Ribosomes are the sites of protein synthesis. In both prokaryotes and eukaryotes, ribosomes consist of a large subunit associated with a small subunit (Fig. 9-2). Each subunit is a complex consisting of rRNA and many types of proteins. Because eukaryotic ribosomes contain a larger quantity of rRNA and more types of proteins, eukaryotic ribosomes are 80S while prokaryotic ribosomes are 70S. Although ribosomes contain many types of proteins, they are rich in RNA, with proteins only covering parts of the surface (2/3 are RNA and 1/3 is protein). In particular, the space between the two subunits, which is the site of protein synthesis, is predominantly composed of RNA.

Ribosomes bind mRNA, interact with aminoacyl-tRNA, and perform enzymatic reactions such as cleaving the ester bond between tRNA and the polypeptide chain and forming peptide bonds between the polypeptide chain and amino acids, thus, proceeding protein synthesis. These important functions of ribosomes are performed by rRNA. Ribosomes are considered ribozymes consisting of RNA with enzymatic activity.

Fig. 9-2 Escherichia coli ribosome

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