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Meiosis

Meiosis. Chapter 9. Asexual Reproduction. Parent splits, fragments, or buds Mitosis in eukaryotes Produces clones Advantage: Rapid and efficient No need for a mate Disadvantage: Lack of genetic variability. Sexual Reproduction. Gametes join to form zygote Increases genetic variability

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Meiosis

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  1. Meiosis Chapter 9

  2. Asexual Reproduction • Parent splits, fragments, or buds • Mitosis in eukaryotes • Produces clones • Advantage: • Rapid and efficient • No need for a mate • Disadvantage: • Lack of genetic variability

  3. Sexual Reproduction • Gametes join to form zygote • Increases genetic variability • Offspring are different from either parent and each other • This may increase survival of some; may decrease chances for others • Somatic (body) cells have 2 sets of chromosomes – diploid (2n) • Gametes have 1 set – haploid (n) • Some plants are polyploid – 3 or more sets of chromosomes

  4. Meiosis • Cell division to produce haploid gametes from diploid somatic cells • Also called ‘reduction division’ • Consists of 2 divisions – meiosis I and meiosis II • Produces 4 cells

  5. Meiosis I: Prophase I • Chromosomes condense and synapse to form a tetrad • Tetrad contains a homologous pair – one maternal homologue, one paternal homologue (each homologue is 2 sister chromatids) • Crossing-over occurs – DNA segments are exchanged • This genetic recombination increases variation

  6. Meiosis I: Metaphase I • Homologous chromosomes are lined at the equatorial plate

  7. Meiosis I: Anaphase I • Tetrads separate and homologous chromosomes (still with 2 sister chromatids connected) move to opposite poles • Separation is random between maternal and paternal homologues (independent assortment)

  8. Meiosis I: Telophase I • Each nucleus now contains a haploid number of chromosomes even though each one is made of 2 sister chromatids • Cytokinesis divides the cell into 2 • Interkinesis occurs – between meiosis I and meiosis II • DNA is NOT copied • Very brief

  9. Meiosis II: Prophase II • Chromosomes condense but do not pair (no pairs exist)

  10. Meiosis II: Metaphase II • Chromosomes line up at equatorial plate • Chromosomes are NOT in tetrads

  11. Meiosis II: Anaphase II • Sister chromatids are separated and pulled to opposite poles

  12. Meiosis II: Telophase II • Unduplicated single chromosomes • Nuclear envelope re-forms • Cytokinesis occurs

  13. Gametogenesis • Spermatogenesis • 4 haploid sperm cells from each cell • Oogenesis • 1 haploid ovum (egg) cell from each cell • There is an uneven division of cytoplasm during cytokinesis • Polar bodies generally disintegrate

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