IDENTIFICATION OF JOINT MOLECULES THAT FORM FREQUENTLY BETWEEN HOMOLOGS BUT RARELY BETWEEN SISTER CHROMATIDS DURING YEAST MEIOSIS

We have investigated DNA interactions between homologs and between sister chromatids during meiosis in S. cerevisiae. We have detected a DNA species containing information from both parental chromosomes at a specific hotspot for melotic recombination and double strand breaks (DSBs). These joint molecules are a prominent feature of melotic prophase. They appear to be a major intermediate stage in DSB-promoted recombination, because they occur with appropriate timing and require known recombination functions. Other possibilities cannot be completely dismissed, however. Most or all joint molecules contain two full-length nonrecombinant strands from each parental duplex and thus do not consist of single Holliday junctions. Joint molecules form between sister chromatids at approximately 10% the interhomolog level. Also, joint molecule formation is aberrant in a mutant defective in the HOP1 gene, which encodes a melotic chromosome structure component. General models for discrimination between homologs and sisters during meiosis are discussed.