Well, DNA has a four-letter code. The order in which those letters​—A, T, G, and C—​appear forms “words” called codons. Codons are arranged in “stories” called genes. Each gene contains, on average, 27,000 letters. These genes and the long stretches between them are compiled into chapters of a sort​—the individual chromosomes.

Remember, the DNA contains the genes, the instructions for building a unique human body. Each cell has a complete set of instructions.