When a gene "switches on," it eventually makes a protein, but it does not do so directly. First, the gene codes an intermediary molecule called mRNA. To transfer a gene's information from DNA to mRNA, base pairing is used. However, there is one change: An adenine base (A) in the DNA matches with a new base called uracil (U) in the mRNA. This difference helps to distinguish mRNA from DNA.

mRNA travels from the nucleus into the cytoplasm to cell organelles called ribosomes. There it directs the assembly of amino acids that fold into a unique protein.