The double helix of DNA is a molecule consisting of two complementary strands that can be thought of as a ladder that is twisted. There are subunits of DNA called nucleotides. A nucleotide contains a five- carbon sugar known as deoxyribose along with a phosphate group and one of four bases--adenine, guanine, cytosine and thymine. A complementary nucleotide joins the base of the first nucleotide but follows a specific set of base-pairing rules. Adenine pairs with thymine and cytosine pairs with guanine. The base pair in the center is held together by hydrogen bonds. If you can visualize the sides of the ladder are represented by the sugar and phosphate groups and the rungs of the ladder are the complementary bases, this is a representation of how DNA is constructed.