In the early 1980s the basic obstacle to work on DNA was quantity: obtaining enough material to study a gene meant cloning in bacteria, a process of weeks. Trace samples from a crime scene or a fossil were usually unusable.

The polymerase chain reaction Kary Mullis described in 1983 reduces the problem to a cycle. Heating the mixture separates DNA's two strands; cooling lets short primers bind to the ends of the target region; a polymerase enzyme synthesises new strands from those points. Each cycle doubles the number of copies — thirty cycles means more than a billion. What made the method practical was Taq polymerase, obtained from *Thermus aquaticus*, a bacterium living in hot springs, which is not destroyed at high temperature; that an extremophile had been studied was the precondition of a biotechnology revolution thirty years later.

Its effect crossed disciplines. The sequencing pipeline of the Human Genome Project, DNA fingerprinting in forensics, paternity testing, diagnosis of inherited disease, ancient DNA work (including the identification of the Denisovans) and clinical virology all rest on PCR. The RT-PCR used worldwide as the standard diagnostic in the COVID-19 pandemic of 2020 is the same cycle adapted to RNA. Mullis received the Nobel Prize in Chemistry in 1993; that most components of the method already existed and the contribution was the idea combining them is a point that keeps the question of authorship alive.