What Must Happen Before a Cell Can Begin Mitosis
The transition from interphase to mitosis is one of the most tightly regulated events in cellular life. For a cell to safely divide its nucleus and subsequently its cytoplasm, it must complete a series of precise checks and molecular preparations. These requirements make sure genetic material is duplicated accurately, any damage is repaired, and the cellular environment supports the energetic and structural demands of division. Understanding what must happen before a cell can begin mitosis reveals the elegance of biological control systems and the consequences when they malfunction Simple as that..
The cell cycle is traditionally divided into four main phases: G1, S, G2, and M (mitosis). Consider this: before a cell can even consider entering mitosis, it must pass through a series of control points known as checkpoints. While mitosis itself occupies a relatively short window, the preceding phases—particularly G1 and G2—serve as critical preparation periods. These checkpoints function as molecular "quality control" stations, evaluating both internal and external conditions before allowing progression And that's really what it comes down to..
The first major requirement occurs at the G1 checkpoint, often called the restriction point in mammalian cells. Practically speaking, at this stage, the cell assesses its size, nutrient availability, growth factor signals, and DNA integrity. If conditions are favorable and the cell has reached an adequate size, it commits to entering the S phase. This commitment is irreversible under normal conditions and is mediated by the activation of cyclin-dependent kinases (CDKs) bound to regulatory cyclins. Without passing this threshold, the cell may exit the cycle into a quiescent state called G0, where it remains metabolically active but does not prepare for division.
Not the most exciting part, but easily the most useful.
Following G1, the cell enters S phase, during which DNA replication occurs. In practice, this phase is fundamental because mitosis separates sister chromatids, which are exact copies produced during DNA synthesis. Still, before progressing further, the cell must confirm that its genome has been fully and accurately replicated. Replication origins fire in a coordinated manner, and licensing factors such as CDC6 and CDT1 confirm that each segment of DNA is copied exactly once per cycle. Any failure to complete replication or the presence of stalled replication forks will trigger checkpoint pathways that halt cycle progression until the issues are resolved.
Counterintuitive, but true.
Once S phase concludes, the cell enters G2, a gap period dedicated to preparation for mitosis. The G2 checkpoint is perhaps the most directly relevant to mitosis entry. Here, proteins such as ATM and ATR detect DNA double-strand breaks or replication stress. During G2, the cell continues to grow, synthesizes proteins required for chromosome condensation and spindle formation, and performs a final assessment of the replicated DNA. If damage is detected, signaling cascades lead to the inhibition of CDK1/cyclin B complexes, preventing entry into mitosis until repairs are complete. This safeguard prevents the segregation of damaged DNA, which could lead to mutations or cell death.
A central molecular player in mitosis initiation is the Maturation-Promoting Factor (MPF), a complex of CDK1 and cyclin B. In real terms, the activity of MPF is tightly controlled by phosphorylation and dephosphorylation events. In early G2, MPF remains inactive due to inhibitory phosphates added by kinases such as WEE1.
