DNA is a long polymer or chain of smaller subunits called nucleotides. Each nucleotide has three components. Two of these components are the same in all the nucleotides of the DNA chain; the third component, however, can be one of four different structures. The sequence of these structures in the DNA serves as a way to chemically encode information.
The central component of a DNA nucleotide is a five-carbon sugar called 2-deoxyribose (hence the name deoxyribonucleic acid or DNA). One oxygen and four carbon atoms in this sugar form a five-membered pentagonal ring. The carbons are numbered 1' through 5', where the 1' carbon is immediately adjacent to the oxygen atom in the ring and the 5' carbon is attached to the 4' carbon in the ring. At carbon 3', 2-deoxyribose has a hydroxyl group.
Nucleotides also contain a phosphate (PO4) group. This group is tetrahedral in shape; the phosphorous atom forms the center of a tetrahedron, with oxygen atoms at each corner. One of these oxygen atoms is bonded to the 5' carbon in the 2-deoxyribose sugar. When the nucleotides have been assembled together to form a chain, the phosphate group on one nucleotide will be connected to the 3' carbon on the neighboring nucleotide.
The base is the structural component that differs between the four nucleotides. Two of the DNA bases are pyrimindines, which have only one ring; these are cytosine and thymine, commonly abbreviated C and T. The other two DNA bases are purines, which have two fused rings; these are adenine and guanine, which are commonly abbreviated A and G. All of these bases are heterocyclic, meaning the rings are composed of atoms of two different elements: nitrogen and carbon.
The base in any DNA nucleotide is always attached to the 1' carbon. Bases in the two strands form weak interactions called hydrogen bonds with each other; these hydrogen bonds hold the two strains together. Adenine and thymine can form two hydrogen bonds, so they are complementary to each other and are always found opposite each other in the two strands, while cytosine and guanine can form three hydrogen bonds and are complementary to each other.