Meiosis: Understanding Genetic Variation

Introduction

• Meiosis explains why siblings with the same parents can look different.
• Not to be confused with mitosis; meiosis contributes to genetic variety and produces gametes (sperm and egg cells).

Chromosome Basics

• Humans generally have 46 chromosomes in body cells.
• Gametes (sperm and egg cells) have 23 chromosomes each. Together, they form a fertilized egg with 46 chromosomes.
• Meiosis is a reduction division: starting cell has 46 chromosomes, ending cells have 23 chromosomes.

Pre-Meiosis: Interphase

• Interphase occurs before meiosis and mitosis.
• During interphase, cells grow, replicate DNA, and carry out cell processes.
• Chromosome duplication occurs: 46 chromosomes replicate to become 92 chromatids, but still counted as 46 due to centromeres.

Stages of Meiosis

Meiosis I

• Prophase I:
  • Chromosomes condense, thicken, and line up with homologous pairs.
  • Crossing over occurs, exchanging genetic information leading to recombinant chromosomes.
• Metaphase I:
  • Chromosomes line up in pairs in the middle of the cell.
• Anaphase I:
  • Chromosomes are pulled away by spindle fibers.
• Telophase I:
  • Formation of two new nuclei resulting in two new cells.
  • Followed by cytokinesis, splitting the cytoplasm.

Meiosis II

• Prophase II:
  • Chromosomes and spindles form, no homologous pairs or crossing over.
• Metaphase II:
  • Chromosomes line up in the middle, in a single file line.
• Anaphase II:
  • Chromatids are pulled away to opposite sides.
• Telophase II:
  • Nuclei reform; two cells divide to form four cells.
  • Cytokinesis follows to split the cytoplasm.

Result of Meiosis

• In males, meiosis produces four different sperm cells.
• In females, meiosis produces egg cells.
• Independent assortment and crossing over lead to variety: sperm and egg cells are different from the starting cell and each other.
• Explains why siblings look different despite having the same parents.

Genetic Disorders

• Nondisjunction: chromosomes do not separate correctly, resulting in cells with too many or too few chromosomes.
• Contributes to some genetic disorders; area of active scientific research.

Conclusion

• Meiosis is crucial for genetic diversity.
• The Amoeba Sisters encourage curiosity and continuous learning.