Ribosomes translate genetic information encoded by messenger RNA (mRNA) into proteins. Both prokaryotic and eukaryotic cells have ribosomes. Cells that synthesize large quantities of protein—such as secretory cells in the human pancreas—can contain millions of ribosomes.
Ribosomes are composed of ribosomal RNA (rRNA) and proteins. Ribosomes are not surrounded by a membrane (i.e., despite their specific cell function, they are not an organelle). In eukaryotes, rRNA is transcribed from genes in the nucleolus—a part of the nucleus that specializes in ribosome production. Within the nucleolus, rRNA is combined with proteins that are imported from the cytoplasm. The assembly produces two subunits of a ribosome—the large and small subunits.
These subunits then leave the nucleus through pores in the nuclear envelope. Each one large and small subunit bind to each other once mRNA binds to a site on the small subunit at the start of the translation process. This step forms a functional ribosome.
Ribosomes may assemble in the cytosol—called free ribosomes—or while attached to the outside of the nuclear envelope or endoplasmic reticulum—called bound ribosomes. Generally, free ribosomes synthesize proteins used in the cytoplasm, while bound ribosomes synthesize proteins that are inserted into membranes, packaged into organelles, or are secreted from the cell.
Ribosomes synthesize proteins by bringing together mRNA and transfer RNA (tRNA). Specialized nucleotides of the tRNA, called anticodon loop, bind to the codon of the mRNA. The tRNA carries an amino acid on the other end. In this way, the genetic code from mRNA is translated into a chain of amino acids one codon at a time. Ribosomes also catalyze the formation of peptide bonds between adjacent amino acids, resulting in a polypeptide.
When mRNA binds to the small subunit of the ribosome, tRNA binds to one of three binding sites on the large subunit of the ribosome. The binding sites are called the A (aminoacyl-tRNA), P (peptidyl-tRNA), and E (exit) sites. As the mRNA is translated, new tRNAs are added at the A site, move to the P site, and are released at the E site. The growing polypeptide chain threads through an exit tunnel in the large subunit. When the protein synthesis is complete, the ribosomal subunits dissociate.