Lecture on Meiosis

Introduction to Meiosis

• Meiosis explains why siblings with the same parents can look different.
• Not to be confused with mitosis:
  • Mitosis creates identical body cells (e.g., skin, stomach cells).
  • Important for growth, repair, or replacing worn-out cells.

Key Features of Meiosis

• Contributes to genetic variety.
• Produces gametes (sperm and egg cells) instead of body cells.
• Humans have 46 chromosomes in most body cells but only 23 in gametes.
• A sperm cell (23 chromosomes) and an egg cell (23 chromosomes) combine to form a fertilized egg (46 chromosomes).
• Described as a reduction division:
  • Starting cell: 46 chromosomes.
  • Ending cells (sperm/egg): 23 chromosomes.

Pre-conditions for Meiosis

• Interphase:
  • Growth, DNA replication, and preparation for division.
  • Chromosomes duplicate, but chromosome count remains 46 (by centromere count); turns to 92 chromatids.

Stages of Meiosis

Meiosis I

1. Prophase I:
   • Chromosomes condense and pair with homologous chromosomes.
   • Crossing over occurs: Exchange of genetic material leading to recombinant chromosomes.
2. Metaphase I:
   • Chromosomes align in pairs in the center of the cell.
3. Anaphase I:
   • Homologous chromosomes are pulled apart by spindle fibers.
4. Telophase I and Cytokinesis:
   • Formation of two new nuclei and division into two cells.

Meiosis II

1. Prophase II:
   • Spindle formation, but no crossing over as chromosomes are not in homologous pairs.
2. Metaphase II:
   • Chromosomes line up in single file in the middle.
3. Anaphase II:
   • Chromatids are pulled away by spindle fibers.
4. Telophase II and Cytokinesis:
   • Formation of four distinct cells as nuclei reform.

Outcomes of Meiosis

• In males: Produces four different sperm cells.
• In females: Produces egg cells.
• Each gamete is different due to independent assortment and crossing over.
• Leads to genetic variety among siblings.

Additional Considerations

• Nondisjunction: Errors in chromosome separation leading to genetic disorders.
• Ongoing research into meiosis and its implications for genetic diversity and disorders.

Conclusion

• Meiosis is fundamental to genetic diversity.
• Continuous study reveals more about its role in genetics.

Stay curious!