Practice question

Mammalian mitochondrial ribosome exhibits unusual sedimentation coefficient of about 55S, dissociating into 28S small subunit and 39S large subunit, with total mass approximately 2.7 megadaltons greater than bacterial 70S ribosome yet sedimenting slower. Paradox explained by porous protein-rich architecture containing 69 percent protein versus 33 percent in bacteria, increasing frictional coefficient and lowering density. Mitochondrial genome encodes 12S and 16S rRNAs. Low S value despite high mass exemplifies dependence of sedimentation on shape, density and not solely mass, crucial for understanding ultracentrifugation data.