It’s amazing to consider that all the books ever written in the English language are made up of only the 26 letters in the Latin alphabet. Here is something more staggering: DNA is comprised of only 4 “letters”—the 4 molecules containing adenine, cytosine, guanine or thymine—and yet it encodes all the information to build every organism on earth! The famous twisted ladder structure of DNA is the double-stranded helix, with each “rung” made of two paired nucleotides. Nucleotides, which comprise the letters of the DNA alphabet, each consist of three molecules: a sugar, a phosphate group plus adenine, cytosine, guanine, or thymine. The entire molecules are abbreviated A, C, G, or T. In the DNA alphabet, A only pairs with T, and C only pairs with G to form the rungs of the ladder. The sequence of the nucleotides along the length of DNA determines the information encoded, just like the sequence of letters in a book determines what words you read..