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Understanding the Cell Cycle and Its Stages

May 20, 2025

The Cell Cycle Lecture Notes

Introduction

  • The cell cycle is crucial for cell replication and growth.
  • Interphase and mitosis are key stages.
  • Future discussions will cover cell cycle regulation, proto-oncogenes, tumor suppressor genes, and DNA repair enzymes.

What is a Cell?

  • The basic unit of all living things.
  • Eukaryotic (human) cells consist of:
    • Cell Membrane: A phospholipid bilayer enclosing the cell.
    • Nucleus: Contains genetic material (DNA in chromatin form).
    • Cytoplasm: The fluid and organelles within a cell.

The Cell Cycle

Interphase

  1. G1 Phase (Gap 1):

    • Cell grows, synthesizes proteins and enzymes, and increases organelle count.
    • DNA damage repair occurs here.
    • Majority of cells reside in this phase.

  2. S Phase (Synthesis):

    • DNA replication occurs.
    • Cells go from 2n (46 chromosomes) to 4n (92 chromosomes).
    • Typically lasts about 6 hours.

  3. G2 Phase (Gap 2):

    • Cell continues to grow and prepares for mitosis.
    • Increase in cytoplasm volume is crucial.

Checkpoints

  • G1/S Checkpoint: Ensures DNA is undamaged and conditions are favorable for DNA synthesis.
  • G2/M Checkpoint: Verifies DNA replication accuracy.
  • M Checkpoint: Ensures chromosomes are properly aligned before separation.

Mitosis (M Phase)

  1. Prophase:

    • Chromatin condenses into chromosomes.
    • Nuclear envelope dissolves.
    • Microtubule organizing centers and centrioles appear.

  2. Metaphase:

    • Chromosomes align on the metaphase plate.
    • Microtubules attach to kinetochores on chromosomes.

  3. Anaphase:

    • Sister chromatids separate and move to opposite poles.
    • Involves motor proteins (dynein and kinesin).

  4. Telophase:

    • Chromatids reach poles.
    • Nuclear envelopes reform around each set of chromosomes.
    • Chromosomes begin to decondense into chromatin.

  5. Cytokinesis:

    • Cytoplasm divides, leading to the formation of two identical daughter cells.

Types of Cells Based on Cell Cycle Activity

  • Labile Cells: Continuously dividing (e.g., skin epithelium, GI tract, hematopoietic stem cells).
  • Stable Cells: Divide only when stimulated (e.g., liver cells, kidney tubule epithelium).
  • Permanent Cells: Do not divide after maturity (e.g., neurons, cardiac muscle).

Additional Concepts

  • G0 Phase (Quiescent Phase):
    • Cells that exit the cycle and enter a resting state.
    • Can re-enter the cycle if stimulated.
  • Telomere Shortening:
    • Leads to cellular aging and limits division potential.
  • Cell Senescence:
    • Irreversible cell cycle exit due to damage or telomere shortening.

Conclusion

  • Understanding the cell cycle is key for insights into growth and genetic stability.
  • Various checkpoints ensure fidelity and proper progression through the cycle.

These notes summarize the key points from the lecture on the cell cycle, providing an overview of cellular division processes and the roles of different phases and checkpoints.

Full transcript