_Y_+30 _IMG_01_08-DNA_double_helix.jpg DNA has a double helix structure. Each strand is made of subunits called nucleotides. Each nucleotide is made of a sugar (deoxyribose), a phosphate group, and a nitrogenous base which is either Adenine (A), Guanine (G), Cytosine (C), or Thymine (T). The bases form complementary AT and GC pairs across the helix.