Why wasn't PCR possible until Taq polymerase was discovered?

Polymerase chain reaction (PCR) is a laboratory technique used to amplify a specific segment of DNA. Prior to the discovery of Taq polymerase, PCR was not possible because the DNA polymerase enzymes used in the reaction were heat-sensitive and would denature at the high temperatures required for DNA denaturation.

Taq polymerase, which stands for Thermus aquaticus polymerase, was discovered in the late 1970s in a bacterium called Thermus aquaticus. This bacterium was found living in hot springs and hydrothermal vents where temperatures can exceed 90 degrees Celsius. Taq polymerase, unlike other DNA polymerases used in traditional PCR, is extremely heat-stable and can withstand the high temperatures needed for DNA denaturation.

To perform PCR, the DNA template is first denatured by heating it to around 95 degrees Celsius. This step separates the two strands of the double-stranded DNA molecule. Then, the reaction mixture is rapidly cooled to around 50-60 degrees Celsius, allowing short DNA primers to anneal to each template strand. Finally, the Taq polymerase enzyme is added, and the reaction mixture is heated to the optimal temperature for Taq polymerase activity, typically around 72 degrees Celsius. At this temperature, Taq polymerase extends the primers, synthesizing new DNA strands complementary to the template strands.

The heat stability of Taq polymerase makes it an essential enzyme for PCR because it remains active even at the high temperatures required for DNA denaturation. Other DNA polymerase enzymes used previously would denature at these high temperatures, rendering them ineffective for PCR.

In summary, Taq polymerase's extreme heat stability enables the denaturation and amplification of DNA in PCR, making it a crucial discovery in the development of this groundbreaking technique.