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Meiosis and Alternation of Generations

Meiosis and Alternation of Generations. Chapter 12. Outline. Introduction Asexual Sexual Reproduction The Phases of Meiosis Division I Division II Alternation of Generations. Introduction.

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Meiosis and Alternation of Generations

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  1. Meiosis and Alternation of Generations Chapter 12

  2. Outline • Introduction • Asexual • Sexual Reproduction • The Phases of Meiosis • Division I • Division II • Alternation of Generations

  3. Introduction • Asexual Reproduction - production of cells identical in chromosomes with cells from which they arose • Sexual Reproduction - in nearly all plants • Results in formation of seeds in flowering and cone-bearing plants • Gametesproduced • Egg and sperm unite to form zygote

  4. IntroductionAsexual and Sexual Reproduction

  5. Introduction • Cells -> 2 sets of chromosomes, 1 set from each parent • Members of each chromosome pair identical in: • Length • Amount of DNA • Genes carried • Location of centromere • Homologous Chromosomes = chromosome pairs

  6. Introduction • Results of meiosis: • 4 cells from 2 successive divisions • Cells with half chromosome # of parents • Each cell rarely identical to original cell or each other

  7. The Phases of Meiosis • Before meiosis, DNA molecules of each chromosome double • Each chromosome has identical DNA molecules held together by centromere • Meiosis: • Division I (Meiosis I or Reduction Division) -# of chromosomes reduced to half • Division II (Meiosis II or Equational Division) - no further reduction in chromosome #

  8. The Phases of MeiosisDivision I • Prophase I • Chromosomes coil and condense, and align in homologous pairs • Nuclear envelope and nucleolus disassociate • Each closely associated pair of chromosomes exchange parts = crossing-over Chromosomes coil

  9. The Phases of MeiosisDivision I • Prophase I - Crossing-over • Chiasmataform • Results in exchange of DNA by 2 parents Chromosomes align in pairs; chiasmata visible

  10. The Phases of MeiosisDivision I • Metaphase I • Chromosomes align in pairs at equator • Spindle formation completed

  11. The Phases of MeiosisDivision I • Anaphase I • One whole chromosome from each pair migrates to pole • Telophase I • Original cell becomes 2 cells or 2 nuclei

  12. The Phases of MeiosisDivision II • Similar to mitosis • Prophase II • Chromosomes become shorter and thicker

  13. The Phases of MeiosisDivision II • Similar to mitosis • Metaphase II • Centromeres become aligned along equator • New spindles completed

  14. The Phases of MeiosisDivision II • Similar to mitosis • Anaphase II • Centromeres and chromatids of each chromosome separate, and migrate to opposite poles

  15. The Phases of MeiosisDivision II • Telophase II • Coils of chromatids relax and chromosomes become longer and thinner • Nuclear envelope and nucleoli reappear for each group of chromosomes • New cell walls form Chromosomes at poles Formation of cell walls

  16. Alternation of Generations • Haploid (1x) - cell with 1 set of chromosomes • Gametes • Diploid (2x) - cell with 2 sets of chromosomes • Zygote • Polyploid - cell with >2 sets of chromosomes • Triploid (3x) - 3 sets of chromosomes • Homologous chromosomes cannot pair properly, thus gametes typically inviable • Navel oranges, seedless watermelons • Tetraploid (4x) - 4 sets of chromosomes • Potatoes, pasta wheat

  17. Alternation of Generations • Alternation of Generations - life cycle involving sexual reproduction alternating between 2nsporophyte phase and 1n gametophyte phase • Sporophytes develop from zygotes and produce sporocytes • Sporocyte undergoes meiosis - produces 4 1nspores

  18. Alternation of Generations • Gametophytes develop from spores • Form cells or sexual structures in which gametes are formed by mitosis • Fertilization produces zygote • Fertilization (Syngamy) = fusion of gametes

  19. Alternation of Generations • First cell of gametophyte generation = spore, and last cell = gamete • Any gametophyte cell = 1n • First cell of sporophyte generation = zygote, and last cell = sporocyte • Any sporocyte cell = 2n

  20. Alternation of Generations • Change from sporophyte to gametophyte generation occurs from meiosis • Change from gametophyte to sporophyte occurs from fertilization

  21. Review • Introduction • Asexual • Sexual Reproduction • The Phases of Meiosis • Division I • Division II • Alternation of Generations

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