The Polymerase Chain Reaction, which amplifies specific DNA sequences out of mixtures (starting with as little as a single molecule), has revolutionized molecular biology, enabling DNA-based tests that once took months to be performed in an afternoon. But even an afternoon is pretty slow for some purposes, such as diagnostic kits for infectious agents. A team of impatient researchers at Lawrence Livermore National Lab has now managed to cut the time needed for a PCR reaction down from a few hours to less than three minutes.

PCR relies on a cyclical amplification process: high temperatures reset the DNA-copying reaction, lower ones let a new round of reactions start, and they proceed at an intermediate step. The proteins that catalyze these reactions are actually very fast; the delay comes from the time neeed to shift the reactions between these temperatures. Small machines called thermocyclers heat and chill metal blocks as quickly as they can, but it still takes minutes to get through a single cycle. When a typical PCR reaction runs for 30 cycles, that can soak up a lot of time. This not only slows individual PCR reactions down, but also means that the thermocycler isn't available for anyone else's use.

The Livermore team tackled the heating and cooling very simply: their device has two reservoirs of water kept at the high and low temperatures needed during the cycle. The water is pumped through a foamed copper block that contains the sample, enabling it to quickly equillibrate to the target temperature. They also eliminated the time spent at the intermediate temperature, figuring the samples will pass through there long enough on their way between the two extremes.

It doesn't work as well as a normal PCR reaction, but it makes up for that in speed: a short fragment of DNA can be amplified in two minutes, 18 seconds. Boosting the number of temperature cycles up improved the efficiency with a minimal effect on the time—it was still under 5 minutes. There still needs to be a way to assay for the product of a reaction (the authors, in this case, ran a simple gel), but this could turn PCR from an all-afternoon ordeal into something that might be done within an hour, which makes it something that could help diagnose a person's infection while they are still in the office or ER, instead of sending them home and waiting for the test results to come in.

Analyst, 2011. DOI: 10.1039/c1an15365j (About DOIs).