(Description) Figure 15 The ribosome is organised into a small subunit and a large subunit. It is useful to be aware that the sizes of the subunits and their component ribosomal RNAs (rRNAs) are described by a so-called sedimentation coefficient 'S', which depends on their relative molecular mass and shape (S stands for Svedberg unit). The small and large subunits are referred to as 30S and 50S, respectively, for prokaryotic ribosomes; and 40S and 60S, respectively, for eukaryotic ribosomes. The complete ribosome is referred to as 70S for prokaryotic ribosomes and 80S for eukaryotic ribosomes.

A tour of the cell

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This diagram summarises the structural organisation of prokaryotic and eukaryotic ribosomes. Prokaryotic ribosomes are 70S particles comprising a small, 30S subunit and a large, 50S subunit. The 30S subunit is made up of 16S rRNA and 21 proteins; and the 50S subunit is made up of 23S rRNA, 5S rRNA and 34 proteins. Eukaryotic ribosomes are 80S particles comprising a small, 40S subunit and a large, 60S subunit. The 40S subunit is made up of 18S rRNA and 33 proteins; and the 50S subunit is made up of 28S rRNA, 5.8S rRNA, 5S rRNA and 50 proteins.

4.4 Ribosomes: the sites of protein synthesis