The code on the DNA molecule is used to determine how the polypeptide molecule is constructed. In the nucleus, a complementary copy of the code from a gene is made by building a molecule of a different type of nucleic acid, called messenger RNA (mRNA), using one strand of the DNA as a template. The mRNA leaves the nucleus, and attaches to a ribosome in the cytoplasm. In the cytoplasm, there are molecules of transfer RNA (tRNA). These have a triplet of bases at one end and a region where an amino acids can attach at the other. THere are at least 20 different sorts of tRNA molecules, each with a particular triplet of bases at one end and able to attach to a specific amino acid at the other. The tRNA molecules pick up their specific amino acids from the cytoplasm and bring them to the mRNA on the ribosome. The triplet of bases (an anticodon) of each tRNA links up with a complementary triplet (a codon) on the mRNA molecule. Two tRNA molecules fit into the ribosome at any one time. This brings two amino acids side by side and a peptide bond is formed between them. usually, several ribosomes work on the same mRNA strand at the same time. They are visible as polyribosomes. So the base sequence on the DNA molecule determines the base sequence on the mRNA, which determines which tRNA molecules can link up with them. Since each type of tRNA molecule is specific for just one amino acid, this determines the sequence in which the amino acids are lined together as the polypeptide molecule us made. The first stage in this process is called transcription. The last stage is called translation.