Synthetic Circuits: Switches, Oscillators and Why They Misbehave in Cells · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.

Circuits that meet the living cell

Science · Microbiology · ages 23-24
Name ______________________   Date ____________

  1. What sets the switching threshold of a toggle?

    • The size of the culture flask
    • Promoter strength, repressor levels, and binding affinity
    • The color of the fluorescent reporter

  2. What makes a two repressor toggle bistable?

    • Two repressors blocking each other, so one side wins and stays on
    • One repressor working alone at high speed
    • Two activators firing at fixed intervals

  3. Attaching a downstream module can change the behavior of the module driving it.

    Circle one:   True   False

  4. A circuit works on day one and fades after many generations of growth. What is the most likely cause?

    • The threshold was calculated with the wrong units
    • Mutations breaking circuit genes, with freeloaders outgrowing producers
    • The cells learned to ignore all promoters

  5. You attach a downstream reporter and the driver signal sags. What happened?

    • The reporter mutated within minutes
    • The inducer evaporated from the flask
    • The downstream part consumed the shared signal, dragging the driver down

  6. Which wiring holds states and which wiring oscillates?

    • Positive feedback holds states, negative feedback oscillates
    • Negative feedback holds states, positive feedback oscillates
    • Both wirings always oscillate

  7. Your toggle flips at a tiny unwanted signal. Which change raises its switching threshold?

    • Strengthen the promoter driving the repressor
    • Delete one of the two repressor genes
    • Dilute the culture with fresh medium

  8. A student blames a faded 50 generation culture on the toggle threshold drifting, while non producers dominate the flask. What is the likelier cause?

    • Threshold drift in every cell at once
    • Mutation plus selection: broken freeloaders outgrew the producers
    • Retroactivity from a reporter added years ago
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Answer key

For grown-ups. Fold this page away before handing over the rest.

Circuits that meet the living cell W1-mt_xRJGSHidqo-s1

  1. Promoter strength, repressor levels, and binding affinity · Those three quantities decide how much signal it takes to flip the winner.
  2. Two repressors blocking each other, so one side wins and stays on · Mutual repression creates two stable states with one winner each.
  3. True · The downstream load consumes the shared signal and drags the driver down.
  4. Mutations breaking circuit genes, with freeloaders outgrowing producers · Broken circuits shed their burden, and selection favors the cells that dropped it.
  5. The downstream part consumed the shared signal, dragging the driver down · That sag is load retroactivity: the new load drinks from the same signal pool.
  6. Positive feedback holds states, negative feedback oscillates · Self reinforcement locks a winner in, while self negation keeps flipping the state.
  7. Strengthen the promoter driving the repressor · A stronger promoter makes more repressor, so flipping the winner takes a larger signal.
  8. Mutation plus selection: broken freeloaders outgrew the producers · Dominance by non producers after many generations is the signature of evolutionary loss, not threshold drift.
Worksheet · LightMySky