The nucleus contains the hereditary material deoxyribonucleic acid or DNA, which makes up the chromosomes and genes. However, this blueprint, or genetic code, does not leave the nucleus. Another nucleic acid known as messenger RNA, can copy or transcribe the DNA code. Either side of the DNA double helix can serve as the template. This process is called transcription and the mRNA can move out of the nucleus to the cytoplasm, where it attaches to a ribosome. Here, translation occurs. Every three bases along the mRNA transcript is called a codon--it is a triplet and each one represents a particular amino acid. For example, AUG is a triplet that means "start" and signals the beginning of translation. It is also the code for methionine, an amino acid. Every three bases is translated, with the help of transfer RNA. Free nucleotides are present in the cell and the transfer RNA has the job of delivering the correct ones into position, as the messenger RNA is "read" and translated. One by one,each amino acid joins the long chain of amino acids, called a polypeptide, until a "stop" codon is reached. This signals the end of translation. The polypeptide is now synthesized and must detach and fold into a functional three dimensional shape and is called a protein.