4.7 Regulation of the Cell Cycle

G₁ checkpoint (most important): is the cell big enough, are nutrients and growth factors present, is DNA undamaged? G₂ checkpoint: was DNA replicated correctly? M (spindle) checkpoint: are all chromosomes attached to spindle fibers?

Cells failing a checkpoint pause for repair or undergo apoptosis (programmed cell death).

Cyclin-dependent kinases (CDKs) are always present but only active when bound to cyclins, whose levels rise and fall through the cycle. Cyclin-CDK complexes like MPF push cells past checkpoints.

p53 is a tumor suppressor that halts the cycle when DNA is damaged. Mutated p53 lets damaged cells keep dividing. Proto-oncogenes (gas pedals) become oncogenes when overactive.

Cancer = loss of cell-cycle control: oncogene (accelerator stuck) + broken tumor suppressor (brakes fail).

Worked example: Cyclin levels peak right before mitosis then crash. Why the crash? → Cyclin is degraded so CDK turns off, allowing the cell to exit mitosis.