Given the importance of this technique it is amazing how simple they underlying principles are. The PCR reaction just uses the cellular machinery of DNA replication and a cycling of reaction temperatures to achieve amplification of the desired DNA target sequence. This cycling of temperatures, or thermocycling, doubles the amount of target DNA after every cycle leading to the astonishing amplification of even extremely minute amounts of starting material. This is how we are able to convict criminals from the smallest amounts of DNA left at a crime scene or even amplify up DNA sequences from long extinct species.