4 Metaphase I 4 Anaphase I 4 Telophase I 2/4 Prophase II 2 Metaphase II 2 Anaphase II 2 Telophase II 2 Cytokinesis 2

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1 Meiosis What therefore God hath joined together, let not man put asunder Mark 10:6-9 Stage Table 1 Meiosis (Without Crossover Event) # Chromosomes Per Cell Prophase I 4 Metaphase I 4 Anaphase I 4 Telophase I 2/4 Prophase II 2 Metaphase II 2 Anaphase II 2 Telophase II 2 Cytokinesis 2 Stage Diagram See text for details. See throughout for the details of Metaphase and the end result. Table 2 Meiosis (With Crossover) # Chromosomes Per Cell Prophase I 4 Metaphase I 4 Anaphase I 4 Telophase I 2/4 Prophase II 2 Metaphase II 2 Anaphase II 2 Telophase II 2 Cytokinesis 2 Diagram See text for details. See throughout for the details of Metaphase and the end result. Data Analyses and Conclusions 1. What is the purpose of meiosis? The main purpose of meiosis is to reduce the number of chromosomes from the diploid (2n) number to the haploid (n) number in order to produce gametes to take place in sexual reproduction Catholic Initiatives in Math and Science, LLC All Rights Reserved 1

2 2. In the cell you modeled, what was the diploid number (2n) for the parent cell? What was the haploid number? In the parent cell the diploid was 2n=4. The daughter cells after meiosis were haploid, n=2. 3. Metaphase is drastically different between Mitosis and Meiosis I: a. Describe what is observed at the metaphase plate b. How is Metaphase similar between Mitosis and Meiosis I? c. How is Metaphase different between Mitosis and Meiosis I? d. What does this difference lead to during the next step? COMPARISON OF METAPHASE BETWEEN MITOSIS AND MEIOSIS MITOSIS MEIOSIS I Describe what is observed at the Metaphase Plate How are they the same? How are they different? What is the result of difference? A duplicated chromosome consists of 2 sister chromatids connected by a centromere. They migrate to the middle of the cell. The maternal and paternal The maternal and paternal homologous chromosomal pairs homologous chromosomal pairs line up at the plate but they do line up at the equatorial plate. NOT form a tetrad. The Form a tetrad. The tetrad is homologous chromosomal pairs attached to a spindle fiber from are linked to separate spindle each pole. fibers to the poles. In the next step in Meiosis I, the ploidy is reduced Catholic Initiatives in Math and Science, LLC All Rights Reserved 2

3 4. Review the figure below which portrays a cell in Metaphase I. Which scenario, if any, is most likely? Which Law or Principle informs your answer and explain? Any of the above options may occur. The orientation of the duplicated sister chromatids is independent of the homologous chromatids in the tetrad. Once attached to the spindle fiber they may flip as they line up. This increases the genetic diversity found in the final four gametes and is another source is genetic variation. This is called the LAW OF INDEPENDENT ASSORTMENT. 5. Meiosis has two consecutive cell divisions termed Meiosis I and Meiosis II. Meiosis I starts with a parent cell with a ploidy that is diploid. In this model, the diploid number was 2n=4. At what point, was the ploidy reduced to haploid (n=2), at the end of Meiosis I or at the end of Meiosis II? Explain. The haploid number is reached in Meiosis I. Metaphase II sorts the homologous pairs. When they separate to the two poles, the ploidy is reduced from diploid to haploid. 6. End Result No Crossover: a. Describe, in detail, the end result of Mitosis and Meiosis. b. How are they similar? c. How are they different? What have similarities and differences. Compare and contrast mitosis and meiosis in the table below Catholic Initiatives in Math and Science, LLC All Rights Reserved 3

4 COMPARISON OF END RESULT BETWEEN MITOSIS AND MEIOSIS Assuming No Crossover MITOSIS MEIOSIS Describe End Result How are they the similar? How are they different? (# of Cells; Ploidy, etc.) Both are a type of cell division. Both begin with a parent cell that is diploid. Chromosomes sort randomly on the equator. Four daughter cells produced that are Two daughter cells produced that haploid. Two cycles of division occur, are diploid. Only one cycle of Meiosis I and Meiosis II. Crossover may division occurs. No crossover. occur. Homologous chromosomes form tetrad in Metaphase I. 7. Consider each bead to represent a gene. In the crossover event explain what took place between the homologous pairs. Answers will vary. For example, 3 genes (3 beads) from the paternal homologous chromosome were exchanged for 3 genes (3 beads) on the maternal homologous chromosome. There was no net change in the number of genes in the paternal or maternal chromosomes. 8. Compare the resulting 4 cells of meiosis With and without a Crossover Event: a. Describe the chromosomes in each of the 4 cells WITH and WITHOUT a Crossover Event b. How are the gametes similar? c. How are the gametes different? 2016 Catholic Initiatives in Math and Science, LLC All Rights Reserved 4

5 Describe the chromosomes in gametes WITH Crossover. COMPARISON OF END RESULT WITH AND WITHOUT CROSSOVER No Crossover CROSSOVER Describe the chromosomes in gametes WITHOUT Crossover. How are the gametes the same? How are the gametes different? Four haploid cells result Chromosomes, although reduced in number, are identical the homologous chromosomes in the parent nucleus Two of the four daughter nuclei contain recombinant chromosomes, different than the ones in the parent nucleus 9. Why is it necessary to reduce the number of chromosomes in gametes but not in skin cells? One gamete must join with a second gamete in fertilization, at which time the chromosomal content merges. If both gametes were diploid, the result would be polyploidy and the individual would not survive. Gametes must contain half the genetic content since they merge. Skin cells merely divide to replace old cells. The diploid state and genetic content must be duplicated exactly Catholic Initiatives in Math and Science, LLC All Rights Reserved 5

6 10. Review the figure below. Assume these are 4 female gametes produced from one parent cell undergoing meiosis, resulting in 4 egg cells. What would be the result if each egg could be fertilized with the SAME male gamete, the sperm? Would the four individuals be identical? Explain. Fertilization would result in four distinct individuals. Meiosis sorts chromosomes randomly and as can be seen in the figure, none of the four have identical combinations. Each individual would be different. Point out to the student(s) that crossover events add another layer of genetic diversity Catholic Initiatives in Math and Science, LLC All Rights Reserved 6