Understanding Meiosis

Introduction to Meiosis

• Meiosis is a process that contributes to genetic diversity.
• It is distinct from mitosis, which creates identical body cells for growth and repair.
• Meiosis produces gametes (sperm and egg cells), each with 23 chromosomes.
• Gametes combine to form a fertilized egg with 46 chromosomes.

Chromosomes and Genetic Material

• Humans have 46 chromosomes in most body cells, but gametes have 23.
• Meiosis is known as a reduction division, starting with 46 chromosomes and ending with 23.
• Interphase:
  • Occurs before meiosis begins.
  • Cells grow, replicate DNA, and carry out cell processes.
  • Chromosomes duplicate, creating 92 chromatids but still counted as 46 chromosomes (counted by centromeres).

Stages of Meiosis

• Meiosis involves two divisions, each with stages similar to mitosis (PMAT), repeated twice:
  • Prophase 1:
    • Chromosomes condense and pair up with homologous pairs (same size, same gene types).
    • Crossing Over occurs – exchange of genetic material between homologous chromosomes, leading to recombinant chromosomes.
  • Metaphase 1:
    • Chromosome pairs align in the middle of the cell.
  • Anaphase 1:
    • Chromosomes are pulled away by spindle fibers.
  • Telophase 1 and Cytokinesis:
    • Formation of two new nuclei and division into two cells.
  • Prophase 2:
    • Less eventful; no crossing over, spindles form.
  • Metaphase 2:
    • Chromosomes line up single file in the middle.
  • Anaphase 2:
    • Chromatids are pulled away by spindle fibers.
  • Telophase 2 and Cytokinesis:
    • Nuclei reform, resulting in four cells.

Gamete Formation and Genetic Variability

• Meiosis results in sperm cells (males) and egg cells (females).
• Independent assortment and crossing over lead to genetic diversity.
• Each gamete is unique, explaining sibling differences.

Nondisjunction and Genetic Disorders

• Sometimes chromosomes do not separate properly during meiosis, called nondisjunction.
• This can result in cells with too many or too few chromosomes, potentially leading to genetic disorders.

Conclusion

• Meiosis is essential for genetic diversity and reproduction.
• Scientists continue to study meiosis, especially issues like nondisjunction, to understand genetic disorders better.