Meiosis and Sexual Life Cycles

Asexual vs Sexual Reproduction

  • Asexual   * Produces clones (genetically identical)   * Single parent   * Little variation in population - only through mutations   * Fast and energy efficient   * Eg. budding, binary fission
  • Sexual   * Meiosis produces gametes (sex cells)   * 2 parents: male/female   * Lots of variation/diversity   * Slower and energy consumptive   * Eg. humans, trees

Chromosomes

  • Somatic (body) cell: 2n = 46 chromosomes
  • Each pair of homologous chromosomes includes 1 chromosome from each parent
  • Autosomes: 22 pairs of chromosomes that do not determine sex
  • Sex chromosomes: X and Y   * Females: XX   * Males: XY
  • Gametes (n=23): 22 autosomes + 1 sex chromosome   * Egg: 22 + X   * Sperm: 22 + X  **or** 22 + Y

Life Cycle

  • Life cycle: reproductive history of organism, from conception → production of own offspring
  • Fertilization and meiosis alternate in sexual life cycles
  • Meiosis: cell division that reduces # of chromosomes (2n → n), creates gametes
  • Fertilization: combine gametes (sperm + egg)   * Fertilized egg = zygote (2n)
  • Zygote divides by mitosis to make multicellular diploid organism

Alternation of Generations

  • Plant and some algae
  • Sporophyte (2n): makes haploid spores by meiosis   * Spore → gametophyte by mitosis
  • Gametophyte (n): makes haploid gametes by mitosis

Meiosis I

  • Interphase: chromosomes replicated
  • Prophase I:   * Synapsis: homologous chromosomes pair up   * Tetrad: 4 sister chromatids   * Crossing over at the chiasmata
  • Metaphase I: Tetrads line up
  • Anaphase I:   * Pairs of homologous chromosomes separate   * (Sister chromatids still attached by centromere)
  • Telophase I & Cytokinesis:   * Haploid set of chromosomes in each cell   * Each chromosome = 2 sister chromatids   * Some species: chromatin & nucleus reforms

Meiosis II

  • Prophase II:   * No interphase   * No crossing over   * Spindle forms
  • Metaphase II:   * Chromosomes line up
  • Anaphase II:   * Sister chromatids separate
  • Telophase II:   * 4 haploid cells   * Nuclei reappear   * Each daughter cell genetically unique

Events Unique to Meiosis I

  • Prophase I: Synapsis and crossing over
  • Metaphase I: pairs of homologous chromosomes line up on metaphase plate
  • Anaphase I: homologous pairs separate → sister chromatids still attached at centromere

Sources of Genetic Variation

  • Crossing Over: exchange genetic material, resulting in recombinant chromosomes
  • Independent Assortment of Chromosomes: random orientation of homologous pairs in Metaphase I
  • Random Fertilization: Any sperm + Any egg

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