Introduction: The process of Meiosis involves 2 nuclear divisions that result in the formation of four haploid cells (2). Meiosis I is preceded by interphase, during which DNA synthesis occurs and each chromosome is made of two chromatids joined at the centromeres. In meiosis I, chromatids of homologous chromosomes may exchange parts by crossing over. During Prophase I homologous chromosomes come together and synapse. A tetrad consisting of four chromatids is also formed (2). In Metaphase I, the crossed over tetrads line up in the center of the cell, and in Anaphase I, the homologous chromosomes separate and move to opposite ends of the cell. In Telophase I, centriole duplication is completed and cytokinesis and results in 2 haploid cells with replicated chromosomes (2). Before Meiosis II, interphase II takes place and no DNA replication occurs. Meiosis II, a second mitotic cell division then takes place (2). In Prophase II, replicated centrioles separate and move to opposite sides of the chromosome groups. During Metaphase II, chromosomes are centered in the middle of each daughter cell. In anaphase II the centromere regions of the chromatids are separated. Lastly, in Telophase II, the chromosomes are at opposite ends of the cell, a nuclear envelope forms, and sister chromatids are separated resulting in four haploid cells (2). …..Sordaria fimicola is fungus that may be used to show the results of crossing over during meiosis. Sordaria throughout most of its life is haploid, but becomes diploid after the fusion of two different types of nuclei, which forms a diploid nucleus (3). In Sordaria meiosis results in the making of eight haploid ascospores found in a sac called an ascus (1). Most asci are found in a perithecium. The life cycle of Sordaria fimicola begins as a spore is discharged through an ascus. The ascospore then undergoes mitosis, which forms a filament. The filament then undergoes mitosis, which forms a mycelium (3). Mycelial fusion and fertilization then takes place. This forms a diploid zygote. The zygote undergoes mitosis to form four haploid nuclei. The nuclei also undergo mitosis and form eight haploid nuclei, which then form eight ascospores. When mycelia of a mutant strain of Sordaria and a wild type of Sordaria undergo meiosis four black and four tan ascospores form. The arrangement of the ascospores reflects whether crossing over has occurred or not. Gametes, egg and sperm, are made during meiosis. Each egg and sperm cell contains half the total chromosomes a normal cell of that species would have. When the egg and sperm unite during fertilization the total chromosome number is restored (3). ….. ….The purpose of this lab is to design a controlled experiment that determines the distance, in map units, from the gene for spore coat color to the centromere of the chromosome in Sordaria fimicola gametes. The independent variable of this experiment is the distance from the gene to centrome. The dependent variables in this experiment are the number of asci in the 4:4 ratio, the number of asci that show crossing over, and the percentage of asci that show crossing over. My hypothesis is: If the percentage of asci that show crossing over is divided 2, then percentage of asci with the 4:4 ratio will be greater than the percentage of asci that cross over, and the distance from the spore coat color gene to the centromere will be small for the asci that cross over, for the same reason. Materials and Methods: Begin by obtaining a crossing medium plate, which should be divided into quarters that are marked alternatively as “+”, “tn,” “+”, “tn.” Next, obtain two strains of Sordaria, the black wild type (+), and the tan mutant type (tn). Then...
References: 1. Benson, Jennifer. Lab: Meiosis in Sordaria Fimicola. 28 October 2008.
2. Biology 8th Edition. Campbell, Neil A. and Jane B. Reece. Prentice Hall, c2005.ISBN 0805367772. Pearson Education 800.848.9500.
3. Bio 325 Genetics Lab: Tetrad analysis in Sordaria fimicola. 30 October 2008
4. Mitosis and Meiosis Lab. 31 October 2008
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