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Overview of the Cell Cycle Stages

May 5, 2025

Lecture Notes: The Cell Cycle

Importance of the Cell Cycle

  • The cell cycle is crucial for cell replication and controlling cell growth.
  • Future lectures will cover regulation, including:
    • Proto-oncogenes
    • Tumor suppressor genes
    • DNA repair enzymes and genes

Basics of a Cell (Eukaryotic)

  • Cell Membrane: Phospholipid bilayer surrounding the cell.
  • Nucleus: Houses genetic material (DNA wrapped around histone proteins forming chromatin).
  • Cytoplasm: Contains organelles and the fluidic environment of the cell.

Interphase

G1 Phase (Gap 1)

  • Cell prepares to replicate by:
    • Increasing organelles (e.g., ribosomes, mitochondria).
    • Synthesizing proteins and enzymes necessary for DNA replication.
    • Repairing DNA damage (e.g., thymidine dimers) to ensure no mistakes are replicated.
  • Most cells exist in the G1 phase for variable durations.

S Phase (Synthesis)

  • DNA replication occurs.
  • DNA polymerases (type 1 and 3) ensure faithful replication.
  • The phase lasts approximately 6 hours.
  • Post-replication, cells transition from 2n to 4n.

G2 Phase (Gap 2)

  • Focuses on cell growth, ensuring sufficient cytoplasm and organelles for two daughter cells.

Types of Cells Based on Replication

  • Labile Cells: Constantly replicating (e.g., skin epithelium, GI tract, hematopoietic stem cells).
  • Stable Cells: Replicate when stimulated (e.g., hepatocytes, kidney tubules, alveolar cells).
  • Permanent Cells: Do not replicate post-differentiation (e.g., neurons, skeletal and cardiac muscle).

Mitosis

P.M.A.T Stages

  • Prophase: Chromatin condenses into chromosomes, nuclear envelope dissolves, centrioles form.
  • Metaphase: Chromosomes align at the metaphase plate, microtubules attach to kinetochores.
  • Anaphase: Sister chromatids are pulled apart to opposite poles by motor proteins (dynein and kinesin).
  • Telophase: Chromosomes de-condense, nuclear envelope reforms, cleavage furrow forms leading to cytokinesis.

Cytokinesis

  • Division of cytoplasm to form two distinct daughter cells, marked by the formation of the cleavage furrow.

The Cell Cycle Checkpoints

  • G1/S Checkpoint: Ensures DNA is ready for replication.
  • G2/M Checkpoint: Verifies successful DNA replication without damage.
  • M Checkpoint: Ensures accurate chromosome alignment and separation during mitosis.

Additional Notes

  • Cells may enter a quiescent G0 phase, where they rest and do not actively cycle.
  • Telomere shortening over time can lead to cellular senescence (irreversible cessation of division).

End of Lecture Summary

This summary captures the key points of the cell cycle and associated processes as discussed in the lecture. Ensure to review the stages of interphase and mitosis for a comprehensive understanding of cell replication.

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