How a few DNA strands become a testable sample
PCR works by making copies of a chosen stretch of DNA, rather than duplicating everything in a cell. The choice is made with primers: short pieces of DNA designed to attach to sequences on either side of the region researchers want to examine. They mark where copying should begin and end.
Three temperature changes drive the copying
A thermal cycler moves the sample through a repeating sequence. First, heat separates the two intertwined DNA strands. Then the sample cools so the primers can bind to their matching sequences. Finally, an enzyme called a polymerase builds new strands by adding DNA building blocks along the exposed templates.
Each newly made strand can serve as a template in the next round. That is what makes the process compound: the number of target-region copies can roughly double from one cycle to the next, so repeated cycles produce a dramatic increase from a very small starting amount. The polymerase must keep working through repeated heating, which is why PCR uses an enzyme suited to high temperatures.
Why targeted copying matters
A crime-scene trace may contain too little DNA for direct analysis. PCR can amplify selected regions until there is enough material for tests that distinguish patterns among samples. The primers determine which regions are copied; the copying itself does not identify whose DNA it is. Instead, it supplies enough of the chosen material for later analysis to make comparisons possible.
The crucial shift is from a nearly invisible trace to a testable quantity: the machine does not create new genetic information, but repeatedly reproduces the small piece investigators need to inspect.
