Lecture on Meiosis by Professor Dave

Introduction to Cell Division

• All living creatures begin as a single cell.
• Initial cell division occurs through mitosis, creating identical cells.
• The origin of the first cell involves meiosis and fertilization.

Meiosis vs. Mitosis

• Meiosis is a type of cell division but differs from mitosis.
  • Mitosis results in two identical cells (diploid - 2n).
  • Meiosis results in four non-identical haploid cells (n).
• Human cells have 46 chromosomes, organized into homologous pairs.
• Homologous chromosomes carry genes for the same characteristics.

Formation of Gametes and Fertilization

• Haploid cells: reproductive cells called gametes (e.g., sperm and egg in humans).
• Gametes have 23 chromosomes each.
• Fertilization merges sperm and egg, forming a diploid cell with 46 chromosomes.
• Sexual reproduction via meiosis and fertilization results in genetic variation.

Meiosis Process

• Comprises two stages: Meiosis I and Meiosis II.
  • Meiosis I: Homologous chromosomes separate.
  • Meiosis II: Separation of sister chromatids.

Meiosis I

• Stages: Prophase I, Metaphase I, Anaphase I, Telophase I, and Cytokinesis.
  • Prophase I:
    • Chromosomes undergo crossing over.
    • Exchange of genetic material between nonsister chromatids.
    • Formation of recombinant chromosomes at chiasmata.
  • Metaphase I:
    • Random alignment of homologous pairs.
  • Anaphase I:
    • Separation of homologous chromosomes.
  • Telophase I and Cytokinesis:
    • Formation of two haploid daughter cells.

Meiosis II

• Stages: Prophase II, Metaphase II, Anaphase II, Telophase II, and Cytokinesis.
  • Similar to mitosis but starting with haploid cells.
  • Prophase II:
    • Formation of spindle apparatus.
  • Metaphase II:
    • Alignment of chromosomes at metaphase plate.
  • Anaphase II:
    • Separation of sister chromatids.
  • Telophase II and Cytokinesis:
    • Formation of four unique haploid cells.

Biological Variation

• Variation arises from random assortment and crossing over.
• Each gamete represents a unique combination of genes.
• Contributes to the diversity of phenotypes in organisms.

Conclusion

• Human life cycle starts with haploid gametes.
• Fertilization leads to a diploid cell, development through mitosis.
• Genetics will determine the traits passed from parents to offspring.

Next topic: Genetics and heredity.