A rigid chain of alternating deoxyribose sugars and phosphates makes up the sides of the DNA ladder. The rungs of the DNA ladder consist of four nitrogenous bases.
Adenine, guanine, thymine and cytosine are the four nitrogenous bases that are attached to a sugar molecule on each side of the ladder. When a phosphate, a sugar and a base form an attachment, they create a subunit of DNA called a nucleotide. Each nitrogenous base is held together by a hydrogen bond. Nitrogen base bonding is very specific, and each base can only pair with the correctly matching base. Adenine and thymine pair together, as do guanine and cytosine pair together. The way these nitrogenous bases are arranged is extremely important. The alignment of these base pairs determines the type of organism being formed, whether it is a plant or an animal. The structure of a DNA strand resembles a right-handed double helix. This compact helical arrangement allows more genetic information to be stored within a single strand of DNA. The purpose of the sides of the ladder, the sugar-phosphate chain, is to keep the genetic code in its proper order. If this structure becomes unstable, the genetic code may be altered and mutations could arise in the cell.