Proteins are produced by stringing amino acids together in the order specified by messenger RNA strands that were transcribed from DNA in the cell nucleus. The process of synthesizing a protein is called translation, and it occurs on ribosomes in the cytoplasm of a cell.
During protein production, ribosomes serve as the site where synthesis takes place, and transfer RNAs serve as the tools that build the growing protein strand. Transfer RNA can attach to both RNA and amino acids. As a ribosome slides down the strand of messenger RNA and exposes each codon one by one, individual molecules of transfer RNA attach the corresponding amino acid to the growing protein chain. Once the entire messenger RNA strand has been read, the completed polypeptide chain is released from the ribosome.
A newly made polypeptide chain is not yet a functional protein. The polypeptide chain must be folded properly into a specific conformation to become a protein. In some cases, multiple polypeptide chains must assemble into a cohesive protein structure before they can function. In some cases, proteins self-assemble, folding into the proper configuration completely on their own. More complex proteins sometimes require molecular chaperones, molecules that assist or stabilize the folding process. During and after the folding process, some proteins are also altered by enzymes or modified by the addition of lipids or carbohydrates to the protein surface.