A plasmid carrying the therapeutic-protein gene is transformed into the host cell line. Transformed cells are expanded in a stirred-tank bioreactor, where they divide and express the recombinant protein — a molecule the host does not make naturally. After harvest, cells are lysed and the crude mixture is loaded onto a Protein A affinity chromatography column, which selectively binds the target (antibodies bind Protein A via their Fc region) while host-cell proteins and DNA flow to waste; a polishing step removes the last impurities.
growth(T,pH) = exp(−(T−Topt)²/2σT²) · exp(−(pH−pHopt)²/2σpH²)
titer(t) = Titer_max · growth · (1 − e^(−k·t))
purity_final = target_bound / (target_bound + impurity_leaked)
- Temperature — CHO cells grow fastest near 37 °C; a few degrees off in either direction slows division and lowers correctly-folded protein yield.
- pH — culture pH drifting from ~7.2 stresses the cells and reduces both growth rate and expression quality.
- Culture duration — longer runs let titer climb toward saturation, but very long runs risk cell death and product degradation in the vessel.
- Affinity chromatography — Protein A resin captures the target with high specificity, so purity jumps sharply at that step even though some product is inevitably lost (yield drops) with every stage.
Real-world relevance: this three-stage upstream/downstream pipeline — gene insertion, bioreactor expression, chromatographic purification — is how real biopharmaceuticals such as insulin, monoclonal antibodies (e.g. trastuzumab) and recombinant vaccines are manufactured at industrial scale under GMP conditions.