Why are the distances between the cuts so different in different people?
In 1986, Alec Jeffreys, a geneticist in England, was studying introns – stretches of DNA located in between the genes. Introns are sequences of “nonsense” or “junk” DNA that do not code for a specific protein. Jeffreys found that some introns are made up of short DNA base pair sequences that repeat over and over again. The number of repetitions varies greatly from person to person; for example, in one person a particular base pair sequence may repeat ten times, while in another person the same base sequence may repeat twenty-five times. Intron lengths vary so much from person to person that after a restriction enzyme has cut the DNA, the resulting set of intron fragments is unique for every individual (except in the case of identical twins). Jeffreys used gel electrophoresis to separate the DNA fragments according to their length. The technique involves placing a sample of the restriction fragments into a “well” in the gel and turning on an electric field. Since all the fragments are negatively charged, they all start to migrate toward the positive end of the gel, but the shorter fragments move faster and so move farther across the gel. The fragments eventually spread themselves into a series of bands (Figure 1), each band composed of fragments of similar length. (Jeffreys had to use radioactive probes and X-ray film to visualize the bands, but you will be using ultraviolent “black” light.) Because every person has a distinctly unique RFLP banding pattern, Jeffreys called this technique “DNA fingerprinting.” 1 Restriction enzymes are found in bacteria and probably evolved as a defense against invading viruses. More than 600 restriction enzymes are commercially available. Figure 1: The RFLP banding patterns of a child, the mother, and the alleged father are shown here. Is the alleged father really the biological father of the child? Hint: Half of a person’s genome comes from each parent.