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.