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MEIOSIS

MEIOSIS. Chapter 9. Why must organisms reproduce?. A. Types of Reproduction 1. Asexual Reproduction involves only 1 parent offspring are genetically identical to the parent (clones) advantageous when organism is well-adapted to a stable environment Ex. binary fission, parthenogenesis.

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MEIOSIS

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  1. MEIOSIS Chapter 9

  2. Why must organisms reproduce?

  3. A. Types of Reproduction 1. Asexual Reproduction • involves only 1 parent • offspring are genetically identical to the parent (clones) • advantageous when organism is well-adapted to a stable environment Ex. binary fission, parthenogenesis

  4. 2. Sexual Reproduction • involves 2 parents • parents produce haploid(1n) gametes which fuse to form a diploid(2n) zygote (first cell of offspring). • offspring are genetically diverse • advantageous in a changing environment Haploid gametes are produced by a type of cell division called meiosis.

  5. B. Meiosis Type of cell division that halves the number of chromosomes. • involves 2 nuclear divisions (meiosis I & meiosis II) • 1 diploid (2n) cell  4 haploid (1n) cells • occurs in germ cells In humans, where would germ cells be located?

  6. Germ cell replicates its chromosomes in S phase of interphase. 1. Meiosis I (reduction division) • Prophase I • replicated chromosomes condense • spindle apparatus forms & fibers attach to chromosomes • nucleolus disappears • nuclear membrane breaks down • synapsis & crossing over occurs

  7. Synapsis - homologous chromosomes (homologs)pair up. Homologs - chromosomes that carry genes for the same traits in the same sequence. A diploid human cell has 23 homologous pairs.

  8. Crossing over - non sister chromatids of homologs exchange chromosomal material. Chiasma - visible point of cross over between homologs.

  9. Animal cell in prophase I

  10. Metaphase I • paired homologous chromosomes line up along equator of spindle

  11. Anaphase I • homologous chromosomes separate & move to opposite poles

  12. Telophase I • spindle apparatus breaks down • chromosomes decondense • nuclear membranes partially reform around two nuclei Cytokinesis usually occurs between meiosis I & II

  13. 2. Meiosis II (equational division) • Prophase II • chromosomes condense • spindle apparatus forms & fibers attach to chromosomes • nuclear membrane breaks down • Metaphase II • chromosomes line up single-file along equator of spindle

  14. Anaphase II • centromeres part; sister chromatids (now called chromosomes) separate & move toward opposite poles • Telophase II • spindle apparatus breaks down • chromosomes de-condense • nuclear membranes reform • nucleoli reappear Cytokinesis divides two cells into four nonidentical cells.

  15. Meiosis II Four non-identical haploid cells Prophase II Metaphase II Anaphase II Telophase II

  16. C. Gametogenesis in Humans 1. Spermatogenesis (meiosis in the )

  17. Structure of human sperm: • Head • nucleus - 23 chromosomes • acrosome - contains enzymes that help sperm penetrate egg • Midpiece • mitochondria • Tail

  18. 2. Oogenesis (meiosis in the )

  19. Comparison of Oogenesis & Spermatogenesis

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