Rucete ✏ Campbell Biology In a Nutshell
Unit 2 THE CELL — Concept 12.3 The Eukaryotic Cell Cycle Is Regulated by a Molecular Control System
The progression of the eukaryotic cell cycle is controlled by a built-in molecular system that ensures orderly and accurate cell division, responding to both internal conditions and external signals.
Cell Cycle Control System
A cyclically operating set of molecules triggers and coordinates key events in the cell cycle
Comparable to a washing machine timer: progresses on its own but can pause at checkpoints
Major checkpoints occur at G₁, G₂, and M phases
Checkpoints ensure key processes (e.g., DNA replication, chromosome alignment) are properly completed before proceeding
Cyclins and Cyclin-Dependent Kinases (Cdks)
Protein kinases and cyclins regulate the timing of cell cycle events
Cdks are active only when bound to cyclins
MPF (M-phase promoting factor): a cyclin-Cdk complex that drives the cell past the G₂ checkpoint into mitosis
MPF also triggers nuclear envelope breakdown, chromosome condensation, and spindle formation
MPF deactivates itself by degrading its cyclin; Cdk remains inactive until new cyclin accumulates
Internal and External Signals
Internal signals ensure chromosomes are properly attached before moving to anaphase
At the M checkpoint, unattached kinetochores delay separation of sister chromatids
G₁ checkpoint is most important—if no signal is received, the cell enters a non-dividing G₀ phase
External signals include nutrients and growth factors like PDGF (stimulates fibroblast division)
Density-dependent inhibition and anchorage dependence limit normal cell division
Cancer Cells and Cell Cycle Failure
Cancer cells ignore normal control mechanisms
They divide without growth factors and do not exhibit density-dependent inhibition or anchorage dependence
Can divide indefinitely (e.g., HeLa cells) and evade apoptosis
Form tumors:
Benign tumors stay localized
Malignant tumors invade tissues and can metastasize via blood or lymph
Chemotherapy targets actively dividing cells but affects normal dividing cells too
Personalized cancer treatments now target specific signaling pathways or mutated proteins (e.g., HER2, estrogen receptor)
In a Nutshell
A molecular control system using cyclins and Cdks regulates the cell cycle, with internal checkpoints and external signals guiding progression. Disruption of these controls can lead to unchecked division and cancer, but understanding these mechanisms opens the door for targeted therapies.