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Understanding Cell Cycle and Division

May 22, 2025

The Cell Cycle, Mitosis, and Meiosis

Learning Objective

  • Understand the life cycle of a cell and the processes of genetic material duplication and partitioning to produce two daughter cells.

The Cell Cycle

  • Phases of Cell Cycle:
    • G1 Phase:
      • Metabolic preparation for cell division.
      • Restriction point indicates commitment to division.
    • S Phase:
      • DNA synthesis and replication.
      • Chromosomes consist of two sister chromatids.
    • G2 Phase:
      • Metabolic changes assemble cytoplasmic materials for mitosis and cytokinesis.
    • M Phase:
      • Mitosis (nuclear division) and cytokinesis (cell division).
  • Interphase:
    • Period between mitotic divisions (G1, S, G2).

Mitosis

  • Function:
    • Eukaryotic cell division producing two genetically identical daughter cells.
  • Process Overview:
    • Chromosome replication during S phase.
    • Mitotic apparatus aligns and separates sister chromatids.
    • Karyokinesis (nuclear division) followed by cytokinesis (cytoplasm division).
  • Role in Organisms:
    • Asexual reproduction in single-celled organisms.
    • Growth and development from zygote in multicellular organisms.
    • Cell replacement, wound healing, and tumor formation in adults.
  • Stages of Mitosis:
    • Prophase:
      • Nuclear membrane breaks down, centrosomes duplicate and form spindle fibers.
      • Chromosomes condense into sister chromatids.
    • Prometaphase:
      • Chromosomes migrate to metaphase plate.
      • Spindle fibers attach to kinetochores on centromeres.
    • Metaphase:
      • Chromosomes align along metaphase plate.
    • Anaphase:
      • Centromeres divide, sister chromatids (now daughter chromosomes) move to cell ends.
    • Telophase:
      • Nuclear membrane reforms, chromosomes uncoil, spindle fibers disappear.
    • Cytokinesis:
      • Final division into two cells; formation of cell plate in plants or constriction in animals.

Meiosis

  • Function:
    • Produces haploid gametes from diploid cells through one DNA replication and two cell divisions.
  • Phases of Meiosis I:
    • Prophase I:
      • Homologous chromosomes pair and exchange DNA (recombination).
      • Divided into leptotene, zygotene (synapsis), pachytene (crossing over), diplotene, and diakinesis.
    • Prometaphase I:
      • Spindle apparatus forms; chromosomes attach via kinetochores.
    • Metaphase I:
      • Random arrangement of homologous pairs on metaphase plate (source of genetic variation).
    • Anaphase I:
      • Homologous chromosomes separate to opposite cell poles.
    • Telophase I:
      • Chromosomes diffuse, nuclear membrane reforms.
    • Cytokinesis:
      • Formation of two new cells; reduction division (diploid to haploid).
  • Meiosis II:
    • Similar to mitosis; separates sister chromatids into individual chromosomes.
    • Generates genetic diversity through recombination, random chromosome alignment in Meiosis I, and random chromatid alignment in Meiosis II.

Additional Notes

  • Meiosis in females involves special considerations (not detailed in this transcript).
  • Creative Commons License applied to the material.