Meiosis: Reduction Division

What is Meiosis?

• Type of cell division producing gametes (sex cells: sperm and egg).
• Called "reduction division" as it reduces chromosome number by half.
• Two stages: Meiosis I and Meiosis II.

Stages of Meiosis

Meiosis I

• Prophase I:
  • Begins with a diploid cell.
  • Chromatin contains two sets of chromosomes (one from each parent).
  • DNA replicates and condenses into X-shaped chromosomes.
  • Synapsis: Chromosomes pair up with homologous chromosomes forming a tetrad (4 chromatids).
  • Crossing Over (Recombination):
    • Chromatids exchange segments of alleles.
    • Results in genetic variety in offspring.
  • Nuclear membrane disappears.
  • Centrioles move to opposite ends; spindle fibers form.
• Metaphase I:
  • Homologous chromosomes line up at the equator.
  • Attach to spindle fibers from opposite poles.
• Anaphase I:
  • Spindle fibers separate homologous chromosomes to opposite poles.
• Telophase I:
  • Chromosomes at separate poles; still in sister chromatid form.
  • Nuclear membrane reforms; spindle fibers disappear.
  • Cytokinesis: Results in two genetically different haploid cells.

Meiosis II

• DNA does not replicate before this stage.
• Prophase II:
  • Nuclear membrane disappears; spindle fibers form from centrioles.
• Metaphase II:
  • Chromosomes align at equator.
  • Attach to spindle fibers from both poles.
• Anaphase II:
  • Sister chromatids separate and move to opposite poles.
  • Once separated, they are called chromosomes.
• Telophase II:
  • Spindle fibers disappear; nuclear membranes reform.
  • Cytokinesis: Leads to four genetically different haploid cells.

Key Points to Remember

• Starts with a diploid cell and produces haploid gametes.
• Meiosis I:
  • Homologous chromosomes separate, forming two haploid cells.
• Meiosis II:
  • Separation of sister chromatids.
  • Results in four genetically different haploid gametes.
• Prophase I Specifics:
  • Synapsis and tetrad formation.
  • Crossing over leads to genetic differences.
• All gametes produced are haploid.