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Bennett (2017): The logic of synthetic biology: turning cells into computers
This is a good introduction to synthetic biology. I will quote the most important passages here:
The instructions necessary to make decisions are coded in DNA, via networks of interacting genes that regulate one another – i.e. one gene can tell another gene to turn on or off depending on internal or environmental cues.
If glucose is present, the glucose network will not only instruct the cell to eat glucose, but also send a signal that turns off the galactose network responsible for galactose metabolism. By turning off the galactose network, the glucose network has “made a decision” for the cell – to eat glucose before galactose.
In recent years, synthetic biologists have been exploring the idea that cells can behave much like computers. If you compare the logical operations performed by transistors and integrated circuits with the decision-making processes of cells, a stunning similarity emerges. In the example of the yeast above, the two sugars glucose and galactose are analogous to the two input wires of the transistor, while the state of the galactose gene network (on or off) is analogous to the current in the output wire. And there are many more examples of this type of decision-making in cells. It is apparent, then, that cells have the highly evolved ability to sense the world around them, make decisions based on those inputs, and respond accordingly. In one sense, cells are fantastic living computers made not out of electric circuits and transistors, but of genes, proteins, and other chemicals. Yet, the analogy between cells and computers, of course, is far from perfect.
The inexact correspondence between cellular “computation” and digital computation has not dissuaded synthetic biologists from attempting to rewire cells into living computers.
Imagine a synthetic microbe that could be injected into the bloodstream of a cancer patient. Once there, the microbe could search for a tumor, and, once it finds it, begins to colonize it. At this point, the physician, if she deems it appropriate, would inject a special mixture of safe chemicals into the patient. If the chemical mixture has just the right signature, a gene network within the synthetic microbe would produce a drug to kill tumor cells. After the synthetic microbes have released their drug, they self activate another gene network that halts their growth.
Cellular computers made out of genetic logic gates will likely follow a similar trajectory as their silicon-based cousins. We certainly have immediate uses and applications for them that scientists and engineers are working on – such as drug delivery systems, ecological sensors, or self-healing industrial coatings. But if the analogy to electronic computers holds, the future applications of synthetic biology can scarcely be imagined. As time goes on, our ability to create more and more complicated genetic circuits will only increase, bringing with it the capacity to develop more and more fantastic synthetic organisms.
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