FIG. 11.

DSB-induced expansions and contractions of a tandem repeat. (A) To test directly if a DSB can induce rearrangements in tandem repeats in yeast, Pâques et al. (368) tested HO-induced gene conversions with a homologous donor sequence containing an intervening interval including 8 repeats of 375 bp. Perfect copying of the template should introduce the whole intervening repeated locus into the repaired molecule. However, only about half of the repaired chromosomes acquire an unmodified repeated array. The others have a variable number of repeats ranging from 1 to 13 copies. These frequent rearrangements are restricted to the repaired recipient molecule, with the donor template remaining unmodified. Similar result have been obtained with an artificial or real yeast 36-bp minisatellite locus (365) and with a microsatellite CTG locus (402). (B) The rearrangements may result from replication slippage occurring during the semiconservative kind of DNA synthesis predicted by Szostak et al. (494); however, the rearrangement would be expected to be found in the donor template as well as in the recipient. The clustering of the rearrangements in the recipient molecule can be better explained by an SDSA model. (C) In this SDSA model, both 3′ ends initiate DNA synthesis. The newly synthesized strands are then unwound from their template and annealed. Because of the redundant structure, there are many possibilities of annealing, resulting in expansions or contractions. (D) Another possibility is that the kind of DNA synthesis associated with SDSA (bubble migration for example) would easily generate slippage-like events. If DNA synthesis stops before the new strands overlap with the other 3′ end, reinvasion has to occur. Because of the redundant structure, there are many possibilities of reinvasion, which would be responsible for the expansions and contractions that are always found on the recipient molecule because the newly synthesized sequences return to the repaired molecule. Resolution by annealing can occur, but HJs also can be formed and lead to crossovers, as proposed in Fig. 8C. This last feature has the advantage of explaining why the infrequent crossover events found in this experiment were associated with tandem repeat rearrangements as often as were the noncrossover events.