DNA Profiling

The technology of DNA profiling has advanced from its beginnings in the 1980s. Today, DNA profiling primarily examines "short tandem repeats," or STRs. STRs are repetitive DNA elements between two and six bases long that are repeated in tandem, like GATAGATAGATAGATA. These repeat sequences often exist in a chromosomal region called heterochromatin, a largely unused portion of DNA found in each chromosome.

Different STR sequences (also called genetic markers) occur at different loci. While their positions are fixed, the number of repeated units varies within the population, from four to forty depending on the STR. Therefore, one genetic marker may have between four and forty different variations, and each variation is referred to as an allele of that marker. Each person has at most two alleles of each marker, one inherited from each parent. The two alleles for a particular marker may be identical, if both parents had the same form.

The United States Federal Bureau of Investigation has designated thirteen of these sequences to use with STR analysis. These thirteen markers are all four-base repeats, and were chosen because multiple alleles of each exist throughout the population. The FBI system, called CODIS (Combined DNA Indexing System), has become the standard DNA profiling system in use today.

STR analysis begins with sample collection. Because of the often small samples involved and the legal weight that will be given to them, it is vital that the sample not be contaminated by other DNA. This may occur for instance if skin cells from the person collecting the sample are mixed with skin cells under the fingernails of a victim. Once the sample is collected, it must be kept secure at all times, to prevent any possibility of tampering.

In the laboratory, the DNA is isolated and purified, and then multiple copies of it are made using the polymerase chain reaction (PCR). Technicians can specify which DNA sequences to multiply, so that only the thirteen core STR sequences will be amplified (multiple copies produced), leaving the rest of the billions of irrelevant bases alone.

In order to specify which DNA to amplify, "primers" are used. The primers are DNA sequences that recognize a nonrepeated sequence in the genetic markers, and which are used by the DNA polymerase that does the actual copying. After the DNA has been copied, the new DNA molecules are separated by size, by gel electrophoresis. A fluorescent molecule previously attached to each primer will send a light signal to the machine that measures the length of the molecule, or allele.