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  1. 20 µm 100 µm 200 µm (a) Reproduction (b) Growth and development (c) Tissue renewal

  2. 20 µm

  3. 0.5 µm Chromosomes DNA molecules Chromo- some arm Chromosome duplication (including DNA synthesis) Centromere Sister chromatids Separation of sister chromatids Centromere Sister chromatids

  4. Metaphase Anaphase Telophase and Cytokinesis G2 of Interphase Prophase Prometaphase Centrosomes (with centriole pairs) Early mitotic spindle Centromere Chromatin (duplicated) Fragments of nuclear envelope Nonkinetochore microtubules Aster Cleavage furrow Metaphase plate Nucleolus forming Daughter chromosomes Nuclear envelope forming Centrosome at one spindle pole Spindle Nuclear envelope Kinetochore Chromosome, consisting of two sister chromatids Kinetochore microtubule Plasma membrane Nucleolus

  5. 5 µm Pair of homologous replicated chromosomes Centromere Sister chromatids Metaphase chromosome

  6. Key Maternal set of chromosomes (n = 3) 2n = 6 Paternal set of chromosomes (n = 3) Two sister chromatids of one replicated chromosome Centromere Two nonsister chromatids in a homologous pair Pair of homologous chromosomes (one from each set)

  7. Key Haploid gametes (n = 23) Haploid (n) Egg (n) Diploid (2n) Sperm (n) MEIOSIS FERTILIZATION Testis Ovary Diploid zygote (2n = 46) Mitosis and development Multicellular diploid adults (2n = 46)

  8. Key Haploid (n) Haploid unicellular or multicellular organism Haploid multi- cellular organism (gametophyte) Diploid (2n) Gametes n n Mitosis Mitosis Mitosis Mitosis n n n n n n n n Spores n FERTILIZATION MEIOSIS n Gametes Gametes n MEIOSIS FERTILIZATION Zygote MEIOSIS FERTILIZATION 2n 2n 2n 2n Diploid multicellular organism Zygote Diploid multicellular organism (sporophyte) 2n Mitosis Mitosis Zygote (a) Animals (c) Most fungi and some protists (b) Plants and some algae

  9. Interphase Homologous pair of chromosomes in diploid parent cell Chromosomes replicate Homologous pair of replicated chromosomes Sister chromatids Diploid cell with replicated chromosomes Meiosis I Homologous chromosomes separate 1 Haploid cells with replicated chromosomes Meiosis II Sister chromatids separate 2 Haploid cells with unreplicated chromosomes

  10. Telophase I and Cytokinesis Telophase II and Cytokinesis Prophase II Metaphase II Anaphase II Metaphase I Prophase I Anaphase I Centrosome (with centriole pair) Sister chromatids remain attached Centromere (with kinetochore) Sister chromatids Chiasmata Spindle Metaphase plate Sister chromatids separate Haploid daughter cells forming Homologous chromosomes separate Cleavage furrow Homologous chromosomes Microtubule attached to kinetochore Fragments of nuclear envelope

  11. MITOSIS MEIOSIS MEIOSIS I Chiasma Parent cell Chromosome replication Chromosome replication Prophase I Prophase Homologous chromosome pair 2n = 6 Replicated chromosome Metaphase I Metaphase Anaphase I Anaphase Telophase Telophase I Haploid n = 3 Daughter cells of meiosis I MEIOSIS II 2n 2n Daughter cells of mitosis n n n n Daughter cells of meiosis II

  12. Possibility 2 Possibility 1 Two equally probable arrangements of chromosomes at metaphase I Metaphase II Daughter cells Combination 1 Combination 2 Combination 3 Combination 4

  13. Prophase I of meiosis Nonsister chromatids held together during synapsis Pair of homologs Chiasma Centromere TEM Anaphase I Anaphase II Daughter cells Recombinant chromosomes

  14. Meiosis I Nondisjunction Meiosis II Nondisjunction Gametes n – 1 n + 1 n – 1 n n n + 1 n + 1 n – 1 Number of chromosomes (b) Nondisjunction of sister chromatids in meiosis II (a) Nondisjunction of homologous chromosomes in meiosis I