A minisatellite is a section of DNA that consists of a short series of bases 10-60bp. These occur at more than 1000 locations in the human genome. Some minisatellites contain a central (or "core") sequence of letters “GGGCAGGAXG” (where X can be any letter) or more generally a strand bias with purines (Adenosine (A) and Guanine (G))on one strand and pyrimidines (Cytosine (C) and Thymine (T)) on the other. Use the nemonic "pure As Gold" and you will recall that the purines are Adenosine (A) and Guanine (G). Use the nemonic "'CUT'" the Pie" and recall from the C-U-T that pyrimidines are Cytosine (C), Uracil (U)(RNA only for Uracil), and Thymidine (T). It has been proposed that this sequence per se encourages chromosomes to swap DNA. In alternative models, it is the presence of a neighbouring cis-acting meiotic double-strand break hotspot which is the primary cause of minisatellite repeat copy number variations. Somatic changes are suggested to result from replication difficulties (which might include replication slippage, among other phenomena). When such events occur, mistakes are made, this causes minisatellites at over 1000 locations in a persons genome to have slightly different numbers of repeats, thereby making them unique. The most highly mutable minisatellite locus described so far is CEB1 (D2S90) described by Vergnaud.
Some human minisatellites (~1%) have been demonstrated to be hypermutable, with an average mutation rate in the germline higher that 0.5% up to over 20%, making them the most unstable region in the human genome known to date. It is interesting to note that while other genomes (mouse, rat and pig) contain minisatellite-like sequences, none was found to be hypermutable. Since all hypermutable minisatellites contain internal variants, they provide extremely informative systems for analyzing the complex turnover processes that occur at this class of tandem repeat. Minisatellite variant repeat mapping by PCR (MVR-PCR) has been extensively used to chart the interspersion patterns of variant repeats along the array, which provides details on the structure of the alleles before and after mutation.
Studies have revealed distinct mutation processes operating in somatic and germline cells. Somatic instability detected in blood DNA shows simple and rare intra-allelic events two to three orders of magnitude lower than in sperm. In contrast, complex inter-allelic conversion-like events occur in the germline (reviewed by Vergnaud and Denoeud, Genome Research 2000).
Additional analyses of DNA sequences flanking human minisatellites have also revealed an intense and highly localized meiotic crossover hotspot that is centered upstream of the unstable side of minisatellite arrays. Repeat turnover therefore appears to be controlled by recombinational activity in DNA that flanks the repeat array and results in a polarity of mutation. These findings have suggested that minisatellites most probably evolved as bystanders of localized meiotic recombination hotspots in the human genome.