Cell Cycle and Cell Division Lecture

Introduction

• Definition of Cell Cycle: Self-regulated sequence of events for cell growth and division into two daughter cells.
• Continuous process: Cells divide and daughter cells begin the cycle again.

Major Phases of the Cell Cycle

Interphase

• Definition: Period between two successive cell divisions.
• Function: Cell growth and replication of genetic material.
• Sub-phases of Interphase:
  • G1 Phase (Gap 1):
    • Occurs immediately after cell division.
    • Cell growth and nutrient accumulation.
    • Preparation of DNA replication machinery.
    • Critical for successful cell division; defects can lead to cancer.
  • S Phase (Synthesis):
    • DNA replication occurs, doubling genetic material.
    • Formation of chromatids for each chromosome.
    • Checkpoints for DNA integrity and repairs.
  • G2 Phase (Gap 2):
    • Follows DNA replication.
    • Continued cell growth and organelle reorganization.
    • Centrioles formation crucial for cell division.

M Phase (Mitotic Phase)

• Definition: Period of actual cell division.
• Includes nuclear and cytoplasmic division.
• Stages of Mitosis:
  • Prophase:
    • Condensation of genetic material into visible chromosomes.
    • Disintegration of nuclear envelope.
    • Movement of microtubules and centrioles.
  • Metaphase:
    • Chromosomes align at the cell equator.
    • Ideal for karyotyping.
  • Anaphase:
    • Separation of sister chromatids to opposite poles.
  • Telophase:
    • Cytoplasmic division.
    • Reconstitution of nuclear envelope.

Importance of Mitosis

• Increases cell number rapidly (e.g., blood cells).
• Growth and development of organisms and organs.
• Repair and replacement of worn-out tissues.
• Maintains genetic consistency—daughter cells identical to parent cell.

Meiosis

• Occurs only in germ cells (sperm and oocytes).
• Involves two phases: Meiosis I and Meiosis II.
  • Meiosis I: Separation of homologous chromosomes, reducing chromosome number to haploid (23 chromosomes).
  • Meiosis II: Similar to mitosis but with haploid cells.
• Purpose:
  • Achieves haploidy for fertilization, restoring diploid state in offspring.
  • Genetic variation through crossing over.

Cells in G0 Phase

• Description: Cells that have exited the cycle, lost division capacity.
• Termed as "quiescent" stage or terminal differentiation.
• Examples: Mature red blood cells, skeletal muscle cells, most neurons, many cardiac muscle cells, osteocytes.

Conclusion

• The lecture covered the cell cycle's role in growth, division, and maintaining genetic integrity.
• Importance of mitosis and meiosis in cell proliferation, organism development, and genetic consistency and variation.