This frees the polypeptide and tRNA, so they can both release from the ribosome. The two ribosomal subunits dissociate from mRNA and separate back into a small and a large subunit. A cluster of ribosomes simultaneously translating an mRNA molecule. Several ribosomes may translate an mRNA at once, making many copies of a polypeptide. The biological activity of proteins depends upon a precise folding of the polypeptide chain into a native three-dimensional conformation.
Genes determine primary structure , the linear sequence of amino acids.
Primary structure determines how a polypeptide chain will spontaneously coil and fold to form a three-dimensional molecule with secondary and tertiary structure . Some proteins must undergo post-translational modification before they become fully functional in the cell. Sugars, lipids, phosphate groups, or other additives may be attached to some amino acids. Two or more polypeptides may join as subunits of a protein that has quaternary structure. Eukaryotic ribosomes function either free in the cytosol or bound to endomembranes.