But no one ever suspected that these genetic circuits might pulse on an off like an electrical circuit. The reason that this behaviour was obscured is that until recently gene expression was seen as the average of the activity of many cells. Now the gene expression in single cells can be detected, this pulsing behaviour is apparent.
Pulsing enables new systems of control just as it does in electronics. Instead of merely being on-off switches, genetic circuits can be modulated by frequency, amplitude or duration to achieve different effects. An important example is the human tumour suppressor gene p53. This responds differently to gamma radiation and UV light for example. Gamma radiation produces p53 activity pulses whose number is controlled by dose, whereas UV produces a sustained non-pulsing response, with amplitude and duration dependent on dose.
Reinforcing the parallels between genetic signalling and electrical circuits, Levine et al in Science write of “a simple ‘AC to DC’ convertor’ in the bacterium Bacillus subtilis.
Joe H. Levine et al, ‘Functional Roles of Pulsing in Genetic Circuits’, Science. 2013, Vol 342, pp. 1193-1200.