Making just one of the millions of proteins we manufacture every day involves copying the recipe from DNA, ferrying the building block to a kind of biological knitting machine, threading the blocks into a chain, folding the chain into complex shapes and finally adding various chemical identity tags. At every stage, things can go wrong.
So a second system of quality control – involving enzymes, chaperone cells and scavengers – monitors every step. But the endless checking is expensive. Just proofreading the recipe for proteins can use up 3% of a cell’s energy budget. What we call ageing is what happens when these errors start to pile up. Arteries and skin become less flexible, hair turns grey, and cancerous cells that would normally be destroyed at an early stage are allowed to develop.
But even more damaging is the Jekyll and Hyde nature of oxygen. We need it to turn our food into energy, but as a by-product it gives off poisonous free radicals that zap DNA, proteins and membranes. So our cells have developed an array of antioxidants to mop them up.
But gradually even these defenders start to falter – the DNA in every cell takes an average 10 000 free radical hits a day – and the results also show up as ageing.