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DNA Goes to Court

Within the last century, detectives increasingly have turned to scientific evidence to help solve crimes. The distinctiveness of the human fingerprint was first described in 1892, the same year the legendary Sherlock Holmes was created. Soon to follow were the techniques for ABO blood typing and leukocyte antigen tissue typing. DNA fingerprinting, first introduced in U.S. courts in 1988, was considered to be the greatest forensic advance since classical fingerprinting. Recently, due primarily to the O.J. Simpson murder trial, formerly esoteric topics such as the polymerase chain reaction and restriction fragment length polymorphisms are found even in popular news magazines. DNA typing is based on an observation made in 1984 by Sir Alec Jeffreys. Jeffreys found something rather odd in the non-coding region of the human genome: multiple copies of short nucleotide sequences, 3 to 30 base pairs long, repeated one after another 20 to 100 times [e.g., GACTGACTGACT]. These groups of repeat sequences, called minisatellites or VNTRs (variable number of tandem repeats), are now known to be widely scattered throughout the human genome. Everyone has these repeat units in their DNA, but the number of these regions at different loci are different in each individual. Only identical twins end up with the same numbers and patterns of VNTRs. Figure 1 Length polymorphism due to VNTRs.

Although DNA molecules from different individuals are more alike than they are different, there are areas of the human genome that exhibit a great deal of diversity. Such regions are called hypervariable or "polymorphic" (meaning many forms). These regions provide the markers for genetic disease diagnosis and DNA typing. Many DNA polymorphisms are found in the parts of the genome that do not code for protein. In humans, this non-coding DNA constitutes approximately 95 percent of the genome! DNA polymorphisms are divided into two categories: sequence polymorphisms, such as occur within the genes of the human leukocyte antigen (HLA) complex; and length polymorphisms, exemplified by the VNTR loci. In the case of VNTRs, the polymorphism is the difference in the number of times the sequence is repeated. For example, an individual heterozygous for a VNTR locus may have 3 copies of the repeat on one chromosome and 7 copies, at that same position, on the homologous chromosome. Thus, when a DNA sample is cut with an endonuclease (one that does not cleave the repeat unit) and probed with a radioactive VNTR, the length of the resultant restriction fragment is a function of the number of copies of the tandem repeats within the fragment (Fig. 1). The number of repeat units can range from a few to a few hundred, so that any VNTR locus can exist in one of several forms, or alleles. An allele is one of two or more alternative forms of a single genetic locus. Since there are many different numbers of repeat units for any one locus among different people, the fragment pattern revealed by analysis of multiple VNTR loci constitutes a nearly unique genetic profile for every individual. Go to next page...

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