The instructions for building a protein originally come from DNA, which is the store for all hereditary information.
During protein synthesis, this information is copied (transcribed) from DNA and onto messenger RNA (mRNA), in the nucleus of the cell.
mRNA then leaves the nucleus and enters into the cytoplasm where it attaches to a ribosome. Once there, transfer RNA (tRNA) begins to read (translate) the information on the attached mRNA and, corresponding to the instructions, fetches amino acids from the pool of free amino acids in the cytoplasm, and brings them up to the ribosome where they are linked to each other into a chain or polymer, forming the primary structure of the protein.
