Lecture on Meiosis

Introduction

• Meiosis vs Mitosis:
  • Meiosis contributes to genetic variety, producing sperm and egg cells (gametes).
  • Mitosis makes identical body cells, important for growth and repair.
• Chromosome Numbers:
  • Humans have 46 chromosomes in most body cells.
  • Human sperm and egg cells have 23 chromosomes each.

Overview of Meiosis

• Reduction Division:
  • Starts with a cell having 46 chromosomes, ends with gametes having 23.
  • Involves two divisions (PMAT twice) compared to one in mitosis.

Interphase

• Occurs before meiosis begins.
• Cell grows, replicates DNA, and carries out cell processes.
• Chromosomes duplicate (46 chromosomes, 92 chromatids but still counted as 46 by centromeres).

Stages of Meiosis

• Prophase I:

  • Chromosomes condense, pair up with homologous partners.
  • Crossing over occurs, contributing to genetic diversity.

• Metaphase I:

  • Chromosomes line up in pairs in the middle of the cell.

• Anaphase I:

  • Chromosomes are pulled away by spindle fibers.

• Telophase I and Cytokinesis:

  • Two nuclei form, cytoplasm divides leading to two new cells.

Meiosis II

• Prophase II:

  • No homologous pairs or crossing over.
  • Spindles form.

• Metaphase II:

  • Chromosomes line up in a single file line in the middle.

• Anaphase II:

  • Chromatids are pulled away by spindle fibers.

• Telophase II and Cytokinesis:

  • Nuclei reform, cells divide into four.

Conclusion

• Outcome of Meiosis:

  • Results in four genetically diverse gametes.
  • Explains sibling genetic differences despite having the same parents.

• Nondisjunction:

  • Errors in chromosome separation can lead to genetic disorders, an area of ongoing scientific research.

Note

• Meiosis in males produces sperm cells; in females, it produces egg cells.
• Independent assortment and crossing over contribute to genetic variety.

---

Stay Curious!