Often only small amounts of DNA are available for forensic analysis so the STRs at each genetic locus are copied many times using the polymerase chain reaction (PCR) to get enough DNA to make a profile. ... Specific primers are used during PCR that attaches a fluorescent tag to the copied STRs.
To produce a DNA profile, scientists examine STRs at ten, or more, genetic loci. These genetic loci are usually on different chromosomes.