Ribosomes are a complex of RNA and protein that bind to and processively move down (from 5’ to 3’ end) a strand of mRNA, picking up aminoacyl-tRNAs, checking to see if they are complementary to the RNA tri-nucleotide being “read” at the moment, and adding them to the new polypeptide chain if they are. The RNA part of the ribosomes are generated by the organism’s general purpose RNA polymerase in prokaryotes, and generated by the RNA polymerases I and III in eukaryotes. Recall that RNA Pol II is used by eukaryotes to generate protein-coding mRNA’s. Although the numbers of RNA strands and protein subunits differ between the prokaryote and eukaryote, the mechanism for translation is remarkably well conserved.
Figure 1. Two views of a prokaryotic ribosome. The large ribosomal subunit (50S) is shown in red, while the small ribosomal subunit (30S) is shown in blue. 3D images generated from data in the RCSB Protein Data Bank.