Deoxyribonucleic acid (;
DNA) is a
molecule that carries most of the
genetic instructions used in the development, functioning and
reproduction of all known living
organisms and many
viruses. DNA is a
nucleic acid; alongside
proteins and
carbohydrates, nucleic acids compose the three major
macromolecules essential for all known forms of
life. Most DNA molecules consist of two
biopolymer strands coiled around each other to form a
double helix. The two DNA strands are known as
polynucleotides since they are composed of
simpler units called
nucleotides. Each nucleotide is composed of a
nitrogen-containing nucleobase—either
cytosine (C),
guanine (G),
adenine (A), or
thymine (T)—as well as a
monosaccharide sugar called
deoxyribose and a
phosphate group. The nucleotides are joined to one another in a chain by
covalent bonds between the sugar of one nucleotide and the phosphate of the next, resulting in an alternating
sugar-phosphate backbone. According to
base pairing rules (A with T, and C with G),
hydrogen bonds bind the nitrogenous bases of the two separate polynucleotide strands to make double-stranded DNA. The total amount of related DNA
base pairs on Earth is estimated at 5.0 x 10
37, and weighs 50 billion
tonnes. In comparison, the total
mass of the
biosphere has been estimated to be as much as 4 TtC (trillion tons of
carbon).
