Cell Cycle Lecture Notes

Introduction to the Cell Cycle

• Importance: Critical for cell replication and controlling cell growth.
• Phases: Interphase and Mitosis.
• Future Topics: Regulation of the cell cycle, proto-oncogenes, tumor suppressor genes, and DNA repair enzymes.

Basic Structure of a Cell

• Three Essential Components:
  • Cell Membrane: Phospholipid bilayer surrounding the cell.
  • Nucleus: Houses genetic material (DNA wrapped around histone proteins).
  • Cytoplasm: Fluid within the cell containing organelles.

Interphase

G1 Phase (Gap 1 Phase)

• Purpose: Prepare for replication; primary growth phase.
• Key Activities:
  • Increase number of organelles (e.g., ribosomes, mitochondria).
  • Synthesize proteins and enzymes for DNA replication.
  • Repair DNA damage (e.g., thymidine dimers).
• Cell Types:
  • Labile Cells: Constantly proliferating (e.g., skin, GI tract, hematopoietic stem cells).
  • Stable Cells: Replicate with strong stimulus (e.g., liver, kidney tubules).
  • Permanent Cells: Do not undergo mitosis (e.g., neurons, cardiac muscle).

S Phase (Synthesis Phase)

• Activity: DNA replication; ensures each new cell has a complete set of chromosomes.
• Enzymes: DNA polymerases (type 1 and 3).
• Duration: Approximately 6 hours.

G2 Phase (Gap 2 Phase)

• Focus: Cell growth; prepares for mitosis.
• Key Activity: Increase cell size by expanding cytoplasm.

Mitosis (M Phase)

• Phases: Prophase, Metaphase, Anaphase, Telophase, followed by Cytokinesis.

Prophase

• Events:
  • Chromatin condenses into chromosomes.
  • Nuclear envelope dissolves.
  • Microtubule organization centers form (centrioles).

Metaphase

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

Anaphase

• Events:
  • Sister chromatids are pulled apart to opposite poles.
  • Motor proteins (dynein and kinesin) involved in movement.

Telophase & Cytokinesis

• Events:
  • Nuclear envelope reforms around chromosomes at each pole.
  • Chromosomes begin to de-condense.
  • Cell divides by forming a cleavage furrow (constriction ring).
  • Cytokinesis completes the division into two cells.

Additional Concepts

• G0 Phase (Quiescent Phase): Cells rest and do not actively divide.
• Cell Senescence: Irreversible exit from cell cycle due to telomere shortening with age.

Checkpoints in the Cell Cycle

• G1/S Checkpoint: Verifies DNA integrity before synthesis.
• G2/M Checkpoint: Ensures error-free DNA replication before mitosis.
• Metaphase Checkpoint: Confirms proper alignment and attachment of chromosomes before anaphase.

Summary

• Interphase: Comprised of G1, S, and G2 phases.
• Mitosis: Divides replicated DNA into two cells.
• Cell Cycle Regulation: Controlled by checkpoints to ensure accuracy and prevent errors.

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Final Remarks

• Understanding the cell cycle is fundamental for comprehending cell growth, repair, and reproduction.
• Importance of accurate replication and division for maintaining healthy organismal function.

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