Molecular geneticist loading a thermal cycler, Institute of Child Health, London, May 2000. Polymerase chain reactions (PCRs) are used to extract DNA from tiny tisue samples. A thermal cycler heats DNA to 95 degrees centigrade; at this temperature, DNA unzips into two separate strands, which the builder enzyme uses to make more DNA copies. Most enzymes are destroyed at this temperature, so the key to PCR is to use a super heat-resistant type of the builder enzyme, which is found in bacteria that live in hot water springs and so can survive hot temperatures.
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