Segregation of non-exchange chromosomes in the budding yeast Saccharomyces cerevisiae.
Rebecca Boumil (1), Benedict Kemp (2), Mara Stewart (3), Dean Dawson (3)
(1) Department of Molecular Biology, Massachusetts General Hospital Boston, MA;
(2) Biology Department Dartmouth College Hanover, NH 03755;
(3) Molecular Microbiology, Tufts University, 136 Harrison Ave., Boston, MA, 02111, USA
One of the key differences between meiotic and mitotic chromosome segregation lies in the behavior of homologous chromosomes at meiosis I. Homologous pairs experience pairing and recombination prior to metaphase I. These exchanges between the pairs are important for linking the chromosomes, which allows them to attach to the spindle in a bipolar orientation. Crossovers between homologs (along with sister chromatid cohesion distal to the crossover) hold homologs together until the signal for anaphase is received. Mutations that eliminate meiotic recombination result in high levels of meiosis I non-disjunction, and consequently, sterility or reduced fertility. However, in many eukaryotes, single chromosome pairs that fail to experience crossovers do not segregate randomly. This indicates that mechanisms beyond exchange can facilitate meiosis I segregation. Our experiments are intended to elucidate the mechanisms used to partition non-exchange chromosomes in budding yeast. The results demonstrate that the centromeres of non-exchange chromosomes are actively paired in prophase of meiosis I. This centromere pairing appears to hold non-exchange chromosomes together and orient the kinetochores toward opposite poles. We propose that pairing of non-exchange chromosomes is a default process that occurs after exchange homologs have synapsed. Experiments are underway to test the timing of non-exchange pairing and the genes required for this process.
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