TR

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