A DNA molecule is built from repeating units called nucleotides. Each nucleotide has three parts: a phosphate group, a sugar (deoxyribose), and one of four nitrogen bases, which are adenine (A), thymine (T), guanine (G) and cytosine (C).
According to the National Human Genome Research Institute, the nucleotides link into two strands whose alternating sugar and phosphate groups form the sides of a twisted ladder, the double helix. Each rung is a pair of bases held together by hydrogen bonds, with A always pairing with T and C with G, and the order of those bases carries the molecule’s genetic instructions.
Source: National Human Genome Research Institute, Deoxyribonucleic Acid (DNA) Fact Sheet
Image: DNA structure with key and labels, by Zephyris, CC BY-SA 3.0, via Wikimedia Commons.