What exactly tells (makes) a cell divide?

Question

I know about the stages of cell division but can't figure out what mechanism starts the process.

Answer 1

In a cell, there is a chemical called cyclin (the name should give you a big hint). Think of cell division as a big clock - at certain times, there is a little blocker that stops if it says no-go, or lets the hand by when it says go. A cell has these little blockers called checkpoints. At each checkpoint, if a cell is not ready to proceed to the next phase, then it will not. However, once the checkpoint is passed, then the next phase(s) will proceed.

The way checkpoints are passed is Cyclin and CDK (Cyclin Dependent Kinases). The levels of Cyclin and CDk's indicate when the cell should pass the next checkpoint.