Lecture Notes: The Cell Cycle

Introduction

• Importance of small molecules and hormones as messengers in cellular functions.
• Signals often regulate the cell cycle, the series of stages for cell division.
• Cell cycle is crucial for proliferation and replacement of cells.

DNA Arrangement and Replication

• Eukaryotic cells: multiple linear DNA molecules (chromosomes). Humans have 46 chromosomes.
• Chromosomes consist of DNA wrapped around histones, forming nucleosomes.
• DNA replication results in two identical sister chromatids, attached by a centromere.
• During cell division, sister chromatids separate into daughter cells.

Cell Cycle Phases

1. Interphase
   • G1 Phase (First Gap): Cell growth; some cells stay here long (or in G Zero).
   • S Phase (Synthesis): DNA replication occurs, taking 10-12 hours in humans.
   • G2 Phase (Second Gap): Cell growth and preparation for mitosis, taking 4-6 hours.
2. M Phase (Mitotic Phase): Cell divides into two daughter cells, taking about 1 hour.

Regulation of the Cell Cycle

• Cell Cycle Control System:
  • Regulated by signaling molecules in the cytoplasm.
  • Includes checkpoints that require specific signals to proceed.
  • Key checkpoints: during S phase, G1 phase, end of G2 phase, and during M phase.
• Key Regulators:
  • Protein Kinases: Enzymes that activate/deactivate proteins via phosphorylation.
  • Cyclins: Proteins that vary in concentration; bind to kinases to activate them.
  • Cyclin-dependent Kinases (CDKs): Kinases activated by cyclins.
  • MPF Complexes: Formed by cyclins and kinases, passing the G2 checkpoint.

Checkpoints

• G1 Checkpoint (Restriction Point): Determines if a cell will divide; absence of a signal leads to G Zero phase.
• M Phase Checkpoint: Governs separation of sister chromatids.

Importance of Regulation

• Regulation ensures appropriate cell division rates.
• Density-dependent Inhibition: Cells stop dividing when they fill a container.
• Surface proteins and receptors inhibit division when space is filled.

Cancer and the Cell Cycle

• Cancer: Uncontrolled cell division forming tumors.
• Cancer cells do not respond to normal regulatory signals.
• Caused by genetic mutations affecting cell cycle regulation.
• Cancer treatment relies on understanding cell cycle regulation.

Conclusion

• Understanding cell division and regulation is fundamental for addressing cancer and ensuring healthy cellular functions.