Meiosis Formation of Gametes (Eggs & Sperm) 1 Facts About Meiosis ü 4 haploid daughter cells are produced (each contain half the number of chromosomes as the original cell) ü Produces gametes (eggs & sperm) ü Occurs in the testes in males (Spermatogenesis) ü Occurs in the ovaries in females (Oogenesis) 2 Meiosis: Two Part Cell Division Sister chromatids separate Homologous Chromosomes separate Meiosis Meiosis I II Diploid Diploid Haploid 3 Why Do we Need Meiosis? ü sexual reproduction ü two haploid (1n) gametes are brought together through fertilization to form a diploid (2n) zygote 4 Fertilization 2n = 6 1n =3 5 Meiosis I: Reduction Division Telophase I Late Prophase I Anaphase I (haploid) Metaphase I Nucleus Spindle fibers Early Prophase I Nuclear (Chromosome number doubled envelope because has gone through S phase of interphase) 6 Prophase I Early prophase Late prophase ü Homologs pair ü Chromosomes condense. through synapsis ü Spindle forms. forming tetrads. ü Nuclear envelope breaks ü Crossing over down. 7 occurs. Crossing-Over Crossing-over multiplies the already huge number of different gamete types produced by independent assortment 8 Metaphase I Homologous pairs of chromosomes align across from each other along the equator of the cell. Random alignment is called Independent Assortment. 9 Anaphase I Homologs separate and move to opposite poles. Sister chromatids remain attached at their centromeres. 10 Telophase I Nuclear envelopes reassemble. Spindle disappears. Cleavage furrow forms and cytokinesis divides cell into two. 11 Prophase II Nuclear envelope breaks down. Spindle forms. 12 Metaphase II Chromosomes align On the equator of cells. 13 Anaphase II Equator Pole Sister chromatids separate and move to opposite poles. 14 Telophase II Nuclear envelope assembles. Chromosomes decondense. Spindle disappears. Cytokinesis divides cell into two. 15 Results of Meiosis Gametes (egg & sperm) form Four haploid cells with one copy of each chromosome 16 Gametogenesis Oogenesis or Spermatogenesis 17 17 Spermatogenesis ü Occurs in the testes ü Meiosis produces 4 spermatids ü Spermatids mature into sperm ü Men produce about 250,000,000 sperm per day 18 Spermatogenesis in the Testes Spermatid 19 Oogenesis ü Occurs in the ovaries before birth ü Two divisions produce 3 polar bodies that die and 1 egg ü Polar bodies die because of unequal division of cytoplasm ü Immature egg called oocyte ü Starting at puberty, one oocyte matures into an ovum (egg) every 28 days 20 Oogenesis First polar body may divide a (haploid) X a Polar bodies X a die X a X Mitosis Meiosis I Meiosis II A X (if fertilization Oogonium occurs) A (diploid) Primary oocyte X (diploid) A X Ovum (egg) Mature Secondary A egg oocyte X (haploid) Second polar body (haploid) 21 Comparison of Divisions Mitosis Meiosis Number of 1 2 divisions Number of 2 4 daughter cells Genetically identical cells Yes No made? Chromosome # Same as parent Half of parent Germ cell to make Where Somatic cells gametes When Throughout life At sexual maturity Growth and Role Sexual reproduction repair 22 .