The principal role of is to act as a messenger carrying instructions from DNA out of the nucleus for the synthesis of proteins. A serves as the site of protein synthesis in the cytoplasm. is the first step of gene expression, during which a particular segment of DNA is converted into RNA. A is made up of hundreds or thousands of smaller units called amino acids, attached to one another in long chains. is a long linear polymer found in the nucleus of a cell, shaped like a double helix, and associated with the transmission of genetic information. is the process in which mRNA codons are converted into an amino acid sequence.