Color enhanced Transmission Electron Micrograph (TEM) of a single strand of Deoxyribonucleic acid (DNA). DNA is a molecule that carries the genetic instructions used in the growth, development, functioning and reproduction of all known living organisms and many viruses. DNA and ribonucleic acid (RNA) are nucleic acids; alongside proteins, lipids and complex carbohydrates (polysaccharides), they are one of the four major types of macromolecules that are essential for all known forms of life. Most DNA molecules consist of two biopolymer strands coiled around each other to form a double helix. DNA is comprised of two strands of nucleotides linked together in single file. A nucleotide contains the sugar deoxyribose, a phosphate group and one of four organic bases: thymine, adenine, cytosine, and guanine. Magnification unknown.