The Stiff-Spring Door: How a Cell Remembers Without Changing Its DNA
A door with a stiff spring opens only after a hard push, and then it may stay ajar forever. Epigenetic marks behave much the same way: chemical annotations on DNA that alter gene activity without touching the sequence itself. A cell receives an external signal, say a lack of oxygen, and flips a gene into a new state. When the signal fades, the mark often remains, and the cell "remembers" what it went through.