Polymerase Chain Reaction: (PCR)
"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."
(Making PCR, A Story of Biotechnology, University of Chicago Press, 1996)
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.
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.
(see PCRin environmental studies)
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.
- Web Links for PCR, including resources such as itroduction to and history of PCR, as well as many resources for alternatice PCR techniques.
The following resources were originally accessed through the BioSciEd Net (BEN) digital resources collection, which is the National Science Digital Library (NSDL) 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.
- Use PCR and a single hair to produce a DNA fingerprint - this PDF document provides a detailed manual of protocols and instructional information for carrying out an undergraduate laboratory exercise in molecular biology and genetics, in which students use polymerase chain reaction to create DNA fingerprints from their own hair. It includes student outlines, instructor's notes, and suggested questions for laboratory reports. THIS RESOURCE LINK REQUIRES YOU TO REGISTER (for free) WITH BEN.
- Reverse Transcription PCR - this resource, from MicrobeLibrary.org, offers a Flash animation shows how the method of reverse transcription-PCR is performed and some sample data are produced. It uses sound and mouse-over identification to help students learn more and retain the information.