DNA structure.

Each nucleotide consists of a nitrogenous base (T, A, C, or G), the sugar deoxyribose, and a phosphate group.

The 5' end of the polynucleotide is a phosphate, while the 3' end is a hydroxyl group of the sugar.

The phosphate of one nucleotide forms a covalent bond with the sugar of the next, providing a "backbone" from which the bases project.