Technology & Medicine › Digital age › Genetic engineering (recombinant DNA)
PCR (polymerase chain reaction)
1983 (Mullis)· Invention or discovery· Germs, drugs & genetics
Well documentedSolid records, physical evidence or scientific consensus.
A lab technique that copies a chosen stretch of DNA billions of times in a few hours.
See it on the Technology & Medicine map →How it works
Kary Mullis, a chemist at the biotech firm Cetus, conceived it in 1983 while driving in northern California. A tube of DNA is heated to separate its two strands, then cooled so short 'primers' mark the stretch to copy, and an enzyme builds new strands. Each cycle doubles the target; 30 cycles make about a billion copies. From 1988 a heat-proof enzyme from bacteria in Yellowstone's hot springs let machines run the cycles automatically.
Why it mattered
Tiny traces of DNA became readable: it powers forensics, paternity tests, genetic research, ancient-DNA studies and infection tests, including the standard COVID-19 test.
Think of it like
Like a photocopier that copies its own copies: after 30 rounds one page becomes about a billion.
How we know
Cetus's 1985 and 1988 papers in Science, Mullis's patents, his 1993 Nobel Prize in Chemistry, and the 1991 sale of the rights to Roche for $300 million.
Still unknown or debated
Credit is debated: Cetus colleagues did much of the work making it practical, and a similar idea was described in 1971 by Kjell Kleppe and Gobind Khorana.
Connections (3)
▸PCR (polymerase chain reaction) sped up the Human Genome ProjectMostly agreed
The connection
PCR copies a chosen stretch of DNA billions of times in hours. Genome labs used it to fill gaps and to create landmark markers, called sequence-tagged sites, for mapping chromosomes. It was a workhorse rather than the main engine; most DNA for sequencing was copied by growing it in bacteria.
When and how
Kary Mullis at Cetus in California, 1983; marker-based mapping proposed in 1989; used throughout the project's labs, 1990–2003.
How sure are we?
1990s mapping papers from the project describe PCR-based markers; Mullis's 1993 Nobel Prize recognized PCR's central role in genetics.
Debates and open questions
How much PCR specifically sped the project, compared with automated sequencers and computing, is hard to measure.
▸PCR (polymerase chain reaction) became the standard test for the COVID-19 pandemicWell documented
The connection
The standard COVID-19 test was RT-PCR: the virus's RNA is first converted into DNA, then copied by PCR until detectable. Within days of the virus's genome being posted in January 2020, Berlin's Charité hospital designed a test that the WHO shared worldwide. Billions of PCR tests guided diagnosis, travel and quarantine.
When and how
Virus genome shared January 10–11, 2020; Christian Drosten's Charité team in Berlin published a test protocol by January 23; testing then ramped up worldwide.
How we know
WHO-listed protocols, published validation studies and national testing records document PCR's central role.
Debates and open questions
How to interpret positives with very low viral amounts, and why early rollouts were slow in countries like the US, remain points of debate.
▸PCR (polymerase chain reaction) monitors the success of Antiretroviral therapyMostly agreed
The connection
Doctors judge whether HIV treatment works by measuring viral load, the amount of virus in the blood. PCR-based tests, approved in the US from 1996, can detect a few dozen copies per milliliter. 'Undetectable' results show the drugs are working and that the patient cannot pass the virus on sexually; rising levels signal drug resistance.
When and how
Roche's Amplicor viral-load test, FDA-approved in 1996, the same year triple-drug therapy arrived; used in HIV clinics worldwide since.
How sure are we?
WHO and US treatment guidelines require regular viral-load testing; studies such as HPTN 052 and PARTNER used it to show undetectable means untransmittable.
Debates and open questions
In poorer countries viral-load tests are often scarce or slow, so many patients are monitored less closely; cheaper bedside tests are being expanded.