Deoxyribonucleic acid (DNA) is the molecule that carries genetic information essential for the development, functioning, growth, and reproduction of all living organisms and many viruses. DNA is composed of two long strands coiled into a shape called a double helix, resembling a twisted ladder. The backbone of each strand is made of alternating sugar (deoxyribose) and phosphate groups. Attached to each sugar is one of four bases: adenine (A), thymine (T), cytosine (C), or guanine (G). The two strands are connected by specific pairs of these bases, with adenine bonding only to thymine and cytosine only to guanine. The sequence of these bases along the DNA strands encodes biological instructions, such as those for making proteins or RNA molecules. Almost all cells in an organism have DNA, mostly located in the cell nucleus (called nuclear DNA), and a small amount is found in mitochondria (mitochondrial DNA). DNA is passed from parents to offspring during reproduction, forming the genetic blueprint that makes each species—and each individual—unique. In summary, DNA is essentially the instruction manual written in a chemical code that guides the growth, development, and functioning of living organisms and is fundamental to heredity and biological inheritance. Its structure and function were discovered through significant scientific breakthroughs, including the identification of its double helix form.
