Lecture on Meiosis

Introduction to Meiosis

• Meiosis explains why siblings can look different despite having the same parents.
• Meiosis vs Mitosis:
  • Mitosis: Produces identical body cells (e.g., skin, stomach).
  • Meiosis: Contributes to genetic variety, produces gametes (sperm and egg cells).

Chromosome Numbers

• Humans have 46 chromosomes in most body cells.
• Gametes (sperm and egg cells) have 23 chromosomes each.
• Fusion of sperm and egg results in a total of 46 chromosomes in a fertilized egg.

Meiosis as a Reduction Division

• Starts with a cell with 46 chromosomes and ends with gametes having 23 chromosomes.
• Interphase:
  • Occurs before meiosis starts.
  • Involves growth, DNA replication, and cell processes.
  • Duplicates 46 chromosomes, considered 92 chromatids due to counting centromeres.

Meiosis Process

• Meiosis involves two rounds of cell division: Meiosis I and Meiosis II, each with stages PMAT (Prophase, Metaphase, Anaphase, Telophase).

Meiosis I

• Prophase I:
  • Chromosomes condense and line up with homologous pairs.
  • Crossing Over: Exchange of genetic information between homologous chromosomes.
  • Results in recombinant chromosomes contributing to genetic diversity.
• Metaphase I:
  • Chromosomes align in pairs at the middle of the cell.
• Anaphase I:
  • Homologous chromosomes are pulled away by spindle fibers.
• Telophase I:
  • Formation of two new nuclei, resulting in two new cells.
  • Cytokinesis splits the cytoplasm.

Meiosis II

• Prophase II:
  • Chromosomes condense, but no crossing over occurs.
• Metaphase II:
  • Chromosomes align in single file at the middle.
• Anaphase II:
  • Chromatids are pulled away by spindle fibers.
• Telophase II:
  • Nuclei reform, resulting in four cells.
  • Cytokinesis completes cell division.

Outcomes of Meiosis

• In males, results in four sperm cells; in females, one egg cell (often with polar bodies that are not viable).
• Each sperm/egg cell is unique due to independent assortment and crossing over.
• Provides a reason for genetic variety among siblings.

Additional Considerations

• Nondisjunction: Error in separation can lead to genetic disorders.
• Ongoing scientific study to understand and address such issues.

Conclusion

• Meiosis is a key biological process contributing to genetic diversity and is distinct from mitosis.