Deoxyribonucleic acid (DNA) is a molecule that encodes the
genetic instructions used in the development and functioning of
all known living organisms and many viruses. Along with RNA and
proteins, DNA is one of the three major macromolecules essential
for all known forms of life. Genetic information is encoded as a
sequence of nucleotides (guanine, adenine, thymine, and
cytosine) recorded using the letters G, A, T, and C. Most DNA
molecules are double-stranded helices, consisting of two long
polymers of simple units called nucleotides, molecules with
backbones made of alternating sugars (deoxyribose) and phosphate
groups (related to phosphoric acid), with the nucleobases (G, A,
T, C) attached to the sugars. DNA is well-suited for biological
information storage, since the DNA backbone is resistant to
cleavage and the double-stranded structure provides the molecule
with a built-in duplicate of the encoded information. These two
strands run in opposite directions to each other and are
therefore anti-parallel, on