The letters DNA stand for deoxyribonucleic acid. DNA is a molecule that contains the instructions for an organism's development and characteristics. It is found in the nucleus of cells and is made up of four different types of nucleotides: adenine, cytosine, guanine, and thymine. The sequence of these nucleotides determines the genetic code, which is read by cells to produce proteins. Proteins are essential for the structure, function, and regulation of cells and tissues.