Holliday Junctions — Seeing Single DOI: 10.1038/Nrg2051 Crossing Over Is Believed to That Underwent the Crossovers — with a Holliday Junction at One End

RESEARCH HIGHLIGHTS

CHROMOSOME BIOLOGY Holliday junctions — seeing single DOI: 10.1038/nrg2051 Crossing over is believed to that underwent the crossovers — with a Holliday junction at one end. involve an intermediate in restriction enzyme digestion followed However, the other end of the D-loop URLs which homologues are linked by by 2D-gel analysis revealed that, in is cut before the second DNA end is UniProt two crossed-strand structures fission yeast, crossovers between captured (see figure). The authors Mus81 called Holliday junctions. This sister chromatids outnumber those provide evidence that Mus81–Eme1 http://www.expasy.org/uni- model, based on observations between homologues by about 3 to 1. — which cleaves branched DNA prot/P87231 in Saccharomyces cerevisiae, was To investigate the structure of the structures — is required for meiotic Eme1 thought to be universal. But meiotic recombination intermediates, Holliday junction resolution; they http://www.expasy.org/uniprot/ new work from Gerald Smith the authors tested their sensitivity to propose that although it resolves the Q9C103 and colleagues shows that, in resolvase and to high temperature; Holliday junction the D-loop is cut Schizosaccharomyces pombe, cross- they also examined them by electron by a different enzyme. ing over predominantly occurs microscopy. The results confirmed The long-standing model of between sister chromatids, rather that the intermediate involves how meiotic crossing over occurs than between homologues, and that Holliday junctions and, together has been challenged. Although the it involves not double but single with 2D-gel data, revealed that most work in budding yeast is not itself Holliday junctions. of them are not double but single being doubted — indeed, by electron The authors used a natural hot- Holliday junctions. microscopy the authors confirmed spot of recombination — the mbs1 The authors propose a model to double Holliday junctions in budding locus — and marked it with flanking explain how recombination initiated yeast — the widespread applicability restriction sites to allow for physical by a double-strand break might of these findings must now be re- assays of recombination intermedi- involve a single Holliday junction examined. Depending on the aspect ates and crossovers. These markers intermediate. Similar to the current of biology that is under investigation, also allowed them to determine the conventional model, the first DNA either the budding yeast or the fis- parental origin of the DNA molecules end generates a so-called D-loop sion yeast is a better model. The jury is out on whether most organisms resemble the budding or the fission Double Holliday junction Single Holliday junction yeast way of doing crossing over. Magdalena Skipper D-loop D-loop ORIGINAL RESEARCH PAPER Cromie, G. A. & Hyppa, R. W. et al. Single Holliday junctions are intermediates of meiotic recombination. Cell 127, 1167–1178 (2007) FURTHER READING Gerton, J. L. & Hawley, R. S. Homologous chromosome interactions in D-loop meiosis: diversity amidst conservation. Nature Rev. Genet. 6, 477–487 (2005) HJ HJ HJ