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Meiosis

Meiosis. Mom and Dad are both “2n”. How do they make gametes of “n”?. Mom. Dad. 2n. 2n. Process? = MEIOSIS Cell division that halves the chromosome number. Creates gametes for sexual reproduction. n. Process?. Process?. n. Sperm. Ovum. Fertilization. 2n. Child. Meiosis. PMAT I.

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Meiosis

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  1. Meiosis • Mom and Dad are both “2n”. How do they make gametes of “n”? Mom Dad 2n 2n • Process? = MEIOSIS • Cell division that halves the chromosome number. Creates gametes for sexual reproduction. n Process? Process? n Sperm Ovum Fertilization 2n Child

  2. Meiosis PMAT I PMAT II

  3. Meiosis • Meiosis I: 1st Cell Division • Meiosis II: 2nd Cell Division Gametes (become sperm or ovum) Meiosis I Meiosis II Body Cell 2n (Diploid) n (Haploid) n (Haploid)

  4. Meiosis Your Body Cell (2n) Single Chromatids Ignore other organelles for now Gap 1 • Interphase (before division): preparing for cell division • G1 Phase (Gap 1) • Cell is growing • Normal functions • Prepping for DNA duplication • S Phase (Synthesis) • DNA is copied • Single chromatids  Sister chromatids • G2 Phase (Gap 2) • Cell continues growing • Prepping for cell division DNA is uncondensed (not in chromosomes) and is called chromatin Synthesis Gap 2 Your Body Cell (2n) Sister Chromatids

  5. Centrioles: organize cell during division Meiosis • Meiosis I: 1 cell  2 cells • Prophase I • Chromatin condenses into chromosomes • Homologous chromosomes pair up; crossing over occurs • Nuclear envelope breaks down • Spindle fibers form from the centrioles From your dad From your mom Crossing Over Your Body Cell (2n) Sister Chromatids Nuclear Envelope **Recall: In humans, 23 pairs of homologous chromosomes in a human (2n = 46). Here, we are using 2 pairs (2n = 4). Homologous Chromosomes Genetic information exchanges = increased variation Image Source: legacy.owensboro.kctcs.edu

  6. Equator Meiosis • Meiosis I (cont.) • Metaphase I • Spindle fibers attach to centromeres of chromosomes • Homologous pairs line up at equator of cell • Anaphase I • Homologous chromosomes separate and move along spindle fibers to opposite poles Your Body Cell (2n) Sister Chromatids (n) at each pole (n) at each pole Your Body Cell (2n) Sister Chromatids

  7. Meiosis • Meiosis I (cont.) • Telophase I • Spindles break down • Chromosomes uncoil, nuclei reform • Cell divides into two Cytokinesis (n) Chromosomes Sister Chromatids (n) Chromosomes Sister Chromatids

  8. Meiosis • Meiosis II: 2 cells  4 cells • Prophase II • Chromatin condenses into chromosomes (again!) • Nuclear envelopes break down (again!) • Spindle fibers form from centrioles (again!) and attach to chromosomes • Metaphase II • Centromeres of chromosomes line up at equator of each cell (not in pairs!) (n) Chromosomes Sister Chromatids (n) Chromosomes Sister Chromatids Equators (n) Chromosomes Sister Chromatids (n) Chromosomes Sister Chromatids

  9. Meiosis (n) at each pole (n) at each pole • Meiosis II (cont.) • Anaphase II • Centromeres split • Sister chromatids separate & move to opposite poles • Telophase II • 4 nuclei reform • Spindles break down • Chromosomes uncoil • Cells divide (n) Chromosomes Single Chromatids (n) Chromosomes Single Chromatids (n) at each pole (n) at each pole Cytokinesis

  10. Meiosis • Final Products Meiosis Spermatogenesis Oogenesis

  11. Meiosis • Spermatogensis • In males, only a special diploid (2n) cell called spermatocyte in a testis can undergo meiosis to create a sperm. Image Source: www.mun.ca

  12. Meiosis • Oogensis • In females, only a special diploid (2n) cell called an oocyte in an ovary can undergo meiosis to create an ovum. Image Source: www.mun.ca

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