The yeast med1 mutant undergoes both meiotic homolog nondisjunction and precocious separation of sister chromatids.

A mutant at the yeast MED1 locus was isolated in a screen for sporulation-proficient, meiotic-lethal mutants. Synaptonemal complex formation in the med1 mutant is apparently normal and med1 strains undergo meiotic crossing over at approximately 50% of the wild-type level. The med1 mutant undergoes homolog nondisjunction at meiosis I, presumably as a consequence of the decrease in crossing over. In addition, the mutant undergoes precocious separation of sister chromatids, resulting in chromosome missegregation at both meiotic divisions. We suggest that the med1 mutation perturbs chromosome structure, leading to a reduction in recombination and a defect in sister chromatid cohesion.