PCR HOME:
PCR HISTORY:
PCR PROJECT:
WHAT IS PCR?
What is a polymerase? A polymerase is a naturally occurring enzyme, a biological macromolecule that catalyzes the formation and repair of DNA (and RNA). The accurate replication of all living matter depends on this activity -- an activity scientists have learned to manipulate. In the 1980s, Kary Mullis at Cetus Corporation conceived of a way to start and stop a polymerase's action at specific points along a single strand of DNA. What is the chain reaction? Mullis also realized that by harnessing this component of molecular reproduction technology, the target DNA could be exponentially amplified. When other Cetus scientists eventually succeeded in making the polymerase chain reaction perform as desired in a reliable fashion, they had an immensely powerful technique for providing essentially unlimited quantities of the precise genetic material molecular biologists and others required for their work.
What is PCR? Though the simplest and most convenient way to define PCR is as a technique, such compartmentalizing eliminates the history of PCR's invention, thereby covering over the contingent manner of its emergence and the practices and subjects required to make it work. The next simplest answer is to name an individual as the inventor of the concept. The obvious candidate is Kary B. Mullis, who was awarded the 1993 Nobel Prize for chemistry for PCR. However, this terrain is contested. Other scientists, including, Henry Erlich, Norman Arnheim, Randall Saiki, Glen Horn, Corey Levenson, Steven Scharf, Fred Faloona and Tom White, were instrumental in making PCR work. A third argument holds that PCR did not exist until it was made to work in an experimental system. In this view, merely thinking of a concept is not sufficient; a scientific advance must include creating a way to show that the concept has successfully been put into practice.
When Science named PCR and the polymerase that it employs as its first "Molecule of the Year" in 1989, the editor, Daniel Koshland Jr., provided a succinct explanation of PCR. He wrote: "The starting material for PCR, the 'target sequence,' is a gene or segment of DNA. In a matter of hours, this target sequence can be amplified a million fold. The complementary strands of a double-stranded molecule of DNA are separated by heating. Two small pieces of synthetic DNA, each complementing a specific sequence at one end of the target sequence, serve as primers. Each primer binds to its complementary sequence. Polymerases start at each primer and copy the sequence of that strand. Within a short time, exact replicas of the target sequence have been produced. In subsequent cycles, double-stranded molecules of both the original DNA and the copies are separated; primers bind again to complementary sequences and the polymerase replicates them. At the end of many cycles, the pool is greatly enriched in the small pieces of DNA that have the target sequences, and this amplified genetic information is then available for further analysis."
Koshland described described PCR entirely in terms of molecular biological technique. In his "molecule of the year" history, Koshland said absolutely nothing about who invented PCR. In an account he gave to the Smithsonian Institution's Archive of Biotechnology, Kary B. Mullis defined PCR not as a specific technique, or bundle of techniques, but rather as a concept. For Mullis, PCR came into existence at the moment he conceived of it. For him, making the concept work was of secondary importance. He says: "The thing that was the 'Aha!' the 'Eureka!' thing about PCR wasn't just putting those [things] together ...the remarkable part is that you will pull out a little piece of DNA from its context, and that's what you will get amplified. That was the thing that said, 'My God, you could use this to isolate a fragment of DNA from a complex piece of DNA, from its context. That was what I think of as the genius thing.[...]..In a sense, I put together elements that were already there. [ ] You can't make up new elements, usually. The new element, if any, it was the combination, the way they were used. ... The fact that I would do it over and over again, and the fact that I would do it in just the way I did, that made it an invention...the legal wording is 'presents an unanticipated solution to a long-standing problem,' that's an invention and that was clearly PCR."
Mullis's thesis is partially plausible: he is correct that the specific techniques that composed PCR were not new per se. However, his general claim that technical elements are not invented is totally implausible. It is possible to date the technique for making oligonucleotides (short strings of bases of defined length and composition); the development of the electrophoretic gel on which DNA is made to migrate by an electrical current, the means used to separate out strands of different sizes; and the techniques used to transfer these strands to a membrane and detect them. What was original, powerful and significant was the concept that combined -- and reconfigured -- these existing techniques.
Almost everyone now agrees that Kary Mullis thought up the concept of PCR. However, a group of former Cetus scientists and technicians maintains that it only when an experimental system was developed did PCR become a scientific entity. In this view, PCR needed to be more than a series of disparate technical elements, and more than the synthesizing of these elements into a distinctly innovative concept. The concept needed to be practiced, producing results that met scientific standards. As Henry Erlich, a senior scientist at Cetus during PCR's development puts it: "Once PCR had been worked out, i.e. developed, only then was it useful." Erlich and other Cetus scientists seem to agree with Francois Jacob's dictum: "In biology, any study begins with the choice of a 'system.' On this choice depends the experimenter's freedom to maneuver, the nature of the questions he is free to ask, and even, often the type of answer he can obtain."
Further, although Mullis claims that PCR was the solution to a long-standing problem, he never says what that problem was. Another scientist at Cetus, Stephen Scharf, is more perceptive when he says that the truly astonishing thing about PCR is precisely that it wasn't designed to solve a problem; once it existed, problems began to emerge to which it could be applied. One of PCR's distinctive characteristics is unquestionably its extraordinary versatility. That versatility is more than its "applicability" to many different situations. PCR is a tool that has the power to create new situations for its use and those required to use it.
PCR's versatility has been astounding, scientists have produced new contexts and new uses with stunning regularity. These uses have opened new avenues of research, which have in turned proved amenable to new uses of PCR. In less than a decade, PCR has become simultaneously an absolutely routine component of practically every molecular biology laboratory and a constantly changing tool whose potential has shown no signs of leveling off.
Paul Rabinow -- Berkeley, Calif., 1998