Meiosis | Cell Division | Biology

Introduction to Mitosis and Meiosis

• Mitosis: Used for cell division in body development and maintenance. Goal: produce genetically identical daughter cells.
• Meiosis: Specific to production of gametes (sperm and eggs). Goal: produce cells with half the chromosomes.
  • Transforms a diploid cell (two sets of chromosomes) to haploid cells (one set of chromosomes) in humans.
  • Sperm and egg formation results in a complete diploid genome upon fertilization.

Phases of Meiosis

• Comparison to Mitosis: Meiosis involves similar stages but has unique tasks, including separating homologous chromosomes and sister chromatids.
  • Meiosis I: Homologous chromosomes separate.
  • Meiosis II: Sister chromatids separate.
• Outcome: From one cell, up to four gametes (eggs or sperm) are produced. Each division has stages: prophase, metaphase, anaphase, and telophase.

Meiosis I

• Interphase: Cell growth and chromosome duplication.
• Prophase I: Chromosomes condense and pair with homologues.
  • Crossing Over: Exchange of DNA between homologues, facilitated by synaptonemal complex. Results in genetic diversity.
• Metaphase I: Homologue pairs align at metaphase plate.
• Anaphase I: Homologues move to opposite poles; sister chromatids remain attached.
• Telophase I: Chromosomes reach poles, cell divides into two haploid cells.

Meiosis II

• Direct transition from Meiosis I without DNA replication.
• Prophase II: Chromosomes recondense.
• Metaphase II: Chromosomes align individually at metaphase plate.
• Anaphase II: Sister chromatids separate to opposite poles.
• Telophase II: Formation of four haploid gametes.

Genetic Diversity in Meiosis

• Crossing Over: Random exchange points create unique genetic combinations.
• Random Orientation: During Metaphase I, random alignment of homologues leads to diverse gametes.
  • Human Gamete Diversity: Over 8 million different combinations possible merely from random orientation.
• Crossing Over Impact: When combined with crossing over, potential for genetic diversity is effectively infinite.

Meiosis vs. Mitosis

• Mitosis: For growth/maintenance, resulting in identical cells.
• Meiosis: For reproductive cells, resulting in genetic variation.

Special Considerations

• Spermatogenesis: In males, results in four sperm cells.
• Oogenesis: In females, typically produces one egg and polar bodies that usually degenerate.
• Importance for Evolution: Genetic diversity via meiosis essential for evolution and species adaptation.

References

• OpenStax, "The process of meiosis"
• Campbell Biology, "Genetic Variation in Sexual Life Cycles"
• Molecular Biology of the Cell, "Meiosis"