DNA exists as two long, paired strands spiraled into the famous double helix.

Each strand is made up of millions of chemical building blocks called bases. There are only four different chemical bases in DNA (adenine, thymine, cytosine, and guanine), but they can be arranged and rearranged in countless ways. The order in which the bases occur determines the messages to be conveyed, much as specific letters of the alphabet combine to form words and sentences.