Human minisatellites, repeat DNA instability and meiotic recombination

Minisatellites include some of the most variable loci in the human genome a nd are superb for dissecting processes of tandem repeat DNA instability. Si ngle DNA molecule analysis has revealed different mutation processes operat ing in the soma and germline. Low-level somatic instability results in simp le intra-allelic rearrangements. In contrast, high frequency germline insta bility involves complex gene conversions and is therefore recombinational i n nature, almost certainly occurring at meiosis. To determine whether true meiotic crossovers occur at human minisatellites, we have used polymorphism s near the repeat array to recover recombinant DNA molecules directly from sperm DNA. Analysis of minisatellite MS32 has revealed an intense and highl y localised meiotic crossover hotspot centred upstream of the array, the fi rst example of a human hotspot defined at the molecular level. This hotspot extends into the beginning of the repeat array, resulting in unequal and e qual crossovers. Array crossovers occur much less frequently than array con versions but appear to arise by a common process, most likely by alternativ e processing of a recombination initiation complex. The location of MS32 at the boundary of a recombination hotspot suggests that this locus has evolv ed as a by-product of localised meiotic recombination activity, and that mi nisatellites might in general mark recombinationally proficient hotspots or hot domains in the genome. Finally, sperm crossover analysis makes it poss ible to explore the molecular rules that govern human meiotic recombination , and to detect phenomena such as meiotic drive that could provide a possib le connection between recombination and DNA sequence diversity itself.