Ribosomes

Eukaryotic ribosomes are 80S structures composed of two subunits: the large 60S subunit containing 28S, 5.8S, and 5S rRNAs plus 49 proteins, and the small 40S subunit containing 18S rRNA and 33 proteins. Both subunits are assembled in the nucleolus and exported through nuclear pores before joining during protein synthesis.

The three tRNA binding sites—E, P, and A—are formed at the interface of both ribosomal subunits. The initiator tRNA binds at the P or peptidyl site, new tRNAs enter at the A or aminoacyl site, and empty tRNAs exit at the E or exit site. This sequential movement ensures accurate translation of the genetic code.

The rRNA in the large ribosomal subunit catalyzes peptide bond formation between adjacent amino acids during translation. This catalytic activity is essential for linking amino acids into a growing polypeptide chain, making rRNA function as a ribozyme with enzymatic capability in protein synthesis.

Free ribosomes float in the cytoplasm and synthesize proteins used within the cell for cytoplasmic functions. Bound ribosomes attach to the outer nuclear envelope or rough endoplasmic reticulum and synthesize proteins destined for membrane insertion, organellar packaging, or secretion from the cell.

mRNA binds to the small ribosomal subunit, and tRNA molecules with anticodon loops bind to mRNA codons. Each tRNA carries a specific amino acid, translating the genetic code one codon at a time. The ribosome catalyzes peptide bonds between amino acids, forming a polypeptide chain that threads through an exit tunnel.

Cells that synthesize large quantities of protein, such as secretory cells in the human pancreas, require millions of ribosomes to meet their protein production demands. Multiple ribosomes working simultaneously on the same mRNA molecule, called polysomes, enable rapid and efficient protein synthesis.

Once protein synthesis is complete, the ribosomal subunits dissociate from the mRNA and from each other. The individual 40S and 60S subunits can then be recycled and reassemble with new mRNA molecules to begin another round of translation and protein production.