Polymerase chain reaction
Molecular Biology · Biotechnology · Forensic Medicine
Say it: puh-LIM-uh-rays chayn ree-AK-shun
The polymerase chain reaction (PCR) is a lab technique that copies one chosen stretch of DNA over and over until there are billions of copies.
Also: PCR
Definition
What is the polymerase chain reaction?
PCR amplifies a target sequence so that even a tiny sample of DNA can be analyzed. The reaction tube holds the template DNA, two short single-stranded primers that match the two ends of the target, the four DNA nucleotides and a heat-stable DNA polymerase, usually Taq polymerase from the hot-spring bacterium Thermus aquaticus.
A machine called a thermal cycler repeats three temperature steps. Heating to near boiling separates the two DNA strands (denaturation); cooling lets the primers stick to their matching sites (annealing); and warming again lets the polymerase extend the primers, copying each strand (extension). Each cycle roughly doubles the target, so 30 to 40 cycles turn a few molecules into more than a billion. Variants include reverse transcription PCR, which starts from RNA, and quantitative PCR, which measures how much DNA is present as the reaction runs.
Kary Mullis developed PCR in the 1980s and won the 1993 Nobel Prize in Chemistry for it. It is now routine for cloning, preparing DNA for sequencing, diagnosing infections and genetic disorders, and forensic DNA profiling.
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- DNA polymeraseDNA polymerase is an enzyme that builds new DNA by adding nucleotides one at a time to a growing strand, using an existing strand as a template.
- Gel electrophoresisGel electrophoresis is a lab technique that uses an electric field to pull DNA, RNA or proteins through a gel, separating them by size.
- DNA replicationDNA replication is how a cell copies all of its DNA before it divides, so that each daughter cell receives a complete genome.
- Molecular cloningMolecular cloning is a set of laboratory methods for inserting a specific DNA fragment into a vector and copying it many times inside host cells.
- Sanger sequencingSanger sequencing is a method for reading the order of bases in DNA that uses chain-terminating nucleotides, developed by Frederick Sanger in the 1970s.
- Reverse transcriptaseReverse transcriptase is an enzyme that builds DNA from an RNA template, letting retroviruses such as HIV turn their RNA genome into DNA.
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Written for biofideX in plain words, after checking these references:
- NHGRI Talking Glossary: Polymerase Chain Reaction (PCR)
- NHGRI: Polymerase Chain Reaction (PCR) Fact Sheet
- OpenStax Biology 2e, 17.1 Biotechnology
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