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Understanding Meiosis and Mitosis

Mar 29, 2025

Lecture Notes: Meiosis and Mitosis

Introduction

  • Topic: Sexual reproduction in eukaryotic cells via meiosis.
  • Comparison: Mitosis vs. Meiosis
    • Mitosis: Cell division in somatic cells resulting in two genetically identical diploid cells.
    • Meiosis: Cell division in gametes (germ cells), resulting in four genetically different haploid cells.

Key Differences Between Mitosis and Meiosis

  • Mitosis:
    • Produces 2 diploid cells with identical chromosomes (46 in humans).
    • Occurs in somatic cells.
  • Meiosis:
    • Produces 4 haploid cells with half the chromosome number (23 in humans).
    • Occurs in gametes (spermatocytes in males and oocytes in females).

Interphase in Meiosis

  • Process: Similar to mitosis, involving DNA replication during the S-phase.
  • Chromosome Replication: Each homologous chromosome is replicated.
    • Homologous chromosomes carry genes for the same traits.
    • Example: Genes for hair color present on homologous chromosomes.

Meiosis Stages Overview

  • Divided into two stages: Meiosis I and Meiosis II.
  • Focus on Meiosis I in this lecture.

Meiosis I

Prophase I

  • Centrioles: Move to opposite ends; spindle apparatus forms.
  • Chromatin: Condenses into chromosomes; nuclear membrane dissolves.
  • Synapsis and Crossing Over:
    • Synapsis: Homologous chromosomes pair up (side-by-side alignment).
    • Chiasma: Point where chromatids overlap and cross over.
    • Crossing Over: Exchange of genetic material; produces recombinant chromosomes.

Metaphase I

  • Spindle fibers attach to kinetochores.
  • Tetrads (paired homologous chromosomes) align along the equatorial plane.

Anaphase I

  • Disjunction: Separation of homologous chromosomes to opposite poles.
  • Law of Segregation: Random separation of maternal and paternal chromosomes.

Telophase I

  • Nuclear Membrane: Reforms around each set of chromosomes.
  • Cytokinesis: Cytoplasm divides, forming two haploid cells.
  • Reduction Division: Chromosome number reduces from diploid to haploid.

Conclusion

  • Meiosis I results in two haploid cells from one diploid cell.
  • Next Lecture: Focus on Meiosis II.

These notes provide an overview of the processes and differences involved in meiosis, particularly Meiosis I, setting the stage for understanding genetic variation and the formation of gametes in eukaryotic organisms.

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