Novel genes in chromosome structure and recombination

In sexually reproducing organisms, meiosis is the specialized cell division, which starts the generative life cycle, ultimately leading to the production of gametes. The double set of maternal and paternal chromosomes present in vegetative cells is reduced to a single set consisting of paternal and maternal chromosome stretches with very high precision. This precision seems to be surprisingly low in humans, where errors in meiosis increase in frequency with maternal age and can lead to spontaneous abortion or to severe defects such as Down`s syndrome. Over the past 15 years, Saccharomyces cerevisiae has proven to be a very powerful system in revealing molecular meiotic mechanisms. It is now clear that homologous chromosomes recognize each other by comparing their sequences. In yeast and many other organisms, DNA double-strand breaks (DSBs) generated by a meiotic nuclease called Spo11, play a central role in meiosis; first in the recognition of homologous chromosome pairs and subsequently in their physical linkage by stimulating crossing over which leads to the formation of chiasmata. To ensure that every pair of homologous chromosomes is physically linked by at least one chiasma, repair enzymes work together with structural proteins, some of which prevent unproductive chiasma formation between sister chromatids, whereas others prevent premature resolution of useful chiasmata. Following our discovery of the first meiosis-specific cohesin, Rec8, and the fundamental role that cohesion plays in meiosis during our previous FWF project, the field has progressed rapidly in understanding the nature and the role of chiasmata and their resolution through the cell cycle machinery in many organisms, including humans. The aim of the proposed project is to fill the gap in our understanding of how structural proteins and DNA repair enzymes work in tandem to create specialized chromosome structures like the synaptonemal complex (SC), which in turn affect DNA repair and recombination. We propose to undertake a visual screen in S. cerevisiae in order to complete the set of genes known to be required for normal chromosome morphology during yeast meiosis (with the exception of essential genes or genes with redundant functions). The ability of meiotic chromosomes to synapse will be scored primarily, but also axis formation and condensation will be evaluated. A defect in synapsis is a very sensitive indicator of problems with DNA repair or chromosome structure - or both. In addition, such errors activate checkpoint responses, which delay meiotic progression. We expect to identify all the non-essential genes, which when deleted in S. cerevisiae cause a grave defect in meiotic DNA repair, recombination and cell cycle progression. This approach will be complemented by a number of analytical strategies, such as genome-wide high- resolution mapping of structural and repair proteins.

In sexually reproducing organisms, meiosis is the specialized cell division, which starts the generative life cycle, ultimately leading to the production of gametes. The double set of maternal and paternal chromosomes present in vegetative cells is reduced to a single set consisting of paternal and maternal chromosome stretches with very high precision. This precision seems to be surprisingly low in humans, where errors in meiosis increase in frequency with maternal age and can lead to spontaneous abortion or to severe defects such as Down`s syndrome. Over the past 15 years, Saccharomyces cerevisiae has proven to be a very powerful system in revealing molecular meiotic mechanisms. It is now clear that homologous chromosomes recognize each other by comparing their sequences. In yeast and many other organisms, DNA double-strand breaks (DSBs) generated by a meiotic nuclease called Spo11, play a central role in meiosis; first in the recognition of homologous chromosome pairs and subsequently in their physical linkage by stimulating crossing over which leads to the formation of chiasmata. To ensure that every pair of homologous chromosomes is physically linked by at least one chiasma, repair enzymes work together with structural proteins, some of which prevent unproductive chiasma formation between sister chromatids, whereas others prevent premature resolution of useful chiasmata. Following our discovery of the first meiosis-specific cohesin, Rec8, and the fundamental role that cohesion plays in meiosis during our previous FWF project, the field has progressed rapidly in understanding the nature and the role of chiasmata and their resolution through the cell cycle machinery in many organisms, including humans. The aim of the proposed project is to fill the gap in our understanding of how structural proteins and DNA repair enzymes work in tandem to create specialized chromosome structures like the synaptonemal complex (SC), which in turn affect DNA repair and recombination. We propose to undertake a visual screen in S. cerevisiae in order to complete the set of genes known to be required for normal chromosome morphology during yeast meiosis (with the exception of essential genes or genes with redundant functions). The ability of meiotic chromosomes to synapse will be scored primarily, but also axis formation and condensation will be evaluated. A defect in synapsis is a very sensitive indicator of problems with DNA repair or chromosome structure - or both. In addition, such errors activate checkpoint responses, which delay meiotic progression. We expect to identify all the non-essential genes, which when deleted in S. cerevisiae cause a grave defect in meiotic DNA repair, recombination and cell cycle progression. This approach will be complemented by a number of analytical strategies, such as genome-wide high- resolution mapping of structural and repair proteins.