The Double StrandBreak Model

In the Holliday model, recombination starts with singlestrand breaks at the same positions in two homologous DNA molecules. The double-strand-break model, in contrast, begins with double-strand breaks in one of the two aligned DNA molecules (IFigure 12.24a). On both sides of the break, an enzyme nibbles away nucleotides, producing a gap in the DNA, with some single-stranded DNA on each side ( Figure 12.24b). A free 3' end then invades the other unbroken DNA molecule and displaces the homologous strand (I Figure 12.24c). The 3' end of the invading strand is elongated by DNA synthesis, which further displaces the original strand of the unbroken molecule (i Figure 12.24d).

The displaced strand forms a loop that fills the gap in the broken DNA molecule (I Figure 12.24e) and serves as a template for the synthesis of a complementary DNA strand (IFigure 12.24f). The result is that two heterodu-plex DNA molecules are joined by two cross bridges (iFigure 12.24g), in contrast with the single cross bridge produced in the Holliday single-strand-break model.

The interconnected molecules produced in the double-strand-break model can be separated by further cleavage and reunion of the nucleotide strands, in the same way that the Holliday intermediate is separated in the Holliday single-strand-break model (see Figure 12.23g-k). Patched or spliced recombinant products can be produced, depending on whether cleavage is in the vertical or the horizontal plane.

Evidence for the double-strand-break model originally came from results of genetic crosses in yeast that could not be explained by the Holliday model. Subsequent observations in yeast showed that double-strand breaks appear in meiosis during prophase I when crossing over occurs and that mutant strains that are unable to form double-strand breaks do not exhibit meiotic recombination. Although considerable evidence supports the double-strand-break model in yeast, the extent to which it applies to other organisms is not known.

(i) Non-crossover recombinants

(k) Crossover recombinants

(i) Non-crossover recombinants

(k) Crossover recombinants

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