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1/5/2012 Meiosis What is an “n”? • Haploid (n): • Number of chromosomes in a gamete. • Diploid (2n): • Number of chromosomes in a somatic cell • One copy from mother, one copy from father. • Homologous pairs of chromosomes 1 1/5/2012 Two Types of Diploid Cells In Sexual Organisms Somatic • Body Cells • Diploid • Divide by mitosis to make exact copies • In every tissue except reproductive tissue GERM • “Germinating” (reproductive) cells • Diploid • (Most) Divide by meiosis to make gametes (sex cells) (sperm & ova) • In testes & ovaries Mitosis vs. Meiosis •Asexual Reproduction •Sexual Reproduction • Body cells (somatic) • Sex cells (gamete) • DNA replicates once • DNA replicates once • # cell divisions: 1 • # cell divisions: 2 • 2 identical daughter cells • Makes Diploids (2N) • 4 different daughter cells • Makes Haploids (1N) 2 1/5/2012 Meiosis is part of SEXUAL Reproduction (requiring 2 parents) In Meiosis: ONE diploid, germ cell divides to become FOUR • (*circle*) … diploid / • (*circle*) … somatic / haploid gamete (4) daughter cells 3 1/5/2012 In order to go from diploid to haploid, Meiosis Involves Two Divisions Meiosis I • Splits homologous pairs of chromosomes Meiosis II • Splits sister chromatids apart Time To Name The Phases • PROPHASE I • METAPHASE I • ANAPHASE I • TELOPHASE I • PROPHASE II • METAPHASE II • ANAPHASE II • TELOPHASE II 4 1/5/2012 Take out 2 Tables PROPHASE I METAPHASE I ANAPHASE I TELOPHASE I & CYTOKINESIS WHAT IT LOOKS LIKE WHAT’S GOING ON Take Out 2 Tables PROPHASE II METAPHASE II ANAPHASE II TELOPHASE II & CYTOKINESIS WHAT IT LOOKS LIKE WHAT’S GOING ON 5 1/5/2012 PROPHASE I • Nuclear envelope disintegrates • Centrioles migrate, produce spindle fibers pair up CROSS OVER! PROPHASE I CROSSING OVER Exchange of segments of genes between homologous chromosomes 6 1/5/2012 METAPHASE I of chromosomes line up along metaphase plate ANAPHASE I pulled apart to opposite poles 7 1/5/2012 TELOPHASE I & CYTOKINESIS • Chromosomes (no longer homologous pairs!) prepare for meiosis II • Cytoplasm begins to split into first 2 (of 4) daughter cells TELOPHASE I & CYTOKINESIS NOTE: At the end of meiosis I, daughter cells are already ! (because homologous pairs were separated) 8 1/5/2012 2 haploid daughter cells then go through MEIOSIS II In Meiosis II, 2 haploid cells will separate sister chromatids to make 4 gametes! PROPHASE II • Centrioles migrate, produce spindle fibers 9 1/5/2012 METAPHASE II • Chromosomes line up along metaphase plate ANAPHASE II • Sister chromatids pulled apart to opposite poles 10 1/5/2012 TELOPHASE II & CYTOKINESIS • Nuclear envelopes reform around • Cytoplasm splits to separate the • Each of the 4 daughter cells is a new gamete with genetic information different from either parent! Is This Meiosis I or Meiosis II? Why? 11 1/5/2012 Is This Meiosis I or Meiosis II? Why? Meiosis I Meiosis II Check Point: When do the cells become haploid? 12 1/5/2012 Mitosis v. Meiosis , 13 1/5/2012 Finally, Fertilization • Male and female gametes unite • Fusion of two haploid nuclei produces diploid nucleus • Egg + Sperm = Zygote (new life in one diploid cell) Meiosis and Sexual Reproduction Guarantee Variation and Diversity! In Three Ways 1. Independent Assortment • Homologous Chromosomes separate randomly 2. Crossing Over • Homologous pairs exchange gene segments 3. Random Fertilization • Which sperm meets which egg? (game of chance) 14