In prokaryotes, the ribosomal RNA (rRNA) has three types: 23S, 5S, and 16S. In mammals, four types of rRNA have been found : 28S, 5.8S, 5S and 18S. The unit "S" stands for Svedberg, which is a measure of the sedimentation rate. After rRNA molecules are produced in the nucleus, they are transported to the cytoplasm, where they combine with tens of specific proteins to form a ribosome. In prokaryotes, the size of a ribosome is 70S, consisting of two subunits: 50S and 30S. The size of a mammalian ribosome is 80S, comprising a 60S and a 40S subunit. Proteins in the larger subunit are designated as L1, L2, L3, etc. (L = large). In the smaller subunit, proteins are denoted by S1, S2, S3, etc.
Figure 3-C-4. The composition of ribosomes.
During protein synthesis, the ribosome binds to mRNA and tRNA as shown in the following figure. Only the tRNA containing the anticodon which matches mRNA's codon may join the complex.