How DNA Profiling (or DNA Fingerprinting) Works
Amplifying STRs and Determining Their Sizes
DNA profiling starts with isolating DNA from an organism's cells, including from hair roots, saliva, body tissue, and even elephant tusks and dung. A single sample does not provide enough DNA to analyze, so scientists use a technique called the polymerase chain reaction (PCR) to amplify (make billions of copies of) certain regions of an individual's DNA.
To isolate and amplify STRs, scientists use primers complementary to portions of the flanking sequences at different loci (also referred to as markers). This ensures that the regions between the bound primers, which include the repeat units, are amplified. The end product is a DNA sample that contains billions of copies of individual STR fragments. After PCR, DNA samples from each individual are placed in their own wells on an agarose gel. An electrical current is then applied to the gel, causing the negatively charged DNA to move toward the positively charged electrode. The longer the DNA fragment, the more slowly it moves through the gel. By the end of the "run," the fragments are separated out; the longest ones will be closest to the starting point and the shortest ones will be nearest the far end. For example, for two alleles at the same locus, an allele with 9 repeats will migrate more slowly than one with 5 repeats. A sample of DNA containing fragments of known lengths (a DNA ladder) is used as a reference for determining the STR fragment sizes in each sample.