Sister Chromatid and Homolog Interactions in Meiosis

减数分裂是从二倍体前体细胞产生单倍体细胞的过程。它的特点是单轮DNA复制，然后是两轮染色体分离。在第一次减数分裂中，染色体与它们的同源物配对，然后迁出。减数分裂I的一个关键方面是同源染色体之间的交换。这种形式的重组可能是识别过程的一部分，使染色体与其适当的配对对齐。这种比对最终形成一种蛋白质结构，称为联会复合体(SC)，沿着同系物。交叉显然也在提供成对同系物之间的物理联系方面发挥作用，这使得同系物能够正确地附着在纺锤体纤维上，从而调节它们的分离。单靠交叉并不能在同源基因之间提供稳定的连锁。在DNA复制之后，每个同源物由两个相同的姐妹染色单体组成。这些染色单体的内聚力将交叉锁定在适当的位置，允许交叉作为同系物之间的稳定链接，直到后期I，此时姐妹臂的内聚力的释放允许同系物分离。因此，不能完成减数分裂重组的突变体通常表现为低生育或不育。然而，包括酵母在内的一些生物具有确保单对染色体分离的机制，这些染色体在其他正常的减数分裂中未能经历交叉。这个项目解决了两个问题。首先，没有经历过交换的染色体之间的哪些类型的相互作用使它们能够彼此分离？其次，姐妹染色单体如何在减数分裂I中保持关联，从而帮助交叉连接同源物，并确保姐妹俩在减数分裂II之前保持连接？实验将测试测试染色体着丝粒之间的相互作用调节它们在减数分裂I中的分离的模型。细胞学技术将被用来测试一对同源异物之间发生的着丝粒相互作用，该着丝粒相互作用专门用GFP标记。第二个目标，探索姐妹染色单体在减数分裂中的行为，将通过三种方式进行实验。首先，将进行实验以确定在sp11突变体(不启动减数分裂重组)中观察到的减数分裂姐妹凝聚力的丧失是由于对Spo11蛋白的要求还是由于减数分裂姐妹染色单体凝聚力的重组启动的要求。其次，用细胞学方法观察GFP标记的姐妹染色单体的减数分裂行为，以探索姐妹染色单体联合、重组和SC形成之间的关系。最后，GFP标记的染色体将在遗传屏幕上进行监测，以确定参与调节减数分裂中适当的姐妹染色单体行为的新基因。本工作所进行的实验将有助于阐明染色体分离的机制。

Meiosis is the process used to generate haploid cells from diploid precursors. It is characterized by a single round of DNA replication followed by two rounds of chromosome segregation. In the first meiotic division, chromosomes pair with, then migrate away from, their homologues. A critical aspect of meiosis I is crossing-over, or exchange, between homologous chromosomes. This form of recombination may be part of a recognition process that allows chromosomes to align with their proper partner. This alignment culminates in formation of a proteinaceous structure, called the synaptonemal complex (SC), along the homologues. Crossovers also clearly play a role in providing a physical link between paired homologues, which allows the homologues to attach properly to spindle fibers that will mediate their segregation. Crossovers alone do not provide a stable linkage between homologues. Following DNA replication, each homologue is composed of two identical sister chromatids. The cohesion of these chromatids locks crossovers in place allowing the crossovers to act as a stable link between the homologues until anaphase I, at which time release of the sister arm cohesion allows the homologues to disjoin. Accordingly, mutants unable to completemeiotic recombination usually exhibit low fertility or sterility. However some organisms,including yeast, have mechanisms that insure the segregation of single pairs of chromosomesthat have failed to experience a crossover in an otherwise normal meiosis. This project addresses two questions. First, what types of interactions between chromosomes that have failed to experience an exchange enable them to segregate away from one another? Second, how do sister chromatids remain associated in meiosis I, thus helping crossovers to link homologues and ensuring that both sisters will stay joined until meiosis II? Experiments will test the model that interactions between the centromeres of test chromosomes mediate their disjunction at meiosis I. Cytological techniques will be used to test centromeric interactions occur between a pair of homeologues, specifically tagged with GFP. The second objective, to explore the behavior of sister chromatids in meiosis, will be addressed experimentally in three ways. First, experiments will be performed to determine whether the loss of meiotic sister cohesion observed in spo11 mutants (that do not initiate meiotic recombination) is due to a requirement for the Spo11 protein or alternatively a requirement for recombination initiation in meiotic sister chromatid cohesion. Second, the meiotic behavior of GFP-tagged sister chromatids will be observed cytologically in previously characterized meiotic mutants to explore the relationships of sister chromatid association, recombination and SC formation. Finally, the GFP tagged chromosomes will be monitored in a genetic screen to identify new genes involved in mediating proper sister chromatid behavior in meiosis. The experiments conducted in this work will help to elucidate the mechanism of chromosome segregation.