S3.23 Heat, Stick, Duplicate, Repeat: The Story Of The Polymerase Chain Reaction (PCR)
Kat: Hello, and welcome to Genetics Unzipped - the Genetics Society podcast, with me, Dr Kat Arney. In this episode we’re taking a look at the story and the characters behind one of the most transformative - and ubiquitous - techniques in modern molecular biology: the polymerase chain reaction.
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Anyone who has worked with DNA in the laboratory is undoubtedly familiar with the polymerase chain reaction - PCR, as it’s usually known.
Invented in 1985, PCR is an indispensable molecular biology tool that can replicate any stretch of DNA, copying it billions of times in a matter of hours, providing enough DNA to use in sequencing or further research, or for applications like forensics, genetic testing, ancient DNA analysis or medical diagnostics.
The premise of PCR is simple. You take the DNA you would like to amplify (that’s your template); the building blocks of DNA, known as nucleotide bases; polymerase enzymes that can copy DNA; and short sections of DNA called primers which bind to the template DNA and provide a starting point for replication.
Mix ‘em all together in a little plastic test tube, and then pop it in special machine known as a thermal cycler, and wait for the magic to happen.
First, the mixture is heated to over 90°C, so that the strands of DNA lose their double helix structure and separate from each other, known as denaturing. Then, the machine cools everything down, allowing the primers to stick or anneal to the matching sequences on the DNA strands - the exact temperature here will depend on the sequence.
Next, things heat up again, this time to 72°C so the polymerase enzymes can get to work, adding nucleotide bases to the primers and creating new DNA using the original DNA strand as a template. Then you do it again: heat to separate the newly-formed DNA from the template, cool to allow the primers to bind, then raise the temperature to cook up new DNA.
This time, though, not only do you have the original DNA to act as a template for the copying process, you’ve got all the identical pieces of DNA that were made in the first round of replication, so you can make twice as much new DNA from that target sequence. And then, you repeat it again. And again. And again. And again.
Typically these temperature cycles are repeated around 30 times, taking a couple of hours and leaving you with billions of copies of your target DNA sequence.
It’s hard to overstate the transformation that PCR brought to the world of molecular biology and biomedical research. Suddenly, researchers could amplify and study DNA in a way that had been simply impossible before, kickstarting the genetic revolution that’s still going strong today.
The Story of PCR
So where did this revolutionary technology come from? Officially, PCR was invented in 1985 by a colourful character called Kary Mullis, who won a Nobel Prize for the discovery (more on him later). But, as we’ll see, all the components of PCR were in place by the early 1980s - it just took a creative leap to assemble them into one blockbusting technique.
That’s all for now. We’ll be back next time looking back over the best bits of the podcast from the past year. Do let us know your favourites by tweeting @geneticsunzip. And before that, there’s another bonus episode of Genetics Shambles to fill your ears.
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Genetics Unzipped is written and presented by me, Kat Arney, with additional research and scripting by Emily Nordvang. It is produced by First Create the Media for The Genetics Society - one of the oldest learned societies in the world dedicated to supporting and promoting the research, teaching and application of genetics. You can find out more and apply to join at genetics.org.uk. Our theme music was composed by Dan Pollard, and the logo was designed by James Mayall, and audio production was by Hannah Varrall. Thanks for listening, and until next time, goodbye.
All music and sound effects licensed from Envato and Epidemic Sound
Image: Illustration depicting semi-conservative DNA replication. Three generations of DNA are shown. After separation of the DNA double helix, two new complementary DNA strands are synthesised (indicated by a new colour). Complementary base pairing and hydrogen bonding results in formation of a new double helix. Credit: Susan Lockhart. Attribution-NonCommercial 4.0 International (CC BY-NC 4.0)
References:
PCR: Thirty-five years and counting - Science
Professor Arthur Kornberg - What is Biotechnology
The eureka enzyme: the discovery of DNA polymerase - Nature Reviews Molecular Cell Biology
Recollections of Arthur Kornberg (1918–2007) and the beginning of the Stanford Biochemistry Department - Protein Science
Enzymatic synthesis of deoxyribonucleic acid. I. Preparation of substrates and partial purification of an enzyme from Escherichia coli - Journal of Biological Chemistry
Enzymatic synthesis of deoxyribonucleic acid. II. General properties of the reaction - Journal of Biological Chemistry
The biologic synthesis of deoxyribonucleic acid - Arthur Kornberg Nobel Lecture
Sylvy Kornberg: Biography of a Biochemist - The Scientist
Studies on polynucleotides: XCVI. Repair replication of short synthetic DNA's as catalyzed by DNA polymerase - Journal of Molecular Biology
On PCR, LSD, and Science as a Wild Ride - Boom California
The Unusual Origin of the Polymerase Chain Reaction - Scientific American
Primer-directed enzymatic amplification of DNA with a thermostable DNA polymerase - Science
Putting the P in PCR - Roche.com
Deoxyribonucleic acid polymerase from the extreme thermophile Thermus aquaticus (1976) - J. Bact. 127(3):1550–7.
The Discovery of PCR: ProCuRement of Divine Power - Digestive Diseases and Sciences
PCR inventor Kary Mullis dies aged 74 - Chemistry World
Making PCR, A Story of Biotechnology - Paul Rabinow
