First, DNA-helicase (an enzyme) separates the two strands of DNA to allow RNA polymerase (another enzyme) to produce messenger RNA (mRNA) from free nucleotides, coded for by a gene in the DNA. The mRNA then leaves the nucleus and is transported through the cytoplasm by to a ribosome. Within the ribosome transfer RNA (tRNA) carrying amino acids bind to the mRNA where the two RNA have complimentary base pairs (of adenine with uracil and cytosine with guanine) in their codons (sets of three nucleic bases). As each codon equates to a tRNA molecule carrying a specific amino acid, the sequence of bases determines which amino acids are added to the chain in a certain order. This chain of amino acids is then transported to the endoplasmic reticulum, where it is folded and packaged to make the final protein.