DNA is composed of two strands of nucleotides wound around one another in a double helix. The outer longitudinal structure consists of alternating sugars and phosphate groups. Paired nitrogenous bases are in the interior of the molecule. If the molecule unwinds, its structure resembles a ladder with phosphates and sugars forming the sides. The rungs are the nitrogenous bases of nucleotides on opposite sides of the ladder. There are four different bases and only certain pairs are possible in forming the rungs. Adenine is always opposite thymine and guanine is always opposite cytosine. These complementary pairs of nitrogenous bases lie next to one another because their sugar-phosphate chains run in opposite directions. This anti-parallel orientation of the two nucleotide strands brings the bases into position for hydrogen bond formation. The sides of the ladder are twisted in a spiral fashion and make a complete turn every ten base pairs.