Deoxyribonucleic acid, commonly known as DNA, is a molecule that carries the genetic instructions for the development, functioning, growth, and reproduction of all known living organisms and many viruses. It is composed of two long strands that coil around each other to form a double helix. Each strand is made up of smaller units called nucleotides, which consist of a sugar, a phosphate group, and a nitrogenous base. DNA contains the information needed to build proteins, which are essential for various biological functions. The sequence of the nitrogenous bases—adenine, thymine, cytosine, and guanine—determines the genetic code. This code is passed from parents to offspring, making DNA crucial for heredity and evolution.