Life (as we know it on earth today) only comes from preexisting life; we humans always start as fertilised eggs, which come from our parents, who came to be in just the same way; and so on back to the dawn of time. Yet, we all are mortal, and the continuous thread running through this succession is DNA, a self-replicating molecule that stores all the instructions, or perhaps contains all the ingredients, for making a new organism. The DNA molecules, in the form of chromosomes, are stored in cells, and animals and plants are built by cell division followed by cell specialisation. This means that the mechanism and control of cell division is one of the most fundamental aspects of biology. It also, of course, can go wrong with disastrous results, mostly in old age. Neither degeneration nor cancer are welcome. Nobel Laureate in Physiology or Medicine Sir Tim Hunt discusses what we know and what we would like to know.