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Meiosis

This biology lesson explores the process of meiosis, the importance of genetic variation in reproduction, and its implications for evolution. Topics covered include the differences between asexual and sexual reproduction, the human life cycle, karyotypes, and the stages of meiosis. The significance of independent assortment, crossing over, and random fertilization in creating genetic diversity is also discussed.

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Meiosis

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  1. Meiosis AP Biology Crosby High School

  2. Heredity • Offspring gets genes from parents • Determines what/who we are • Nature: Who we are determined by genetics • Nurture: Who we are determined by environment • Genes: • Receive alleles from parents • Located at specific Loci along a chromosome

  3. Reproduction • Asexual: Offspring is exact copy of parent • Usually unicellular organisms • Some Multicellular • i.e. Hydra reproduce asexually through budding • Creates a clone • Sexual: Offspring a mixture of parents • Creates genetic differences • Results in greater variation than Asexual

  4. Human Life Cycle • Somatic Cells: • Contain 46 chromosomes • 2 homologues • Diploid = 2n cells • Gametes • Result of Meiosis • Haploid = n cells • Contain 23 chromosomes • Syngamy of gametes results in a zygote • Meiosis reduces chromosome number

  5. Karyotype • Homologous Chromosomes • Same length, same location of centromere • Each homologue contains one of two alleles • Y chromosomes lack some genes present on X • 23 pairs

  6. Animal Sexual Cycle • Animal • Meiosis produces Haploid gametes • Gametes combine to form Zygote • Zygote divides by Mitosis

  7. Most Fungi, Some Algae • Gametes fuse to form a Diploid Zygote • Meiosis occurs before offspring develop • Produces Gametes by Mitosis

  8. Plants, Some Algae • Alternation of Generations • Multicellular Diploid: Sporophyte • Multicellular Haploid: Gametophyte • Meiosis in Sporophyte produces spores • Spores divide through Mitosis forming Gametophyte • Gametes fertilize to produce zygote (Sporophyte)

  9. Meiosis: Two part Mitosis • Interphase • Meiosis I: Produces two Haploid cells that still contain sister chromatids • Prophase I • Metaphase I • Anaphase I • Telophase I • Meiosis II: Produces four Haploid cells containing single chromosomes • Prophase II • Metaphase II • Anaphase II • Telophase II

  10. Meiosis I Interphase I Prophase I Metaphase I Anaphase I Telophase I Meiosis II Similar to Mitosis Cytokinesis II Meiosis Iand Meiosis II

  11. Mitosis vs. Meiosis • Prophase I • Tetrad forms synaptonemal complex • Non-sister chromatids cross at Chiasmata • Metaphase I • Meiosis: Homologous pairs align on Metaphase plate • Mitosis: Individual duplicated chromosomes align on Metaphase plate • Anaphase I • Sister chromatids remain attached to each other • Meiosis I separates Homologous pairs not sister chromatids

  12. Origins of Genetic Variation • Independent Assortment • 50% chance maternal; 50% chance paternal • Possible combinations = 2n • n = Haploid number of organism • Human possibilities = 223 • Crossing Over • Recombinant Chromosomes • Homologous portions line up and swap with non-sister chromatids • No longer identical Homologues • Random Fertilization • Only 1 of 8 million sperm and 1 of 8 million ovum • 8 million * 8 million = 64 trillion possibilities

  13. Evolution depends on Genetic Variation • Survival of the Fittest • Those best suited to an environment survive to reproduce • The more you reproduce the more present your genes are • Population shifts toward most suited variation

  14. Independent Assortment • Back

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