The main substance in cells responsible for storing hereditary information is DNA, or deoxyribonucleic acid. DNA is a complex molecule composed of two long strands that form a double helix structure. It carries the genetic blueprint for the development, functioning, growth, and reproduction of all known living organisms and many viruses. The sequence of bases in DNA—adenine, thymine, cytosine, and guanine—encodes the instructions for building proteins, which determine an organism's traits. DNA is passed from parents to offspring during reproduction, making it fundamental to inheritance and evolution.