The rungs of the DNA ladder are made of complementary nitrogenous base pairs. DNA contains four different nitrogenous bases: adenine, thymine, cytosine and guanine. The nitrogenous bases are divided into the purines and the pyrimidines; each rung of the DNA molecule contains one purine and one pyrimidine bonded together.
The purines are adenine and guanine; the pyrimidines are thymine and cytosine. The purines are larger than the pyrimidines; therefore, only one of each can fit on a rung. The rules of base pairing dictate that adenine pairs with thymine, and guanine bonds with cytosine.
The sides of the DNA ladder contain alternating sugar and phosphate molecules. The nitrogenous bases are attached to the sides of the ladder through a carbon atom on the sugar molecule. The complex containing a nitrogenous base attached to a sugar molecule is called a nucleoside. The complex resulting from the bonding of the sugar molecule, phosphate molecule and nitrogenous base is called a nucleotide. Repeating nucleotide subunits make up each side of the DNA molecule.