Semisynthetic Organism: Unnatural Base Pair Genetic Firewall

Feed & Transporter

External dNaM-TP/dTPT3-TP feed 70%

PtNTT2 transporter expression 70%

Replication fork speed 1.0x

Live Readout

Specimen cell: UBP retained

Generation

Retention p / gen

98.0%

Population still carrying X:Y

100.0% (144 / 144)

How it works

A semisynthetic organism carries a third, engineered base pair — X:Y (modelled on Romesberg's dNaM:dTPT3) — spliced into an otherwise natural A·T / G·C genome. The cell cannot make the unnatural nucleoside triphosphates itself, so each division needs an external feed plus an algal nucleotide transporter (PtNTT2) to pull them across the membrane:

transport efficiency  e = clamp(feed × transporter × 1.05, 0, 1)
loss rate per gen      L = L_max × (1 − e)          L_max = 35%
retention per gen       p = 1 − L
population fraction    F(g) = p^g   (each cell decides independently, once lost it stays lost)

Feed / transporter sliders — with both near 100% the effective loss rate collapses toward zero and the X:Y pair is copied faithfully generation after generation, exactly like the lab strain's improved retention.
Advance Generation / Auto-play — each generation the visualised specimen cell and a 144-cell population sample independently roll against the retention probability; once a lineage loses X:Y it reverts permanently to a natural T·A pair — DNA repair has no template to restore the unnatural chemistry.
Cut Feed — sets the external supply to zero, so loss rate jumps to L_max: this is the "genetic firewall" — an inherent biocontainment mechanism, since an escaped cell simply cannot propagate its expanded genetic code outside the lab.

Real-world relevance: this is the actual containment strategy behind Synthorx/Sanofi's semisynthetic biologics and the original Scripps Research organism — dependency on a synthetic nutrient, not a kill gene, keeps engineered genetic information from spreading into wild populations.

A / T G / C X : Y (unnatural) reverted / lost

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