The ribosome is the translation machinery

Molecular structure of the E coli ribosome, with large subunit colored red and the small subunit colored blue.
Figure 6 Structure of the E.coli ribosome seen from two different angles, with large 50S subunit in red and small 30S subunit shown in blue. RNA components are darker in color and protein components are lighter.

The ribosome catalyzes peptide bond formation. The ribosome is a large complex assembled from many different components, including ribosomal RNA (rRNA) and protein. The rRNAs are functional RNAs that are never translated into protein.

The ribosome has a large and small subunit. The ribosomal components are described in terms of Svedburg units (S). These are units that indirectly approximate size based on sedimentation during centrifugation. The units are not additive, because sedimentation depends on both the size and shape of a complex. So for example, in prokaryotes the large subunit is 50S, the small subunit is 30S, and the whole ribosome is 70S.

The structure of the E.coli ribosome is shown in Figure 6, with the large subunit colored red and the small subunit colored blue. Prokaryotic ribosomes have 3 ribosomal RNAs (rRNA): 23S rRNA and 5S rRNA in the large subunit, and 16S rRNA in the small subunit. They also have about 50 proteins.

Eukaryotic ribosomes also have a large and small subunit and are structurally very similar. In eukaryotes, the large subunit is 60S, the small subunit is 40S, and the whole ribosome is 80S. Eukaryotic ribosomes have 4 rRNAs and about 80 proteins. The 28S and 5.8S, and 5S rRNAs are part of the large subunit, and the 18S rRNA is part of the small subunit. It is the rRNA components that are catalytically active: they catalyze the peptidyl transferase reaction that builds the RNA molecule. The ribosome is therefore an example of a ribozyme, an RNA molecule that acts as an enzyme.

A comparison of the eukaryotic and prokaryotic ribosomes is summarized in Table 1.


Table 1 Comparison of eukaryotic and prokaryotic ribosomes
 No Data
Eukaryote Ribosome Prokaryote Ribosome
Overall size 80S 70S
Large subunit 60S 50S
Large subunit components 28S rRNA

5.8S rRNA


About 50 proteins

23S rRNA



About 30 proteins

Small subunit 40S 30S
Small subunit 18S rRNA

About 30 proteins

16S rRNA

About 20 proteins

Structure of the ribosome positioned on an mRNA, showing the E, P, and A sites.
Figure 7 EPA sites of the ribosome.

The large and small subunits assemble during translation with the mRNA sandwiched between them. The bases of the mRNA are being positioned within three adjacent sites in the ribosome, called the E, P, and A sites.

A stands for aminoacyl: this is the acceptor site for aminoacyl tRNAs (those carrying an amino acid) to enter the ribosome. P stands for peptidyl: this is the site of the peptidyl transferase reaction that forms peptide bonds between a growing polypeptide and an incoming amino acid. E stands for exit: tRNAs exit from the ribosome via the E site after they have donated their amino acid to the peptide.

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