Cover. Various types of slipped-strand DNAs formed by the disease-associated (CTG)·(CAG) repeats. Slipped homoduplex, S-DNAs, are formed between two strands having the same number of repeats; and heteroduplex slipped intermediates, SI-DNAs, are formed between two strands having different numbers of repeats. Slipped DNAs contained all predicted components including slipped-out repeats and slip-out junctions. In S-DNAs multiple short slip-outs occurred randomly throughout the repeat tract. Strikingly, in SI-DNAs most of the excess repeats slipped-out at preferred locations along the fully base-paired Watson--Crick duplex, forming defined three-way slip-out junctions. Unexpectedly slipped-out CAG (green strands) and slipped-out CTG repeats (red strands) were predominantly in the random-coil and hairpin conformations, respectively. The structural features of SI-DNAs and S-DNAs suggest that they are mutagenic intermediates participating in distinct mutation processes in replicating and non-replicating DNA, respectively. The non-repetitive DNA flanking the repeat is shown in blue. Models represent the electron micrographs of each structure. For details see the paper by Pearson et al. in this issue [Nucleic Acids Res. (2002) 30, 4534--4547].
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