A door with a stiff spring opens only after a hard shove, 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 a signal from outside, say a shortage of oxygen, and flips a gene into a new state. When the signal fades, the mark often stays put, and the cell "remembers" what it went through.
Until now, researchers described such switches by hand, fitting equations to the data. A new paper offers a general principle that derives the thresholds automatically. The idea comes from materials mechanics, where scientists have long known how to describe systems that respond only to the size of a load, not to how fast it is applied. The model is built from three ingredients: a set of possible cell states, the energy stored in each state, and a measure of resistance to change. Require the cell to pick, at every moment, the state with the lowest total cost, and the equations of motion follow.
The payoff is not abstract. The framework can be applied to real biological data, for instance to hypoxia experiments, where cells respond to falling oxygen levels. Activation thresholds that used to be tuned by eye are now computed from the model. That gives a basis for designing therapies aimed at epigenetic marks: to change a cell's fate, you need to know how hard and how long to push on the right "door."
In the bistable model the transition between the two wells happens as a jump at a loading equal to the dissipation threshold, regardless of the barrier height; this is an analogue of Maxwell's rule in phase transitions.
The idea of an epigenetic landscape echoes Conrad Waddington's "Epigenetic Landscape," often visualized as hilly terrain down which a ball rolls, standing for a cell.