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When I was studying microbiology, something didn't make sense to me. We all know ribosomes have two subunits; the large subunit and the small subunit. Such as for eukaryotic cells, the large subunit has 60 S (svedberg) and small subunit has 40 S. When they combine with each other they are considered as 80 S. Why it is 80 S instead 100 S?

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Because Svedberg is not a measure of weight, but it is a measure of a relative sedimentation after centrifugation. "It is defined as the ratio of a particle's sedimentation velocity to the acceleration that is applied to it (causing the sedimentation)." The high number of S are correlated to the shape of the particles as well.

https://en.wikipedia.org/wiki/Sedimentation_coefficient

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