Technicians can specify which DNA sequences to multiply, so that only the thirteen core STR sequences will be amplified (multiple copies produced), leaving the rest of the billions of irrelevant bases alone.

In order to specify which DNA to amplify, "primers" are used. The primers are DNA sequences that recognize a nonrepeated sequence in the genetic markers, and which are used by the DNA polymerase that does the actual copying. After the DNA has been copied, the new DNA molecules are separated by size, by gel electrophoresis. A fluorescent molecule previously attached to each primer will send a light signal to the machine that measures the length of the molecule, or allele.

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