Prokaryotic and eukaryotic ribosomes have similar structures but they are different. Prokaryotic ribosomes are composed of a 50s and 30s subunit, while Eukaryotic ribosomes are composed of a 60s and 40s subunit. Tetracycline binds to the a site of the 30s subunit of Prokaryotes but cannot bind efficiently to the same site in the 40s subunit of eukaryotic ribosomes due to the differences in structure and sequence.