Evidence that spontaneous mitotic recombination occurs at the two-strand stage.

Spontaneous reciprocal mitotic recombination in the yeast Saccharomyces cerevisiae, associated with heteroallelic recombination, occurs almost exclusively at the two-strand stage and involves recombination of unduplicated chromosomes (i.e., during G1) or the unduplicated regions of chromosomes during the S phase of mitosis. The associated heteroallelic recombination frequently reflects the formation of symmetric Holliday structures, is not strongly polarized with respect to conversion at the heteroallelic trp5 sites studied, occasionally results in simultaneous conversion of widely separated genetic markers, and is positively correlated with recombination of flanking markers.