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Meiosis The Production of Reproductive Cells Remember: • Mitosis produced 2 genetically identical cells with 2 copies of every chromosome (one from mom and one from dad) • This is how body (somatic) cells are produced. • REPRODUCTIVE cells do NOT go through MITOSIS because these cells MUST have HALF the number of chromosomes. Mitosis Meiosis Overview • Purpose = so that the number of chromosomes gets reduced by ½ in reproductive cells • Body cells have 1 complete set of genes from each parent SO there are 2 sets of genes in every cell • Diploid = 2 complete sets of genes (chromosomes); Diploid in humans = 46 • Gametes = sex (reproductive) cells = sperm/egg. Gametes only have 1 set of chromosomes in each cell. • Haploid = 1 set of genes (chromosomes); Haploid in humans = 23 Meiosis - Overview MEIOSIS I BIG IDEA • Produce 2 Haploid Cells by SEPARATING HOMOLOGOUS CHROMOSOMES Meiosis I – Interphase • G1-Growth • S-DNA replication takes place • G2Organelles replicate Meiosis I – Prophase I • Homologous = same gene (1 from each parent) • Chromatin condenses into chromosomes • Homologous chromosomes pair up • Crossing over – homologs swap genes to increase genetic diversity Meiosis I – Metaphase I • Homologous chromosomes are moved to the middle Meiosis I – Anaphase I • Homologs are pulled apart • Crossing over is completed when the chromosomes are separated. Error in Meiosis • If the chromosomes fail to separate after crossing over, Nondisjunction will occur. • This can happen during Anaphase I or Anaphase II. • This causes one daughter cell to have both copies of the homologous chromosome and the other to not have a copy of it. • Downs Syndrome (Trisomy 21) is caused by Nondisjunction. Meiosis I – Telophase I • • • • Nuclear envelopes may reform Cytokinesis (2 cells) Cells are NOT identical Cells are HAPLOID 2 Divisions • Unlike Mitosis, there are 2 Divisions in Meiosis. • There is NO Interphase or DNA replication between Meiosis I and Meiosis II MEIOSIS II BIG IDEA • Produce 4 Genetically Different Haploid Cells by SEPARATING SISTER CHROMATIDS Meiosis II – Prophase II • Cells are NOT identical • Haploid cells prepare to divide Meiosis II – Metaphase II • Chromosomes line up in the middle Meiosis II – Anaphase II • Sister chromatids are pulled apart • Nondisjuction can occur Meiosis II – Telophase II • Nuclear envelopes reform • Cytokinesis • 4 genetically DIFFERENT haploid cells Mitosis • • • • Meiosis • Somatic (body) cells • 2 cells • Diploid (2 sets of genes; 1 from each • parent) Genetically Identical (to each other & to • original cell) Gametes (sex cells) 4 cells Haploid (1 set of genes) Genetically Different – Crossing Over – Some have “mom’s” copy; others have “dad’s” Nondisjunction may occur • 2 divisions • Watch the following Mitosis & Meiosis Comparison Video http://www.youtube.com/wat ch?v=Ba9LXKH2ztU Gametes • Gametogenesis is the production of Gametes (aka Meiosis) • The production of sperm is called spermatogenesis. • The production of eggs is called oogenesis. • Gametes are haploid. – 23 chromosomes in human sperm – 23 chromosomes in human eggs Sperm • Spermatogenesis produces 4 viable sperm cells through meiosis. • Sperm contain either an X or Y chromosome. This determines the gender of the offspring. • This occurs every 2-3 days in human males. Eggs • In females, only one large egg is produced through meiosis. • Cytokinesis is uneven. One cell receives the majority of the cytoplasm. • The other 3 cells are called polar bodies and are not involved in reproduction. • Eggs always contain one X chromosome. • Prophase I occurs before birth in human females. Meiosis is completed during puberty. No “new” eggs during lifetime. Oogenesis