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