DNA Fingerprints

Except for identical twins, no two people have exactly the same sequence of bases in their DNA. By detecting the differences in DNA sequences, scientists can distinguish one person from another. As you know, each human has a unique set of fingerprints, a marker of his or her identity. Like all other sexually reproducing species, each human also has a DNA fingerprint, which is a unique array of DNA fragments that were inherited from each parent in a Mendelian pattern. DNA fingerprints are so accurate that even full siblings are readily distinguished from one another.

right comparison of DNA fingerprints

More than 99 percent of the DNA is exactly the same in all humans. But DNA fingerprinting focuses only on the part that tends to differ from one person to the next. Throughout the human genome are tandem repeats - short regions of repeated DNA - that differ substantially among people. For example, the five bases T T T T C are repeated anywhere from four to fifteen times in tandem in different people, and three bases (CGG) are repeated five to fifty times in tandem. By examining many tandem-repeat sites, researchers found out that each person carries a unique combination of repeat numbers.

Researchers detect differences at tandem-repeat sites with gel electrophoresis. In this case, it separates DNA fragments according to their length. Size alone dictates how far each fragment moves through the gel, so tandem repeats of different sizes migrate at different rates. A gel is immersed in a buffer solution, then DNA fragments from individuals are added to the gel. When an electric current is applied to the solution, one end of the gel takes on a negative charge, and the other end a positive charge. DNA molecules carry a negative charge (because of the negatively charged phosphate groups), so they migrate through the gel toward the positively charged pole. They do so at different rates, and so they separate into bands according to length. The smaller the fragment, the farther it will migrate. After a set time, researchers can identify fragments of different lengths by staining the gel or by specifically highlighting fragments that contain tandem repeats.

Here a child's paternity and maternity can be clearly seen written in its DNA profile using three different restriction enzymes. Half of the child's (c) makers come form its mother (m), and half from its father (f). An unrelated individual is shown in the last lane (u).

Figure here shows a series of tandem-repeat DNA fragments that were separated by gel electrophoresis. They are DNA fingerprints from seven individuals and from blood collected at a crime scene. Notice how much their DNA fingerprints differ. Can you identify which one of the patterns exactly matches the pattern from the crime scene? DNA fingerprints help forensic scientists identify criminals and victims, and exclude innocent suspects. A few drops of blood or cells from a hair follicle at a crime scene etc., on a suspect's clothing often yield enough to do the trick.

Law enforcement officials have found an increasing number of uses for profiling in criminal cases, or paternity cases.