Polymorphic tandemly repeated sequences in human DNA
Tandemly repeated tracts of DNA are a ubiquitous feature of eukaryote genomes. One class of tandem repeats, 'minisatellites', have been shown to be highly variable both in overall length and in the internal arrangement of variant versions of the repeating unit along the array. Consequently, both length and internal variation at these loci can be exploited to generate individual-specific profiles of use in forensic science and the establishment of family relationships. Recently it has been demonstrated that short dinucleotide repeats, or 'microsatellites', and other simple tandem repeat arrays can also show length variation. This work describes the isolation and characterization of simple tandem repeat arrays, and their application in a forensic science case. The evolutionary persistence of variability at tandem repeat loci is also explored. Simple tandem repeats isolated were frequently associated with other tandem repeats and interspersed repetitive elements, a phenomenon previously described for minisatellites and perhaps indicative that certain genomic regions show relaxed fidelity in the maintenance of large-scale DNA structure, allowing tandem array expansion and retroposon insertion. Although variability is low relative to minisatellites, microsatellites, owing to limited length, can readily be amplified from highly degraded DNA using the polymerase chain reaction. Consequently it was possible to identify positively the skeletal remains of a murder victim by comparing microsatellite profiles of the skeleton with those of the presumptive parents. Comparative studies of microsatellite and minisatellite loci between man and other primates indicate that in evolutionary terms, the variable state is reasonably persistent at microsatellite loci, whereas highly variable minisatellites show extreme evolutionary transience. Such transience was also demonstrated for two large 'midisatellite' loci, suggesting that highly variable tandem repeat loci are extremely unstable and transient, whereas lower variability leads to evolutionary persistence of the variable state.