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Lesson Overview

Explore the process of meiosis, where the number of chromosomes is halved and genetic diversity is created through events like crossing-over and independent assortment. Learn about the phases of meiosis and the difference between diploid and haploid cells. Discover how gametes form and their role in fertilization. Compare meiosis to mitosis for a better understanding of cell division.

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Lesson Overview

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  1. Lesson Overview 11.4 Meiosis http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter28/animation__how_meiosis_works.html

  2. Chromosome Number Homologous chromosomes – for each chromosome in normal body cells, there is one that comes from the mother and one that comes from the father Ex.

  3. Diploid vs. Haploid Cells DIPLOID CELLS (somatic, or normal body cells) • A cell that contains both sets of homologous chromosomes is diploid, meaning “two sets.” • Represented by the symbol 2n

  4. Diploid vs. Haploid Cells HAPLOID CELLS (gametes, or sex cells) • Some cells contain only a single set of chromosomes, and therefore a single set of genes; such cells are haploid, meaning “one set.” • Gametes of sexually reproducing organisms are HAPLOID • can be written as n

  5. Phases of Meiosis • Meiosis: process in which the number of chromosomes per cell is cut in half • Meiosis usually involves two distinct divisions, meiosis I and meiosis II. • By the end of meiosis II, the ONE diploid cell becomes FOUR haploid cells

  6. Meiosis I • Before meiosis I, the cell goes through INTERPHASE • Each replicated chromosome consists of two identical chromatids joined at the center

  7. PROPHASE I • Chromosomes pair up forming a tetrad, which contains four chromatids • They undergo a process called crossing-over • Crossing-over is important because it is a source of genetic variation

  8. METAPHASE I / ANAPHASE I • Paired homologous chromosomes line up across the center of the cell (metaphase) • Homologous chromosome pair pulled toward opposite ends of the cell (anaphase)

  9. TELOPHASE I / CYTOKINESIS • Nuclear membrane forms around each cluster of chromosomes (telophase) • Two new cells formed (cytokinesis)

  10. MEIOSIS I – IMPORTANT EVENTS 1. In prophase I of meiosis, crossing over occurs 2. During metaphase I of meiosis independent assortment occurs

  11. Meiosis II • The two cells produced by meiosis I now enter a second meiotic division (MEIOSIS II) • NEITHER CELL GOES THROUGH INTERPHASE (no replicating of DNA)

  12. MEIOSIS II • Basically same as phases as mitosis • By the end of meiosis II, the diploid cell becomes four haploid cells (GAMETES) • Male gamete = SPERM • Female gamete = EGG

  13. Gametes to Zygotes Fertilization—the fusion of male and female gametes—generates new combinations of alleles in a zygote

  14. Comparing Meiosis and Mitosis http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter28/animation__comparison_of_meiosis_and_mitosis__quiz_1_.html

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