title: Occurrence of crossed strand-exchange forms in yeast DNA during meiosis.
abstract: The crossed strand-exchange form (Holliday structure, half chiasma) has been predicted as an intermediate in the genetic recombination of eukaryotes. We report here the detection of this form in the yeast plasmid, 2-micron DNA, isolated during meiosis. Physical mapping has previously suggested that two forms of 2-micron DNA arise because of recombination between inverted repeat regions. After appropriate digestion with restriction endonuclease, a crossed strand-exchange form intermediate in this recombination would yield an X-shaped form resistant to loss by branch migration because of nonhomology in sequences flanking the region of homology. We first generated this X-shaped form artificially by reannealing melted restriction fragments of 2-micron DNA. This enabled us to develop a procedure for the physical separation of the X-shaped form by agarose gel electrophoresis. We then used this electrophoretic procedure to isolate a naturally occurring form of identical structure from the 2-micron DNA of meiotic cells. Electron microscopy demonstrated that the exchange junction had the expected configuration of strands and indicated that the junction occurred within the region of homology.
mesh_terms: D003434:Crossing Over, Genetic; D004271:DNA, Fungal / Q000378:metabolism*; D008540:Meiosis; D008854:Microscopy, Electron; D009690:Nucleic Acid Conformation; D010957:Plasmids; D012441:Saccharomyces cerevisiae / Q000378:metabolism*; D012854:Sister Chromatid Exchange
chemical_list: D004271:DNA, Fungal