Regulation of the Cell Cycle

Key Checkpoints

1. G1 to S Phase
   • Regulation before DNA replication.
2. G2 to Mitosis
   • Regulation before mitosis.

Regulatory Proteins

1. Cyclin-Dependent Kinases (CDKs)

   • Adds a phosphate group to other enzymes/proteins.
   • Activates or inactivates the target protein.
   • CDKs are always present but inactive without cyclins.

2. Cyclins

   • Activate CDKs by binding to them.
   • Specific cyclins are produced at specific times.

Role of CDK-Cyclin Complexes

• G1 Phase

  • Cyclin D and Cyclin E are produced.
  • CDK-2 binds to Cyclin E.
  • CDK-4 binds to Cyclin D.
  • CDK-4 Cyclin D complex phosphorylates RB protein, inactivating it and promoting DNA replication.

• S Phase

  • Cyclin A is produced.
  • Cyclin A complexes with CDK-2, activating DNA replication.

• G2 Phase

  • Cyclin B is produced.
  • Cyclin B complexes with CDK-1, promoting mitosis (cell division).

Summary

• Cyclin proteins are crucial for moving past cell cycle checkpoints.
• CDK-cyclin complexes either inhibit blocking proteins or promote necessary proteins for DNA synthesis or mitosis.