The extremely long DNA molecule is actually made of a long string of chemical building blocks called “nucleotides.” There are four different nucleotides, which are labeled adenine (A), thymine (T), guanine (G), and cytosine (C). The human genome is made of a sequence of roughly three billion of these nucleotides, and it is about the same size as the genome of a chimpanzee or a mouse. In contrast, a fruit fly has 180 million, a yeast has 12 million, and the flowering weed “thale cress” has 100 million nucleotides of DNA in its genome.