just a few hours. It is directed by a machine called a thermocycler, which is programmed to alter the temperature of the reaction every few minutes to allow DNA denaturing and synthesis.


Diagram of the mechanics of PCR.
Schematic of the Polymerase Chain Reaction (PCR).(courtisy of Bruce Fouke's lab)


Instrumentation used in PCR -

Image of a thermo cycler used in PCR.
A thermocycler or PCR machine is a laboratory apparatus used for PCR. The device has a thermal block with holes where tubes with the PCR reaction mixtures can be inserted. The cycler then rises and lowers the temperature of the block in discrete, pre-programmed steps. Thermal cyclers are equipped with hot bonnet, which is a heated plate that presses against the lids of the reaction tubes. This prevents condensation of water from the reaction mixtures to the insides of the lids and makes it unnecessary to use PCR oil.


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The following resources were originally accessed through the BioSciEd Net (BEN) digital resources collection, which is the National Science Digital Library (NSDL) (more info) Pathway for biological sciences education. For more teaching resources, please visit BEN to use their searchable database. BEN is free to use, but requires registration.

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