In DNA, the nucleotide bases form a simple four-letter alphabet which can code for the 20 amino acids found in proteins. The language of the genetic code is comprised of codons, words for individual amino acids. Each codon is exactly three bases long. A gene can be thought of as a sentence composed entirely of these three letter words. At first, this alphabet may seem too limited. But using four letters in combinations of three, 64 different combinations are possible. In fact, many amino acids can be described using more than one codon.

The basic mechanism of the genetic code is similar among many organisms. Because of this similarity, it is sometimes possible to take a segment of DNA from one source and predict the same behaviour in another. A segment of human DNA, for example, can be introduced into a host organism, like a bacterium, causing the bacteria to produce the human protein. This is the key to genetic engineering.