DNA has a slightly negative charge due to the presence of phosphate groups found in the sugar-phosphate backbone of a DNA molecule. In addition to a phosphate group, each nucleotide of a DNA molecule features a nitrogen-containing base and a five-carbon sugar.
The phosphate group of a DNA nucleotide is attached to the carbon in the 5', read as "five prime," position of the five-carbon sugar. It is bonded to a hydroxyl group on the next nucleotide through an ester bond called a phosphodiester bond. The composition of the sugar-phosphate backbone renders the exterior of a DNA molecule both negatively charged and hydrophilic, enabling it to form bonds with water molecules.
Each DNA nucleotide contains one of four nitrogen-containing bases: adenine, guanine, cytosine and thymine, and it is the particular base that differentiates the nucleotides from one another. These four bases are further classified into two categories: purines and pyrimidines. Guanine and adenine bases are purines, while thymine and cytosine are pyrimidines. Purines are the larger of the two types of bases and contain five carbon and four nitrogen atoms, while pyrimidines consist of four carbon and two nitrogen atoms each.
DNA forms a double helix shape consisting of two strands connected using a system of nitrogen base pairing. Adenine-containing nucleotides bond with thymine nucleotides, while guanine nucleotides pair with cytosine nucleotides.