DNA, or deoxyribonucleic acid, is the building block of the genetic code. A DNA molecule is composed of two strands of nucleotides wrapped around one another and connected at the bases to form a double helix. DNA is present in all nucleated cells in an animal or plant, with cattle having approximately 2.7 billion nucleotides in the code. DNA is organized into 30 pairs of chromosomes in cattle. We refer to the complete DNA makeup of an animal as its genome.