The cell cycle has four phases — G1 (growth), S (DNA replication, 2N→4N), G2 (growth/repair check) and M (mitosis) — gated by checkpoints that stop the cycle if DNA is damaged.
G1 → [G1/S checkpoint] → S → G2 → [G2/M checkpoint] → M → [spindle checkpoint] → 2 daughter cells
- DNA damage — simulates a lesion appearing mid-cycle; a working checkpoint should arrest the cycle.
- Checkpoint stringency — probability a checkpoint actually catches damage and arrests the cycle instead of letting it slip through.
- Cycle speed — how fast the cell moves through the four phases.
Real-world relevance: cancer arises largely from cells that keep dividing despite damaged DNA — usually because a checkpoint gene (like p53) is itself mutated and stops catching errors.