4.8: Ribosomes

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.

Ribosome Structure and Assembly

Ribosomes are composed of ribosomal RNA (rRNA) and proteins. 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 imported from the cytoplasm. The assembly produces two subunits of a ribosome—the large and the small subunits. These subunits then leave the nucleus through pores in the nuclear envelope. At the start of translation, mRNA binds to a site on the small subunit, and subsequently, a large subunit is also recruited. 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 secreted from the cell.

Protein Synthesis

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. 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; they move to the P site and are released at the E site. Ribosomes catalyze the formation of peptide bonds between adjacent amino acids, resulting in a polypeptide. The growing polypeptide chain threads through an exit tunnel in the large subunit. When the protein synthesis is complete, the ribosomal subunits dissociate.

Eukaryotic ribosomes are protein-synthesizing structures formed inside the nucleolus. The 80S ribosome is divided into large and small subunits.

The large subunit, called the 60S ribosome, consists of 28S, 5.8S, and 5S rRNAs and 49 proteins. The smaller subunit, the 40S ribosome, consists of an 18S rRNA and 33 proteins.

Both subunits are exported out of the nucleus through the nuclear pores. They join for protein synthesis either while floating in the cytoplasm or when attached to the outer nuclear envelope or the rough endoplasmic reticulum.

The mRNA binding site lies within the small ribosomal subunit. The three tRNA binding sites, E, P, and A, are formed at the interface of both ribosomal subunits.

Here, an initiator tRNA is placed at the P or peptidyl site, the new tRNAs are received at the A or aminoacyl site, and the empty tRNAs are released from the E or exit site.

rRNA in the large subunit also catalyzes the peptide bond formation between amino acids.