Cell Cycle Checkpoints, Cyclins, and Cdks

A dividing cell can't afford to rush. If it copies damaged DNA or splits before chromosomes line up correctly, the daughter cells inherit the mistake. So the cycle has three checkpoints, in G1, G2, and M, each asking a different question: is the cell big enough and are conditions favorable (G1), did DNA replication finish without damage (G2), and are all chromosomes properly attached to spindle fibers (M, the spindle assembly checkpoint)? Passing each one is conditional, not automatic.

The timing behind this comes from cyclins, proteins whose concentration rises and falls in a predictable rhythm across the cycle. On their own, cyclins do nothing. They have to bind a Cdk, a cyclin-dependent kinase, which is always present in the cell but inactive without its cyclin partner. When a specific cyclin accumulates enough to bind its Cdk, the resulting cyclin-Cdk complex becomes an active enzyme that phosphorylates other proteins, pushing the cell past that checkpoint and into the next phase. Once its job is done, the cyclin is broken down, the Cdk falls idle again, and a different cyclin starts building for the next checkpoint. That rise-and-crash pattern is what makes cyclin levels act like a built-in clock.

At G2, the checkpoint proteins specifically scan the DNA copied during S phase. If damage turns up, the cycle halts before mitosis starts, giving repair enzymes time to fix the errors. If the damage is too severe to repair, the cell can be directed toward apoptosis instead of dividing with a broken genome. This same checkpoint logic connects forward to mitosis itself, where the M checkpoint hands off control to the machinery that actually separates chromosomes.