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How DNA Profiling (or DNA Fingerprinting) Works

DNA profiling is a forensic technique used to identify individuals based on differences, or variations, in their DNA sequence.

Some regions of the DNA in your cells' chromosomes have a large number of differences among individuals, and even between an individual's two copies. One type of highly variable sequence consists of short tandem repeats (STRs). An example of an STR is the 38-base-pair (bp) fragment below. (DNA is double stranded, but here only one strand is shown.)

-AAGTCTACTACTACTACTACTACTACTACTACGAATCA-

All STRs contain a repeat unit of a few bases, or nucleotides. In this case, the repeat unit is CTA. There are 9 repeats of the CTA sequence. The bases or nucleotides adjacent to the repeats are referred to as flanking sequences. An individual can have two versions, or alleles, of an STR sequence, one from each parent—for example, one allele with 9 CTA units and one with five CTA units. There may be many different alleles among individuals in a population, with each allele having a different number of repeat units.

An individual's DNA profile consists of STRs from several locations, or loci, throughout the genome. A DNA profile can be visualized as a pattern of bands on an agarose gel after electrophoresis, with each STR yielding one or two bands for one individual. Because STRs are so highly variable, the likelihood of two individuals possessing the exact same DNA profile is low. If you sample enough loci, the pattern of bands in an individual’s DNA profile can be considered unique and can be used to identify individuals, much like a fingerprint. In fact, DNA profiling is also called DNA fingerprinting.

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