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Sordaria_Lab - Crossing Over in Sordaria Background...

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Crossing Over in Sordaria Background Sordaria fimicola is an ascomycete fungus that can be used to demonstrate the results of crossing over during meiosis. Sordaria is a haploid organism for most of its life cycle. It becomes diploid only when the fusion of the mycelia of two different strains results in the fusion of the two different types of haploid nuclei to form a diploid nucleus. The diploid nucleus must then undergo meiosis to resume its haploid state. Meiosis, followed by mitosis, in Sordaria results in the formation of eight haploid ascospores contained within a sac called an ascus (plural, asci). Many asci are contained within a fruiting body. When ascospores are mature the ascus ruptures, releasing the ascospores. Each ascospore can develop into a new haploid fungus. The life cycle of Sordaria fimicola is shown in Figure 1. To observe crossing over in Sordaria , one must make hybrids between wild-type and mutant strains of Sordaria . Wild-type (+) Sordaria have black ascospores. One mutant strain has tan spores (tn). When mycelia of these two different strains come
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