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Modeling Meiosis

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Modeling Meiosis Name:_________________________ Modeling Meiosis Meiosis resembles mitosis but serves a very different purpose. Meiosis is a cell division resulting in the halving, or reduction, of chromosome number in each cell. A diploid organism has two sets of chromosomes (2n), while a haploid cell or organism has one set (1n). Meiosis produces gametes (ova and sperm) in animals and spores in fungi, plants, and protists. Three other important characteristics of meiosis are the exchange of genetic material (“crossing over”) between homologous chromosomes, the independent assortment of the chromosomes, and the separation of alleles of the same gene. These characteristics, along with random fertilization, increase the genetic variability in the offspring. These mechanisms are essential to our understanding of genetics and evolution in sexually reproducing organisms. The hallmark of sexual reproduction is the great diversity seen in the gametes and in the resulting offspring produced by fertilization. Meiosis is integral to this process because this type of cell division produces the sex cells, gametes. Before you begin the lab, answer the following questions. 1. How do diploid organisms produce haploid gametes? __________________________________________________________________________________ __________________________________________________________________________________ 2. How does the process increase gamete diversity? ________________________________________ __________________________________________________________________________________ 3. When is the DNA replicated during meiosis? _____________________________________________ 4. Are homologous pairs of chromosomes exact copies of each other? _____________________________ 5.How does the distance between two genes or a gene and a centromere affect crossover frequencies? __________________________________________________________________________________ __________________________________________________________________________________ 6. What is meant by independent assortment? __________________________________________________________________________________ __________________________________________________________________________________ 7. How can you calculate the possible number of different kinds of gametes? ______________________ __________________________________________________________________________________ 8. What happens if a homologous pair of chromosomes fails to separate?______________________________ __________________________________________________________________________________ __________________________________________________________________________________ Background:The fungus Sordaria fimicola exchanges genetic material when two mycelia meet and fuse. The resulting zygote undergoes meiosis to produce asci; each ascus contains eight haploid spores. A single gene determines the spore color. 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. Name:_________________________ 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 called a perithecium. 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 in Figure 7. 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 together and undergo meiosis and then mitosis, the asci that develop will contain four black ascospores and four tan ascospores. The arrangement of the spores directly reflects whether or not crossing over has occurred. Figure 8 Figure 8 In Figure 8, no crossing over has occurred. Two homologous chromosomes line up at metaphase I of meiosis. The two chromatids of one chromosome each carry the gene for tan spore color (tn) and the two chromatids of the other chromosome carry the gene for wild-type spore color (+). The first meiotic division (MI) results in two cells each containing just one type of spore color gene (either tan or wild-type). Therefore, segregation of these genes has occurred at the first meiotic division (MI). The second meiotic division (MII) results in four cells, each with the haploid number of chromosomes (lN). A mitotic division simply duplicates these cells, resulting in 8 spores. They are arranged in the 4:4 pattern. Figure 9 In Figure 9, crossing over has occurred in the region between the gene for spore color and the centromere. The homologous chromosomes separate during meiosis I. This time, the MI results in two cells, each containing both genes (1 tan, 1 wild-type); therefore, the genes for spore color have not yet segregated. Meiosis II (MII) results in segregation of the two types of genes for spore color. A mitotic division results in 8 spores arranged in the 2:2:2:2 or 2:4:2 pattern. Any one of these spore arrangements would indicate that crossing over has occurred between the gene for spore coat color and the centromere. Name:_________________________ Procedure: 1. Use the pictures provided by your teacher to determine the % crossing over. Do this by counting the number of 4:4 asci and the number of recombinant asci and record your data in the table. You must count at least 50 asci. The percentage of asci showing crossover divided by 2 equals the map units in this activity. This is done because each spore produced by meiosis undergoes a mitotic division. 4:4 pattern = parental 2:2:2:2 pattern = crossing over occurred Number of 4:4 Number of Asci showing Total % Asci Gene to Class data Class crossingover Asci showing centromere for gene to average counted crossing distance centromere over (map units) distance (map units) Answer- A map unit is an arbitrary unit of measure used to describe relative distances between linked genes. The number of map units between two genes or between a gene and the centromere is equal to the percentage of recombinants. Evaluating Results 9. Why did you divide the percentage of asci showing crossover (recombinant) by 2? Name:_________________________ 10. The published map distance between the spore color gene and the centromere is 26 map units. How did the class data compare with this distance? Answer this question by completing a chi-square calculation to see if the class data is “close enough” and just due to chance……if there is something majorly wrong with our data. (hint: there are 2 options, normal vs crossing over) Data Chart Normal – 4:4 Asci Crossed over – non 4:4 Asci Observed = class average expressed as # of Asci out of 50 (o) Expected (e) Difference (o-e) Difference squared (o-e)2 (o-e)2/e Sum Accept or reject data? 11. How can you account for any disparities between the class data and the published data? 12. Draw a pair of Sordaria chromosomes in meiosis I and meiosis II and show how you would get a 2:4:2 arrangement of ascospores. 13. What is karyokinesis? Compare and contrast karyokinesis in mitosis and meiosis. 14. The cell cycle in a certain cell type has a duration of 16 hours. The nuclei of 660 cells showed 13 cells in anaphase. What is the approximate duration of anaphase in these cells (in minutes)? Research: What is a Philadelphia Chromosome? What is different in the karyotype of someone with a Philadelphia chromosome versus a “normal” karyotype? What cancer results from a Philadelphia Chromosome? .
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