Protein Synthesis · Paper 2 — Free Biology HL Practice Questions

A research team investigates the production of a therapeutic enzyme, human deoxyribonuclease I (DNase I), in E. coli bacteria. The wild-type human DNASE1 gene is inserted into a bacterial expression plasmid. The protein produced is enzymatically inactive despite being successfully translated. Analysis reveals that the recombinant DNase I lacks removal of an N-terminal signal peptide that targets the protein for secretion in human cells. The team engineers a modified DNASE1 gene coding directly for the mature form of the protein (without the signal peptide). This modified gene is expressed in E. coli. The resulting protein is active but is degraded by intracellular proteases within 30 minutes, whereas human wild-type DNase I is stable for over 24 hours in the bloodstream.

A group of students studies the effects of a novel drug, Modifin, on post-translational modification of the tumour suppressor protein p53 in human cell cultures. p53 is activated by phosphorylation at serine residues in response to DNA damage. Two sets of cells are exposed to UV radiation. Set A receives no Modifin. Set B receives Modifin immediately after UV exposure. Phosphorylated p53 (detected at serine 15) and total p53 are measured over 6 hours. - Set A (control): phosphorylated p53 peaks at 2 hours ($100\,\text{units}$), declines to baseline by 6 hours; total p53 remains constant at $100\,\text{units}$ throughout. - Set B (Modifin-treated): phosphorylated p53 peaks at 2 hours ($100\,\text{units}$), remains at $80\,\text{units}$ at 6 hours; total p53 increases by $50\%$ over 6 hours. - Modifin specifically inhibits phosphatase PP2A, which removes phosphate groups from proteins. - Rate of p53 translation is constant and identical in both sets.