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.
Bound and free ribosomes are structurally identical and interchangeable. Most proteins made by free ribosomes will function in the cytosol.