Denaturation is the alteration of a protein shape through some form of external stress (for example, by applying heat, acid or alkali), in such a way that it will no longer be able to carry out its cellular function. Denatured proteins can exhibit a wide range of characteristics, from loss of solubility to communal aggregation. Proteins are very long strands of amino acids linked together in specific sequences. A protein is created by ribosomes that "read" codons in the gene and assemble the requisite amino acid combination from the genetic instruction, in a process known as translation. The newly created protein strand then undergoes post-translational modification in which additional atoms or molecules are added, for example copper, zinc, iron. Once this post-translational modification process has been completed, the protein begins to fold (spontaneously, and sometimes with enzymatic assistance), curling up on itself so that hydrophobic elements of the protein are buried deep inside the structure and hydrophilic elements end up on the outside. The final shape of a protein determines how it interacts with its environment.