Cell Cycle Regulation

Overview

• The cell cycle is tightly regulated.
• Regulation is crucial at two main checkpoints:
  1. G1 to S Phase: Before DNA replication.
  2. G2 to Mitosis.

Key Proteins Involved

Cyclin-Dependent Kinases (CDKs)

• CDKs are kinases: enzymes that add phosphate groups (+ phosphate) to proteins, which can activate or inactivate them.
• CDKs work in conjunction with cyclins.
• CDKs are always present in the cell in an inactive form.
• Activation requires binding with specific cyclins.

Cyclins

• Cyclins are synthesized at specific times during the cell cycle.
• The presence of specific cyclins is crucial for checkpoint regulation.

Checkpoint Mechanisms

G1 Phase

• Cyclins D and E are produced.
• CDK-2 binds to cyclin E.
• CDK-4 binds to cyclin D.
• The CDK-4/cyclin D complex phosphorylates the RB protein, inactivating its ability to inhibit DNA replication.

S Phase

• Cyclin A is produced.
• Cyclin A forms a complex with CDK-2, aiding in DNA replication activation.

G2 Phase

• Cyclin B is synthesized.
• Cyclin B complexes with CDK-1, initiating mitosis or cell division.

Importance of Cyclins and CDKs

• Cyclins need to be present to:
  • Inhibit proteins that block DNA synthesis.
  • Promote the production of proteins necessary for mitosis.

Conclusion: Understanding the regulation of the cell cycle is essential for studying cellular processes and their implications in areas like cancer research, where cell cycle dysregulation can occur.