In honor of The Scientist’s 40th anniversary, we look back at our coverage of momentous scientific advances over the past four decades. Arguably, no technique has accelerated life science research quite as much as the polymerase chain reaction (PCR). From amplifying crime scene DNA to becoming the gold-standard diagnostic test for a SARS-CoV-2 infection during the COVID-19 pandemic, PCR has revolutionized molecular biology and brought genetic analysis to fields as diverse as archaeology and agriculture. Join us as we dig into our archives to follow PCR’s journey, which even feature a story written by PCR inventor Kary Mullis himself.
1989: Licensing Disagreements Highlight Scientists’ Hunger for PCR Technology
After the invention of PCR in 1983 by Kary Mullis at Cetus Corporation, scientists flocked to the new technique like moths to a flame. These included researchers in academia using the technology to better understand how cancer cells respond to chemotherapy as well as industry researchers hoping to use PCR to develop diagnostic tests. An April 1989 article in The Scientist mentioned that Cetus Corporation’s licensing practices were impeding scientists from getting the equipment they needed to conduct their experiments in a timely manner. Cetus Corporation, however, disagreed. In this response published in The Scientist just a couple of months later in June, they pointed to the nearly 400 published papers that have cited the use of PCR and the clear paths to licensing the technology to develop commercial products.
1991: Innovative PCR Alternatives Begin Proliferating
By 1991, scientists had begun developing new and competing DNA amplification methods. One system called Q-beta replicase relies on the replicase enzyme from the RNA virus Q-beta. Rather than amplifying DNA like PCR, Q-beta replicase amplifies RNA complementary to a specific DNA sequence. Ligase chain reaction (LCR), another method that was gaining popularity, amplifies DNA using a ligase rather than a polymerase. While LCR cannot amplify unknown sequences, it has higher fidelity than PCR or Q-beta replicase because it will stop the reaction if its target sequence is not present. Other DNA amplification techniques gaining popularity included nucleic acid sequence-based amplification and signal amplification.
2003: PCR Inventor Kary Mullis Shares a Precious Lab Notebook Entry
In 2003, Kary Mullis took The Scientist readers behind the scenes of the night in 1983 when he came up with the idea for PCR. He included a photo of his lab notebook that depicts his very first experiment, labeled “PCR01.” He used no controls and only one reaction tube. To give the experiment the highest chance of success, he cut out his template DNA using restriction enzymes and loaded up the reaction with an abundance of primers. Not sure what he would find in the morning, he poured himself a beer from the isotope fridge into a 500mL beaker and then left for the night. The next afternoon he ran the reaction on a gel, and the rest is history.
2006: Mr. Cycle: An Automated PCR Prototype
In 2006, The Scientist looked back at the first semi-automated PCR machine, which was developed by Cetus Corporation in 1985. To perform PCR, scientists needed to manually shift their reaction tubes into water baths held at different temperatures. Instead, “Mr. Cycle” (a play on “Mr. Coffee,” which was the first automatic drip coffee machine) automatically moved tubes between a hot and a cold water bath. The PCR process was still not entirely automatic since researchers still needed to add more polymerase after each cycle as the hot temperatures would denature the enzyme. Soon after, the development of the heat-stable Thermus aquaticus polymerase (Taq polymerase) would remove the need for that step.
2013: Celebrating 30 years of PCR
In a 2013 webinar, Kary Mullis; Stephen Bustin, a molecular diagnostics researcher at Anglia Ruskin University; and Reginald Beer, a medical diagnostics scientist at Lawrence Livermore National Laboratory, reflected on the past 30 years of PCR and its promise for the future. Mullis described the fateful night he had the idea for PCR while driving past California buckeye shrubs in Mendocino county. The next morning, he said, “There were diagrams of PCR reactions on every surface that would take pencil or crayon in my cabin. I woke up in a new world.” Bustin discussed quantitative PCR (qPCR), which at the time was celebrating its 21-year anniversary. He explained how important it is that qPCR experiments are reproducible and transparent, citing how plaintiffs in a trial tried to link autism with the measles, mumps, and rubella vaccine, but Bustin said that he “was able to show that the authors’ conclusions were based on extremely poorly executed qPCR experiments.” Beer covered digital PCR, explaining how this method, which uses microfluidics to place zero or one DNA molecules into wells, is much more accurate than standard PCR or qPCR. He highlighted its potential in Alzheimer’s disease diagnostics and cancer screening.
2023: A Lookback at the Team that Built PCR
In 2023, The Scientist spoke with Henry Erlich, one of the members of the team that developed PCR and who is now a molecular geneticist emeritus at the Benioff UCSF Children’s Hospital Oakland Research Institute. Mullis’s first attempts to amplify a 110 base pair section of the human β-globin gene resulted in a smear of DNA on the resulting gel. Erlich, who led the human genetics team at Cetus Corporation, worked in parallel with Mullis, and his team also saw the same DNA smear. After many rounds of trial and error, Erlich and his colleagues had a ‘Eureka’ moment when their Southern blot showed a 110 base pair band corresponding to the β-globin gene. “We thought, ‘Oh my god, this thing could actually work,’” he recalled.
2024: Optimizing PCR: Proven Tips and Troubleshooting Tricks
Even though PCR has been around for over 40 years, it can still give scientists a hard time when they’re faced with complex DNA sequences. In this article from 2024, The Scientist outlines the many different types of PCR experiments that have been developed over the years and their advantages in various situations. These include when scientists only have trace amounts of DNA available or need to amplify an extra-long DNA molecule. It also details how to handle DNA templates with GC-rich regions, primer design, picking the best polymerase for the specific needs of the experiment, and what to do when everything goes wrong—from a blank gel to one with a persistent smear.
2026: The Renaissance of Digital PCR
In the 1990s, Bert Vogelstein and Kenneth Kinzler, cancer researchers at Johns Hopkins University, invented digital PCR. The method works by performing limiting dilutions to isolate just one DNA molecule per reaction. Scientists then add a fluorescent probe to the reaction wells that hybridizes with the template DNA sequence and emits light, which is detected and then used to quantify the amount of DNA in the well. Because digital PCR uses such small reaction volumes, it is a very precise and sensitive method. This makes it a valuable method to use when amplifying very rare DNA sequences, such as those that are found in rare cancers. Digital PCR is also used in infectious disease diagnostics and in non-invasive prenatal testing.
From its start as an idea on a dark California highway to its place in every molecular biology lab in the world, PCR has made an indelible impact on the life sciences that will reverberate for decades to come.
