Molecular Genetics Practice Exam

The concept being tested is bistability created by mutual repression. A synthetic toggle switch uses two transcriptional repressors that inhibit each other’s expression. This reciprocal repression forms two stable states: one where the first gene is on and the second is off, and another where the second gene is on and the first is off. Because the system has two stable equilibria, it can “sit” in either state until a sufficiently strong input shifts it to the other, effectively toggling between states. This architecture relies on strong, cooperative repression to create distinct, enduring states. If two genes were simply mutually activating, you’d tend toward runaway expression rather than stable toggling. An oscillator usually needs a time delay and a particular feedback arrangement to produce periodic cycling, not just two stable states. And a circuit that permanently expresses a single gene doesn’t provide a switch between alternative states. So the description with mutual repression leading to two stable expression patterns best captures a toggle switch.