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 sporulat ion-proficient, meiotic-lethal mutants. Synaptonemal complex formation in the med1 mutant is apparently normal and med1 strains undergo meio tic crossing over at approximately 50% of the wild-type level. The med 1 mutant undergoes homolog nondisjunction at meiosis I, presumably as a consequence of the decrease in crossing over. In addition, the mutan t undergoes precocious separation of sister chromatids, resulting in c hromosome missegregation at both meiotic divisions. We suggest that th e med1 mutation perturbs chromosome structure, leading to a reduction in recombination and a defect in sister chromatid cohesion.