Cell Cycle: Meiosis Module 11

Overview of Meiosis

• Meiosis reduces chromosome number by half, from diploid (2n = 46) to haploid (n = 23).
• Occurs solely in reproductive cells of sexually reproducing organisms.
• Key function: ensures genetic variability and maintains chromosome number across generations.

Phases of Meiosis

Meiosis I: Reductional Division

• Reduces chromosome number from diploid to haploid.

Phases of Meiosis I

• Prophase I:

  • Chromosomes coil and become visible (Leptonema).
  • Homologous chromosomes pair (Zygonema), forming bivalents (two homologous chromosomes).
  • Crossing-over occurs (Pachynema), ensuring genetic diversity.
  • Homologous chromosomes begin to separate; chiasmata become visible (Diplonema).
  • Chromosomes fully condense; chiasmata move to ends; nuclear envelope breaks down (Diakinesis).

• Metaphase I:

  • Homologous chromosomes align on the metaphase plate.
  • Spindle fibers attach from opposite poles to chromosomes.

• Anaphase I:

  • Homologous chromosomes separate and move to opposite poles.
  • Sister chromatids remain attached at the centromere.

• Telophase I:

  • Homologous chromosomes complete migration to poles.
  • Nuclear envelope reforms; cytokinesis creates two haploid daughter cells.

Meiosis II: Equational Division

• Similar to mitosis, separates sister chromatids while maintaining haploid number.

Phases of Meiosis II

• Prophase II:

  • Spindle apparatus reforms; chromosomes condense in preparation for division.

• Metaphase II:

  • Chromosomes align on the metaphase plate; kinetochores face opposite poles.

• Anaphase II:

  • Centromeres split; sister chromatids move to opposite poles as individual chromosomes.

• Telophase II:

  • Nuclear envelopes reform around separated chromosomes.
  • Cytokinesis occurs, resulting in four non-identical haploid daughter cells.

Key Terms

• Meiosis: Cell division producing four genetically diverse haploid cells from one diploid parent cell.
• Diploid (2n): Cells with two complete chromosome sets (46 in humans).
• Haploid (n): Cells with one complete chromosome set (23 in humans).
• Homologous Chromosomes: Chromosome pairs from each parent, similar in structure and genetic content.
• Synapsis: Pairing of homologous chromosomes.
• Bivalent/Tetrad: Homologous chromosome pair consisting of four chromatids.
• Crossing-over: Exchange of genetic material between homologous chromosomes.
• Chiasma: Points at which homologous non-sister chromatids cross over.
• Nondisjunction: Failure of chromosome separation leading to genetic disorders.
• Cytokinesis: Division of the cytoplasm, resulting in distinct cells.

Key Points to Remember

• Purpose of meiosis: reduce chromosome number and create genetic diversity via recombination.
• Differences between Meiosis I & II:
  • Meiosis I reduces chromosome number; Meiosis II separates sister chromatids.
• Errors in meiosis (nondisjunction, translocation, deletion) can lead to significant genetic disorders.
• Crossing-over during Prophase I is vital for genetic variation in offspring, crucial for sexual reproduction.

Comparison: Mitosis vs. Meiosis

• Number of Cell Divisions: Mitosis (1), Meiosis (2).
• Number of Daughter Cells: Mitosis (2), Meiosis (4).
• Genetic Identity: Mitosis (identical to parent), Meiosis (genetically different from parent).
• Chromosome Number: Mitosis (diploid, 46), Meiosis (haploid, 23).
• Location of Occurrence: Mitosis (somatic cells), Meiosis (gametes).
• Timing: Mitosis (throughout life); Meiosis (during sexual maturity).
• Primary Role: Mitosis (growth, repair); Meiosis (sexual reproduction, genetic diversity).

Common Errors in Meiosis

• Nondisjunction: Leads to genetic disorders such as:
  • Down Syndrome: Extra copy of chromosome 21.
  • Klinefelter Syndrome: Additional X chromosome (XXY).
  • Turner Syndrome: Missing X chromosome in females (XO).

Test your understanding of the different phases of meiosis with this quiz. Learn about the key events that occur during meiotic cell division, which reduces the chromosome number from diploid to haploid, specifically in reproductive cells. Perfect for reinforcing concepts covered in Module 11.