Ribosomes

Eukaryotic ribosomes, called 80S ribosomes, consist of a large 60S subunit and a small 40S subunit. The 60S subunit contains 28S, 5.8S, and 5S rRNAs plus approximately 49 proteins. The 40S subunit contains 18S rRNA and approximately 33 proteins. Both subunits are assembled in the nucleolus and exported through nuclear pores.

Ribosomal subunits are assembled in the nucleolus, a specialized region within the nucleus where rRNA is transcribed and combined with imported proteins. After assembly, both subunits are exported from the nucleus through nuclear pores in the nuclear envelope, allowing them to reach the cytoplasm for protein synthesis.

Free ribosomes synthesize proteins in the cytosol for use within the cell. Bound ribosomes attach to the outer nuclear envelope or rough endoplasmic reticulum and synthesize proteins destined for insertion into membranes, packaging into organelles, or secretion from the cell. Both types perform protein synthesis but differ in location and protein destination.

The three tRNA binding sites—A (aminoacyl), P (peptidyl), and E (exit)—are located at the interface between ribosomal subunits. During translation, the initiator tRNA binds to the P site first. New tRNAs enter at the A site, move to the P site during elongation, and exit at the E site. This sequential movement ensures accurate codon-to-amino acid translation.

The rRNA in the large ribosomal subunit catalyzes peptide bond formation between adjacent amino acids. This catalytic activity allows the growing polypeptide chain to form as tRNAs deliver successive amino acids. The rRNA essentially acts as an enzyme, facilitating the chemical reaction that links amino acids together during protein synthesis.

When mRNA binds to the mRNA binding site on the small ribosomal subunit, it recruits the large subunit to form a functional ribosome. The small subunit recognizes the start codon, and the large subunit joins to complete the ribosome structure. This assembly brings the three tRNA binding sites into alignment, enabling translation to begin.

As amino acids are linked by peptide bonds, the growing polypeptide chain threads through an exit tunnel in the large ribosomal subunit. This tunnel protects the nascent protein during synthesis. When translation is complete and the stop codon is reached, the ribosomal subunits dissociate, releasing the finished polypeptide chain.