Solution

ID: past-exam-of-the-biochemistry-course-of-the-university-of-cambridge/2024/paper-5/section-2/i/solution

The major problems are unequal recovered totals, absence of replication/error estimates, and ambiguity about what pellets. Wild-type supernatant plus pellet signal is roughly 860 units without drug and 1,100 with drug; the mutant totals also differ. Equal starting actin should give comparable total recovery after correcting for fraction volumes and loading. The graph does not specify such corrections, so comparing raw pellet heights alone is misleading. Fractions are more informative but still require comparable staining response and unbiased recovery.
The mutant's poor drug-free pelleting is biologically important and should not be hidden by calling it simply resistant. Independent preparations and replicate assays would establish whether that baseline is reproducible. One drug concentration and one nominal steady-state time cannot establish potency, binding affinity, assembly kinetics or attainment of a true steady state. The gel confirms an actin-sized band but does not preserve actin filament structure: denaturation occurs after separation. Short actin filaments can remain in the supernatant, and aggregates can pellet. Direct imaging, mass-balance controls, a monomer-only control and a linear densitometry calibration would make the interpretation substantially stronger.

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