1983

Polymerase Chain Reaction

Har Gobind Khorana (1922–2011), Kjell Kleppe (1934–1988), Kary Banks Mullis (b. 1944)

Like many great discoveries, the polymerase chain reaction (PCR)—the rapid synthesis of large quantities of a given DNA segment—seems simple, and like many great discoveries, it had been anticipated. DNA polymerase was a known enzyme that would copy an existing DNA segment. Indian-American chemist Har Gobind Khorana and Norwegian biochemist Kjell Kleppe had used it in the late 1960s to make several copies of a DNA sequence, proving that it could be done outside of a living cell. American biochemist Kary Banks Mullis added a key insight—if the reaction temperature was cycled up and down, it might be possible to amplify very small samples of DNA in an unheard-of fashion. Mullis was awarded the Nobel Prize in Chemistry in 1983 for his discovery.

Heating the DNA unravels the hydrogen bonds holding the double helix together, leaving it open for the polymerase. Once the solution cools, the enzyme can assemble a DNA sequence from individual nucleotide building blocks, but it needs “primers,” short stretches of preassembled DNA that are complementary to the starting and ending regions of the whole sequence being copied. Far from being a complication, though, these primers allow a researcher to zero in on exactly the sequence of interest. The newly formed DNA sequences go on to serve as templates for still more copies, leading to an exponential increase of newly synthesized DNA.

Mullis had some trouble convincing his coworkers that he’d gotten his ideas to work, but his colleagues Randall Saiki and Henry Erlich were able to improve the technique. The original DNA polymerase was inactivated in each heating cycle and had to be added fresh, but a variant of the enzyme Taq, from an organism that lived in boiling hot springs, was rock-stable. Taq polymerase PCR became a huge success, the subject of both huge financial deals and huge patent lawsuits. Later versions are now indispensable in every field that touches on DNA—and they are numerous, with anthropology, archaeology, genetics, forensics, medicine, biotechnology, and molecular biology among the most prominent. PCR (and fast DNA sequencing) has remade the world.

Morning Glory Pool in Yellowstone National Park, the sort of habitat where the Taq-containing bacterium was found. Extreme environments breed extreme life-forms, which contain extreme enzymes.