The image in the center shows the structure of DNA. Note that there are two strands: one shown in blue, one in yellow. Other examples of a helix include yarn, a phone cord, or a spiral staircase. Each chain of the double helix is made up of repeating units called nucleotides. A single nucleotide is composed of three functional groups: a sugar, a triphosphate, and a nitrogenous (nitrogen-containing) base, as shown below.