The Basics...

PCR (Polymerase Chain Reaction) is used to make copies of a section of DNA. It is typically used when you have only a small sample of DNA and you need more copies so you can run your tests or experiment. For example, an investigator may find a hair or small blood stain at a crime scene that contains a small amount of DNA. Before analyzing the DNA, the lab can make millions of copies of it in order to have more DNA to study.

The Details...

PCR Animation - watch this McGraw-Hill animation to learn how PCR works.
PCR Virtual Lab - you can complete a virtual PCR lab at this site to simulate the lab procedure and see what happens inside the PCR machine. (tip - hold down the mouse button when transferring liquid with the micro-pipette)