The model is called a double helix because two long strands twist around each other like a twisted ladder. The rails of the ladder are made of alternating sugar and phosphate molecules. The steps of the ladder are made of two bases joined together with either two or three weak hydrogen bonds.
DNA is basically a long polymer of nucleotides. It's backbone is made of sugars and phosphate groups joined by ester bonds. Attached to each sugar is one of four types of molecules called bases, or the rungs. The 4 bases for DNA are Adenine, Thymine, Guanine, & Cytosine. Adenine and Thymine is a base pair and form two H-bonds. Guanine and Cytosine is the other base pair, but forming three H-bonds. These H-bonds stabilize the two helices.
