Cellular protein synthesis by ribosomes. Cutaway illustration of the inside of a cell, showing proteins (one at upper right) being synthesized by ribosomes (one at centre). The instructions for building the proteins are encoded in strands of messenger ribonucleic acid (mRNA) that originates in the cell nucleus (lower left), and is a copy of the information coded in DNA (deoxyribonucleic acid) in the nucleus. Transfer RNA brings amino acids to a ribosome, where they are joined in a sequence determined by the mRNA. Different sequences produce different proteins.