PCR--The technique that changed everything. It is now possible to work with a single molecule of DNA!

With two primers [local], it is trivial to use DNA polymerase and the 4 dNTPs to produce both strands of the intervening sequence in vitro. However, this only doubles the amount of the DNA between the primers. The Polymerase Chain Reaction takes advantage of heat-stable polymerases to combine primer-directed DNA synthesis with repetitive cycles of denaturation and reannealing to double the amount of DNA with each cycle [local].

Explanation [local] of the polymerase chain reaction. Once you understand it, it seems simple.

Animation of the polymerase chain reaction.