Meiosis: Phases of Cell Cycle - Module 11

Meiosis Overview

• Meiosis is a type of cell division that reduces the chromosome number by half, transitioning from diploid (46 chromosomes) to haploid (23 chromosomes).
• Occurs only in reproductive cells of sexually reproducing organisms.
• Ensures genetic variability and maintains chromosome number across generations.

Meiosis Phases

Meiosis I (Reductional Division)

• Reduces chromosome number from diploid (46) to haploid (23).
• Phases of Meiosis I:
  • Prophase I:
    • Leptonema: Chromosomes coil and become visible.
    • Zygonema: Homologous chromosomes undergo synapsis, forming bivalents (two homologous chromosomes, four chromatids).
    • Pachynema: Chromosomes thicken, crossing-over occurs, increasing genetic variability.
    • Diplonema: Homologous chromosomes begin separating, with visible chiasmata (points of crossing-over).
    • Diakinesis: Chromosomes completely condense, chiasmata move to ends, and the nuclear envelope breaks down.
  • Metaphase I:
    • Homologous chromosomes align on the metaphase plate with spindle fibers attached from opposite poles.
  • Anaphase I:
    • Homologous chromosomes separate to opposite poles, sister chromatids remain attached at the centromere.
  • Telophase I:
    • Homologous chromosomes migrate to poles, nuclear envelope reforms, and cytokinesis produces two haploid daughter cells.

Meiosis II (Equational Division)

• Similar to mitosis; separates sister chromatids while maintaining haploid number (23 chromosomes).
• Phases of Meiosis II:
  • Prophase II:
    • Spindle apparatus re-forms; chromosomes condense for division.
  • Metaphase II:
    • Chromosomes align on the metaphase plate, kinetochores of sister chromatids face opposite poles.
  • Anaphase II:
    • Centromeres separate, chromatids move to opposite poles becoming individual chromosomes.
  • Telophase II:
    • Nuclear envelopes reform around chromosomes; cytokinesis yields four genetically distinct haploid daughter cells.

Key Terms and Definitions

• Meiosis: Cell division producing four genetically diverse haploid cells from a diploid parent.
• Diploid (2n): Cell with two complete chromosome sets (46 in humans).
• Haploid (n): Cell with one complete chromosome set (23 in humans).
• Homologous Chromosomes: Chromosome pairs from each parent sharing similar shape, size, and genetic content.
• Synapsis: Pairing of homologous chromosomes in meiosis.
• Bivalent/Tetrad: Pair of homologous chromosomes (four chromatids).
• Crossing-over: Genetic material exchange between homologous chromosomes enhancing diversity.
• Chiasma: Sites of genetic material exchange during crossing-over.
• Nondisjunction: Improper separation during meiosis causing disorders like Down Syndrome.
• Cytokinesis: Cytoplasmic division completing cell division, yielding four genetically distinct haploid cells.

Key Points to Remember

• Meiosis purpose: Reduce chromosome numbers and enhance genetic diversity through recombination.
• Differences between Meiosis I and II involve chromosomal reduction vs. separation of sister chromatids.
• Genetic disorders arise from errors like nondisjunction, impacting human health.
• Crossing-over is critical for genetic variation, a key benefit of sexual reproduction.

Mitosis vs Meiosis

• Number of Cell Divisions:
  • Mitosis: 1
  • Meiosis: 2
• Number of Daughter Cells:
  • Mitosis: 2
  • Meiosis: 4
• Genetic Identity:
  • Mitosis: Genetically identical to parent.
  • Meiosis: Genetically different due to crossing-over.
• Chromosome Number:
  • Mitosis: Same as parent (diploid, 46).
  • Meiosis: Half of parent (haploid, 23).
• Location:
  • Mitosis: Somatic cells.
  • Meiosis: Gametes (sex cells).
• Timing:
  • Mitosis: Throughout life for growth/repair.
  • Meiosis: During sexual maturity for reproduction.
• Role:
  • Mitosis: Growth, repair, asexual reproduction.
  • Meiosis: Sexual reproduction, genetic diversity.

Common Errors in Meiosis

• Nondisjunction: Failure to separate chromosomes or chromatids, leading to genetic disorders:
  • Down Syndrome: Extra chromosome 21.
  • Klinefelter Syndrome: Additional X chromosome (XXY).
  • Turner Syndrome: Missing X chromosome in females (XO).

Test your understanding of the different phases of the meiotic cell division with this quiz from Module 11. Learn to identify and describe the events that occur during this critical process, which reduces the chromosome number from diploid to haploid. Dive into the specifics of meiosis as you prepare for your studies.