Meiosis: A Source of Genetic Variation

Introduction

• Meiosis vs. Mitosis:
  • Mitosis creates identical body cells (e.g., skin, stomach cells) for growth and repair.
  • Meiosis contributes to genetic variety by creating gametes (sperm and egg cells).
• Chromosome Numbers:
  • Humans have 46 chromosomes in most body cells.
  • Human sperm and egg cells have 23 chromosomes each, combining to form the 46 chromosome zygote.

Interphase

• Occurs before meiosis (and mitosis):
  • Cell growth, DNA replication, cell processes.
• Chromosome Duplication:
  • Starting cell has 46 chromosomes; duplicates DNA.
  • 46 chromosomes replicate to 92 chromatids (but still counted as 46 chromosomes by centromeres).

Meiosis Overview

• Two Divisions: Meiosis involves two rounds of division, each with PMAT stages.
  • Meiosis I: Reduction division from 46 to 23 chromosomes.
  • Meiosis II: Similar to mitosis, separates chromatids.

Meiosis I Stages

• Prophase I:
  • Chromosomes condense and pair up with homologous chromosomes.
  • Crossing Over: Exchange of genetic material between homologous chromosomes.
• Metaphase I:
  • Chromosome pairs align in the middle of the cell.
• Anaphase I:
  • Chromosome pairs pulled apart by spindle fibers.
• Telophase I and Cytokinesis:
  • Formation of two new nuclei and division into two cells.

Meiosis II Stages

• Prophase II:
  • Spindle fibers form; no crossing over.
• Metaphase II:
  • Chromosomes align in a single file in the middle.
• Anaphase II:
  • Chromatids pulled apart to opposite poles.
• Telophase II and Cytokinesis:
  • Nuclei reform, resulting in four cells with 23 chromosomes each.

Genetic Variation

• Independent Assortment and Crossing Over:
  • Lead to genetic variety among gametes.
  • Each sperm or egg is unique.

Nondisjunction

• Errors in Chromosome Separation:
  • Can result in genetic disorders due to receiving the incorrect number of chromosomes.

Conclusion

• Meiosis is crucial for genetic diversity, explaining why siblings can look different despite sharing the same parents.
• Scientific Study: Researchers study meiosis to understand and address genetic disorders.