Question: how do cells know when to divide ?

Alexis Barr answered on 17 Jun 2013:

The simple answer is when other cells tell them to! Cells can produce chemicals known as Growth Factors. These bind to the surface of cells and send a chemical message into the cell to tell them to divide. When the signal from the Growth Factor is gone, the cells should stop dividing. One of the problem with cancer cells is that they don’t need Growth Factors to divide any more, so they keep dividing even when no Growth Factors are around.