Cell Cycle Regulation and Checkpoints part 4

Cell Cycle Regulation

  • The cell cycle varies in frequency based on cell type and developmental stage.
  • Dividing cells require signals from other cells.

Internal Regulation

  • Experiments involving cell fusion:
    • Cells at different stages of the cell cycle were fused to investigate internal regulation.
    • Cytoplasmic factors influence the nucleus of the other cell.

Experiment 1: S Phase and G1 Phase Fusion

  • S phase cell (duplicating DNA) fused with a G1 phase cell.
  • The G1 cell immediately started duplicating its DNA due to molecules present in the S phase cytoplasm.

Experiment 2: Mitotic (M) Phase and G1 Phase Fusion

  • M phase cell (condensed DNA, spindle setup) fused with a G1 phase cell.
  • The G1 cell was converted to M phase, condensing its DNA and setting up a mitotic spindle even before DNA duplication.

Conclusion

  • Molecules in the cytoplasm control progression through cell cycle phases.

Regulatory Proteins: Cyclins and Cyclin-Dependent Kinases (CDKs)

  • Cyclins and CDKs regulate the transition between cell cycle phases.
    • G1 to S
    • S to G2
    • G2 to M

Kinases

  • Kinases: Enzymes that phosphorylate other molecules (add a phosphate group).

Cyclin-Dependent Kinases

  • Activity depends on binding to a cyclin.
  • Cyclin + CDK = Activated enzyme.

Cyclins

  • Concentration varies with the cell cycle stages.
  • Different cyclins and CDKs mediate transitions between different stages.

CDK Concentration

  • CDK concentration remains fairly constant throughout the cell cycle.
  • Activity is tied to the presence of specific cyclins.

Maturation Promoting Factor (MPF)

  • Cyclin-CDK pair that helps move cells from G2 into M phase.
    • MPF activity is high from late G2 into M.
    • Cyclin degrades within M phase, allowing the cell to exit and daughter cells to enter G1.

Cyclin and CDK Activity

  • Different cyclin-CDK pairs mediate movement between different cell cycle stages.
  • When a particular cyclin + CDK pair enters a G1 cell's cytoplasm, it signals that the cell is in a different phase, like S or M.

MPF as Cyclin-CDK Complex

  • Triggers a cell's passage through G2 and into M phase.

Activity of MPF

  • Cyclin concentration increases as you transit through G2.
  • Maximal MPF activity occurs when cyclin concentration is maximal.
  • As cyclin degrades, MPF activity decreases.
  • Phosphorylation of proteins, such as those involved in nuclear lamina fragmentation (for nuclear envelope disintegration) and DNA condensation (condensin activation).

Speed of Response

  • Having proteins readily available in the cytosol allows for fast responses.
  • Phosphorylation quickly activates inactive proteins.
  • Cyclin-dependent kinases waiting for cyclin expression allows the cell to transition quickly through cell cycle phases.

Regulation and Checkpoints

G1 Checkpoint

  • Regulated by:
    • Nutrient Availability.
    • Signals from other cells (mitogens).
    • Cell Size.
  • Absence of adequate nutrients or signals blocks cell cycle progression.
  • DNA Damage: Another reason for G1 arrest. Prevents replication of damaged DNA.

G2 Checkpoint

  • Cell size must be adequate.
  • Successful chromosome replication in S phase is crucial.
  • Ensures two copies of each chromosome have been made correctly.
  • Provides another chance to stop and fix damage.

Metaphase Checkpoint

  • Ensures all chromosomes are attached to the spindle.
  • Prevents premature separation of chromosomes.

Purpose of Mitosis

  • Mitosis and cytokinesis should result in identical cells.
    • Identical to each other.
    • Identical to the parent cell.

Internal and External Controls

  • Internal controls: DNA damage.
  • External controls: Mitogens (cytokines).

The "Go-Ahead" Signal

  • Mitogens or growth factors are the "go-ahead" signal.
  • Their absence leads to cell cycle arrest.
  • Cells arrest until they receive the proper signals for a cell to proceed down the cell cycle.

M Phase Checkpoint

  • Ensures all chromosomes are aligned at the metaphase plate.
  • The block is removed when all chromosomes are captured, allowing progression into anaphase.

Experiment with Fibroblasts

  • Human tissue cut up and treated with enzymes to create a cell suspension (fibroblasts).
  • Without platelet-derived growth factor (a cell signaling molecule), cells don't divide.
  • Adding the growth factor stimulates cell division.

Cancer Cells and Cell Cycle Control

  • Cancer cells may not adhere to normal cell cycle signals.

Aneuploidy

  • Tumor cells often have abnormal karyotypes.
  • Aneuploidy: Not the true number of chromosomes; having too many or too few of certain chromosomes.