Polymerase Chain Reaction (PCR)

Polymerase chain reaction, or PCR, is a technique that scientists use to rapidly make millions of copies of deoxyribonucleic acid, or DNA, which is the genetic material found inside every living organism. Kary Mullis, who shared the 1993 Nobel Prize in Chemistry for his contributions to PCR, developed the idea for its underlying biochemical mechanism in the early 1980s while working at Cetus Corporation, a company in Emeryville, California, that used biology to create tools for medical treatments, including cancer. Over the next several years, Mullis and his Cetus colleagues developed the technology, publishing the first study to reference PCR in the journal Science in 1985. Cetus scientists further enhanced PCR by adding Taq polymerase, an enzyme that helps copy DNA and remains stable at high temperatures. Prior to the development of PCR, researchers used slow, difficult methods to study small amounts of DNA. PCR facilitated the rapid and affordable amplification of DNA, helping with tasks like diagnosing diseases, solving crimes, and testing food and water for bacterial contamination.

1. Before PCR
2. Making PCR
3. Impacts

Before PCR

Before the invention of PCR, researchers faced many challenges in studying DNA. To obtain enough DNA, they could either try to collect more of it from the source or make copies of it through a process called cloning. To clone DNA, scientists used specialized enzymes to insert a fragment of DNA into a living cell such as the bacterium Escherichia coli. They would then feed and grow the bacteria on a dish. As the bacteria reproduced, they would duplicate their own DNA and the inserted DNA as well. Scientists could then harvest and purify the duplicated DNA fragment for use in experiments. However, cloning was laborious and time consuming. PCR made the process of duplicating DNA much faster and more efficient.

The development of PCR built on prior scientific discoveries, including several related to how cells naturally replicate their DNA. In 1956, Arthur Kornberg, a scientist who studied bacteria at Washington University in St. Louis, Missouri, and his colleagues isolated from bacteria an enzyme that was capable of copying DNA. They called it DNA polymerase. Then, in 1958, Matthew Meselson and Franklin Stahl working at the California Institute of Technology in Pasadena, California, performed an experiment showing how cells use DNA polymerase to copy DNA through a semi-conservative mechanism. Semi-conservative means that each strand of the DNA helix serves as a template for the synthesis of a complementary strand. In 1966, researcher Thomas Brock and colleagues from Indiana University in Bloomington, Indiana, isolated Taq polymerase, a heat-stable DNA polymerase, from thermophilic, or heat-loving, bacteria called Thermus aquaticus that lived in hydrothermal vents and were able to withstand extremely hot temperatures. That meant that Taq polymerase was able to function at temperatures that normally resulted in enzyme denaturation, a feature that became important later in the context of the development of PCR. Finally, in 1971, Har Gobind Khorana, Kjell Kleppe, and their colleagues, who researched biochemistry at the University of Wisconsin in Madison, Wisconsin, published a method of amplifying DNA that used short, human-made primers and DNA polymerase. A primer is a short strand of DNA that binds to a complementary sequence on a template strand and provides a starting point for DNA synthesis.

Making PCR

According to Mullis’s 1990 article in the magazine Scientific American titled “The Unusual Origin of the Polymerase Chain Reaction,” he first had the idea for PCR during a night drive from Berkeley, California, to Mendocino, California, in 1983. His idea was to copy DNA by using short DNA strands to mark the ends of a desired sequence and use a DNA polymerase to repeatedly replicate that region through cycles, in a process conceptually similar to the 1971 publication by Khorana and Kleppe. Over the next several months, Mullis began testing the idea in the Cetus Corporation lab and performed early experiments where he assessed the efficacy of different concentrations of reactants, reaction temperatures, and processing times. In his article, Mullis states that, because most DNA polymerases cannot operate in the hot temperatures necessary for proper isolation of the DNA, he used Taq polymerase, thus allowing PCR to proceed through repeated heating cycles and process large amounts of DNA without the need to add fresh enzyme each time. By the spring of 1984, Mullis was writing the patent for his technology, and, in June 1984, he presented a poster about PCR at the annual Cetus Scientific Meeting, a research conference for employees of the company.

The structure of DNA is key to how PCR works. DNA consists of nucleotides, which serve as the building blocks of DNA. There are four types of nucleotides, represented by the letters A, for adenine, T, for thymine, C, for cytosine, and G, for guanine. DNA is made up of two strands of interlinked nucleotides that pair together and form a helix. In the DNA structure, A always pairs with T, and C always pairs with G. During cell division, cells replicate their DNA by first separating the two strands of DNA and then adding complementary nucleotides to each separated strand. PCR functions similarly to natural DNA replication, except that PCR takes place in a machine called a thermocycler, and researchers target specific sequences to copy.

To amplify DNA using PCR, scientists first separate the two strands of DNA. That is done by heating up the DNA to ninety-five degrees Celsius, which breaks the bonds holding the two strands together. Once the strands are separated, annealing occurs at a much lower temperature, usually fifty-five to seventy-two degrees Celsius. Annealing is the process of primers binding to their complementary sequence on the original DNA to initiate the copying process. For example, if a strand of DNA has the nucleotides CAT, the primer has the complementary nucleotides GTA. Once the primers bind, the researchers raise the temperature to seventy-five degrees Celsius to allow DNA polymerase, the enzyme that adds new nucleotides, to extend the newly synthesized DNA strands. Each cycle doubles the amount of DNA, so at the end of the first round, two DNA strands have become four. Researchers can repeat the cycle up to forty times.

Cetus filed the patent for PCR on 28 March 1985 and took steps to advertise the technology to the broader scientific community. Researchers involved in its creation, including research assistant Randall Saiki, presented PCR at the annual Meeting of the American Society for Human Genetics in October 1985. The group also published an application paper for the technology in the journal Science in December 1985. There, they described using PCR on human blood proteins to identify differences in DNA between individuals with and without sickle cell anemia, which is a genetic blood disorder in which red blood cells are abnormally shaped. Mullis received the Nobel Prize in Chemistry in 1993, which he shared with Michael Smith, who developed techniques to intentionally insert mutations, or sequence changes, into DNA. Though Mullis received the Nobel Prize and much of the recognition for the invention, according to an article by Stephen Scharf, one of the workers at Cetus at that time, many people participated in the actual development of PCR. Mullis may have been the first to think of the idea, but other people who worked at Cetus refined his work and came up with the solution of adding Taq polymerase, leading to debates about the true ownership of the invention of PCR.

PCR has continued to evolve since its first implementation in 1983, and scientists have created a variety of specialized PCR methods that broaden its applications. Techniques like quantitative PCR, also known as real-time PCR, make it possible to measure and characterize genetic material as the thermocycler amplifies it. Researchers use quantitative PCR for analyzing bacterial, viral, and fungal infections, as well as detecting mutations and other genetic variations in DNA. Digital PCR splits DNA into thousands of pieces, runs PCR on every individual piece, and measures how much DNA is present in a sample. Scientists have developed microfluidic PCR, also called point-of-care PCR, to make PCR portable. Instead of sending samples to a lab, researchers can receive real-time PCR results in thirty minutes or less from just a small sample. Scientists also developed isothermal amplification, a method that does not need to change temperatures to amplify DNA.

Impacts

As of 2026, scientists use PCR for many purposes, including diagnosing diseases and solving crimes. For diagnosing a disease, a healthcare provider takes a small sample of blood, saliva, or mucus, which contains DNA, from an individual. A scientist, usually working in a lab setting, then adds a primer that attaches only to the specific DNA sequence of interest, such as a viral sequence. If the PCR technique yields amplification of that sequence, the scientist can determine if the desired DNA sequence is present in the sample. That principle is the basis of COVID-19 diagnostic tests. Furthermore, in criminal investigations, forensic researchers will collect samples of blood, saliva, or mucus from a crime scene and then use PCR to generate enough DNA to analyze. Researchers will then compare the sequence of that DNA sample to a suspect’s or a database of criminals’ DNA sequences. If researchers find a match, they can link someone to a crime. Scientists can also use PCR to find bacteria in food that can make people sick. Additionally, because PCR works quickly and accurately, it helps doctors identify the offending pathogen, allowing them to choose the right treatment faster and avoid using the wrong medicine. Researchers also use PCR to study serious illnesses like cancer and to check for inherited conditions, even before a baby is born.

By allowing scientists to rapidly amplify small amounts of DNA, PCR addresses the challenges associated with analyzing tiny or degraded DNA samples. The technology facilitates genetic analysis in situations where there is a limited amount of DNA, including viral diagnostic tests, forensic crime analysis, and screening for genetic disorders. As of 2026, PCR remains an often-used tool in science and healthcare, supporting the study of diseases and biological systems.

Sources

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