DIFFERENCES AND SIMILARITIES BETWEEN VARIOUS TANDEM REPEAT SEQUENCES - MINISATELLITES AND MICROSATELLITES

Tandemly repetitive DNA sequences are abundantly interspersed in the g enome of practically all eukaryotic species studied. The relative occu rrence of one type of repetitive sequence and its location in the geno me appear to be species specific. A common property of repetitive sequ ences within the living world is their ability to give rise to variant s with increased or reduced number of repeats. This instability depend s upon numerous parameters whose exact role is unclear: the number of repeats, their sequence content, their chromosomal location, the misma tch repair capability of the cell, the developmental stage of the cell (mitotic or meiotic) and/or the sex of the transmitting parent. It is now apparent that mutations in repetitive sequences are a common caus e of human disease, including cancer and disorders which may exhibit a dominant mode of inheritance. Two mechanisms have been proposed to ex plain the instability of repetitive sequences: DNA polymerase slippage , which may account for the instability of short repeats and unequal r ecombination which reshuffles repeat variants and maintains repeat het erogeneity in minisatellites. The purpose of this review is to show th at no general rule can explain the instability of repetitive sequence. Each sequence of repeats is under the influence of local and general biological activities that determine its level of instability.