A cell does what its DNA tells it to. Put standard genetic parts on a plasmid in the
right order and you get a cell that glows only under the conditions you chose. Then you
test it against every input combination and see whether it really does.
1
A transcription unit is promoter → RBS → coding sequence → terminator.
In that order. No promoter and nothing is transcribed. No RBS and the RNA is made but no
protein comes off it. No terminator and transcription runs on into whatever sits next on the ring.
2
A NOT gate is an input driving a repressor. Arabinose switches
on pBAD, pBAD makes cI, cI shuts off pCI, and whatever pCI was driving goes dark. Every other
gate you build here is made from that one move.
3
The chassis matters. This E. coli carries no lacI
and no tetR of its own, and gets araC from the plasmid backbone. So pLac and pTet run
until you supply LacI or TetR; pBAD stays off until you add arabinose.
4
Promoters leak and cells get tired. Nothing is fully
off, so a long chain of inverters accumulates leak and the output gets muddy. Every unit competes
for the same ribosomes, so a crowded plasmid expresses everything weakly and grows slowly.
Fewer parts is a real result, not a shortcut.
WHAT THIS ISThese are the standard teaching parts — pBAD, pLac, pTet,
lambda cI, GFP, RFP — the ones in every undergraduate parts kit, used here to show how genetic
parts compose into logic. Nothing here describes making anything hazardous. Real work with even
these parts happens in a licensed lab, under institutional biosafety review, with the strain and
the waste stream accounted for.
DRAG turn the plasmid ← → move round the ring ↑ ↓ pick a part ENTER place BACKSPACE clear O origin SPACE run the plate
S submit R restart