4.7 - Regulation of the Cell Cycle

Importance of Cell Cycle Regulation

Proper timing and rate of cell division are essential for normal growth, development, and maintenance of tissues. The cell cycle is regulated by a system of internal controls and checkpoints that ensure each stage is completed accurately before moving to the next phase.

Key Components in Cell Cycle Regulation

• Cyclins and Cyclin-Dependent Kinases (CDKs):

  • Cyclins: Proteins whose levels fluctuate cyclically throughout the cell cycle.

  • CDKs: Enzymes that, when activated by binding to cyclins, can phosphorylate other proteins to drive the cell cycle forward.

  • Function: Cyclins and CDKs work together to transduce signals that either promote or halt cell cycle progression at various checkpoints.

• Cell Cycle Control System:

  • Mechanism: A set of molecules that operate cyclically to trigger and coordinate critical events in the cell cycle.

  • Checkpoints: Specific points where stop and go-ahead signals can regulate the cycle, ensuring each phase is properly completed before proceeding.

Major Checkpoints in the Cell Cycle

1. G1 Checkpoint (Towards the end of G1 phase):

   • Criteria:

     • Adequate supply of energy and raw materials.

     • Enough space in the environment for more cells.

   • Decision Point: Determines whether the cell will proceed with the cell cycle or enter the G0 phase (a nondividing state).

2. G2 Checkpoint (Towards the end of G2 phase):

   • Criteria:

     • Complete replication of DNA.

     • DNA checked for errors and repaired if necessary.

   • Function: Ensures the cell is ready to enter mitosis.

3. M Checkpoint (Between metaphase and anaphase):

   • Criteria:

     • All chromosomes must be properly attached to the spindle apparatus.

   • Function: Ensures that sister chromatids will separate correctly.

Cancer and the Cell Cycle

• Unregulated Cell Cycling:

  • Cancer Cells: Characterized by uncontrolled and continuous cell division due to the failure of cell cycle regulation mechanisms.

  • Growth Factors: Unlike normal cells, cancer cells do not require external growth factors to divide.

• Avoidance of Apoptosis:

  • Mechanism: Cancer cells evade programmed cell death (apoptosis), which normally eliminates cells with severe DNA damage or errors.

  • Consequences: This evasion allows cancer cells to survive and proliferate despite significant genetic abnormalities.

• Metastasis:

  • Definition: The process by which cancer cells spread from the original tumor site to distant locations in the body.

  • Types of Tumors:

    • Malignant Tumors: Capable of metastasis, invading new tissues and forming secondary tumors.

    • Benign Tumors: Remain localized at the original site and do not spread.