How does a cell know when to stop dividing?

A cell does not simply divide without limit. When it encounters damaged DNA, an energy shortage, or excessive oxidative stress, it can apply a kind of emergency brake: a permanent halt to division called senescence. This protects the tissue by preventing damaged cells from passing their errors on to daughter cells.

That emergency brake has a downside. In the short term the standstill is useful, but when too many cells remain in this state for a long time they accumulate in tissues. They then secrete signalling molecules that damage surrounding cells. As a result, accumulated senescent cells are associated with diseases such as atherosclerosis, joint degeneration, and cancer. The same pattern is well documented in kidney damage: a temporary halt to division protects the kidney during acute injury, but if that halt lasts too long it actually contributes to permanent kidney disease.

The cell has multiple checkpoints in its division cycle, and each checkpoint can respond to different signals. Some viruses, such as cytomegalovirus, actively force a cell to stop at a specific checkpoint so that the virus can replicate its own genetic material. Replication stress, in which the machinery that copies DNA is damaged by reactive oxygen species, leads to arrest at a different point.

In cancer research this mechanism is used as a point of attack. Experimental compounds, including a derivative of a plant acid and an extract from liquorice, can force cancer cells to stop dividing and subsequently die. Plant-derived alkaloids also block the division of colorectal cancer cells in laboratory research. This has been demonstrated exclusively in cell culture and in animal models; whether it works in humans has not yet been investigated.