Genetics & Molecular · 1985
Polymerase Chain Reaction (PCR)
In the early 1980s, studying a specific stretch of DNA meant cloning it into a bacterial vector, growing up sufficient cells, and extracting the product through a laborious series of steps that took days to weeks and required substantial starting material. Detecting a pathogen's genetic sequence in a clinical sample, or identifying a single-nucleotide variant in a patient's genome, was a research exercise rather than a routine clinical act. The sensitivity of available hybridization methods was limited by the amount of target DNA present in any given specimen.
Kary Mullis, a biochemist working at Cetus Corporation in Emeryville, California, conceived the idea of polymerase chain reaction during a drive on Highway 128 in Mendocino County in 1983. The insight was that if primers flanking a target sequence could be extended by a polymerase and the product denatured and re-primed repeatedly, the target would double with each thermal cycle, reaching millions of copies in a matter of hours. The method was first described in the scientific literature in 1985, in a paper from the Cetus group reporting its application to sickle cell anemia diagnosis. The initial protocols used a DNA polymerase that was destroyed by each heating step and had to be replenished manually each cycle. That limitation was solved when Saiki and colleagues at Cetus substituted Taq polymerase, isolated from the thermophilic bacterium Thermus aquaticus, which survived denaturation temperatures intact.
Randall Saiki, also at Cetus, was central to refining the early PCR protocols and to the Taq substitution that made the thermal cycler a fully automated instrument rather than a bench procedure requiring constant attention. Once automation was practical, PCR spread through molecular biology laboratories with unusual speed. By the early 1990s, clinical applications were multiplying faster than regulatory frameworks could process them: HIV viral load quantification, direct detection of Mycobacterium tuberculosis in sputum, diagnosis of herpes simplex encephalitis from cerebrospinal fluid, and prenatal identification of single-gene disorders from chorionic villus samples were all becoming clinically available.
Mullis shared the 1993 Nobel Prize in Chemistry with Michael Smith, who had independently developed site-directed mutagenesis. The Nobel Committee's citation for Mullis emphasized that PCR had transformed molecular biology from a labor-intensive research discipline into a technique accessible to any equipped laboratory. Forensic applications followed in parallel with clinical ones: DNA profiling from trace samples became central to criminal investigation and exoneration work, and PCR's ability to amplify degraded DNA made it applicable to archaeological and historical specimens.
The method's reach became most visible to the general public during the COVID-19 pandemic, when RT-PCR for SARS-CoV-2 RNA was deployed as the reference standard for diagnosis in hundreds of millions of tests globally. The same thermal cycling principle Mullis sketched out on a California road in 1983 was running in mass-throughput laboratory platforms forty years later. Rapid point-of-care PCR platforms, which return results in under an hour from swab to answer, are now standard for influenza, Group A streptococcus, and respiratory panel testing in emergency and outpatient settings.
Key People
- Kary Mullis — Biochemist who conceived and developed the PCR method
- Randall Saiki — Cetus Corporation researcher who refined early PCR protocols and introduced Taq polymerase
- Henry Erlich — Cetus scientist who led early PCR applications in genetics and diagnostics
- Michael Smith — Biochemist who shared the 1993 Nobel Prize in Chemistry with Mullis
Science. 1985;230(4732):1350-1354.
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- 1975 · Southern blot: DNA detection by gel transfer and hybridization (Genetics & Molecular)