The Svedberg (S) value of eukaryotic ribosomes is:
Sedimentation coefficient measured in Svedberg units depends on molecular weight, density and shape, not strictly additive, so sum of subunit S values does not equal particle S value. Cytoplasmic ribosomes of eukaryotes consist of forty S small subunit containing eighteen S rRNA and approximately thirty three proteins and sixty S large subunit containing twenty eight S, five point eight S and five S rRNA plus about forty seven proteins, together associating into eighty S monosome of three point two megadaltons and about twenty five nanometers diameter when translating. Prokaryotic ribosomes are smaller thirty S and fifty S joining into seventy S. Mitochondrial ribosomes sediment near fifty five S despite containing two subunits twenty eight S and thirty nine S, while chloroplast resemble seventy S. Values fifty S, seventy S, ninety S thus refer to bacterial subunits, prokaryotic ribosomes, or immature nucleolar pre-ribosomes, not mature functional eukaryotic cytosolic translation machine. This size distinction underlies differential antibiotic sensitivity: aminoglycosides target thirty S decoding, chloramphenicol fifty S peptidyl transferase, cycloheximide sixty S translocation, enabling selective inhibition without affecting host cytosolic translation, important for therapy.
Ref: Warner Cell Biol; eukaryotic 80S 40S+60S not additive, distinct from bacterial 70S.