cell cycle checkpoints
linked to Cyclins & CDK
- The G1/S Checkpoint (The Restriction Point)
This is the primary decision-making point where the cell commits to dividing or exits to a resting state ((G_{0})). • DNA Integrity: Is the genomic DNA free of damage? (If damaged, proteins like p53 halt the process to initiate repair). • Cell Size: Has the cell grown large enough to support eventual division? • Nutrient Reserves: Are there enough energy resources and molecular building blocks available? • External Signals: Has the cell received external cues (growth factors) from neighboring cells? • Drivers Checked: Successful pairing and activation of Cyclin D + CDK4/6.
- The G2/M Checkpoint
This checkpoint acts as the safety gate ensuring the cell doesn't begin mitosis with corrupted or missing genetic blueprints. • Replication Completeness: Has the entire genome been completely and perfectly copied during the S phase? • DNA Damage Post-Replication: Are there any replication errors or double-strand breaks in the newly synthesized DNA? • Protein Reserves: Are there enough structural proteins (like tubulin) prepared to assemble the cell-splitting machinery? • Drivers Checked: Activation of Cyclin A + CDK1 and the conversion into the Cyclin B + CDK1 complex to form active MPF (Maturation-Promoting Factor).
- The M Checkpoint (Spindle Assembly Checkpoint / SAC)
This checkpoint occurs during Metaphase of the M Phase and regulates the irreversible transition to Anaphase. • Chromosome Alignment: Are all chromosomes perfectly lined up along the central metaphase plate? • Kinetochore Attachment: Is every single chromosome securely attached to spindle microtubules from opposite poles? • Spindle Tension Check: Is proper mechanical tension sensed across the sister chromatids? (This ensures equal chromosomal distribution and prevents conditions like aneuploidy). • Drivers Checked: Downregulation or destruction of Cyclin B by the APC/C complex to initiate the final physical split.
