Medicine & USMLE


Cell Division
  1. Mitosis
  2. Meiosis


Meiosis is the process by which germ cells divide into our gametes, the sperm and the egg. Through meiosis one, a single diploid germ cell splits apart into two haploid cells. The phases of meiosis one are nearly the same as mitosis, with a few key differences. In prophase one, crossing over occurs. In metaphase one, homologous chromosome pairs align side-by-side. In anaphase one, homologous pairs are separated. After meiosis one, meiosis two occurs. In meiosis two, the two haploid cells are further split into four haploid cells, which become the resulting gametes.The events in each phase of meiosis two are the exact same as those of mitosis!

Key Points

  • Meiosis
    • Sexual reproduction in germ cells (gametocytes)
      • Contrast vs. mitosis (occurs in somatic cells)
      • Starts with 1 diploid cell, ends with 4 haploid cells
    • Meiosis I
      • Start: 1 diploid cell
      • End: 2 haploid cells
      • Phases
        • Prophase I
          • Spindle apparatus forms, chromosomes condense, nuclear membrane dissolves (same as prophase of mitosis)
          • Homologous chromosomes cross over
            • intertwine and exchange DNA
              • Responsible for genetic diversity of offspring
        • Metaphase I
          • Homologous pairs align side-by-side at metaphase plate
            • Two lines of chromosomes, with homologous chromosomes laterally paired
            • Contrast vs. mitosis (chromosomes line up single file)
        • Anaphase I
          • Homologous pairs pulled apart
            • Homologous chromosomes pulled apart
            • Split is random, resulting gametes do not have identical gene sequences
          • Contrast to mitosis (sister chromatids are pulled apart)
          • Nondisjunction may occur here
        • Telophase I
          • Nuclear membranes form, chromosomes begin to decondense, microtubules dissolve, cytokinesis (same as telophase of mitosis)
          • 2 genetically different haploid cells produced
    • Meiosis ll
      • Start: 2 haploid cells
      • End: 4 haploid cells
        • These are gametes (germ cells)
      • Phases
        • Similar to Mitosis, except with 23 chromosomes
        • Prophase ll
          • Nuclear membrane dissolves, centrosomes migrate and spindle apparatus forms
        • Metaphase ll
          • Chromosomes aligned on equatorial plate
        • Anaphase ll
          • Sister chromatids are pulled apart by spindle
            • Note: centromeres divided so each sister has its own centromere (number of chromosomes doubles)
          • Nondisjunction may occur here as well
        • Telophase ll
          • Nuclear membranes form around each set of chromosomes, cytokinesis
          • End with 4 haploid germ cells