The four bases of DNA are adenine (A), thymine (T), guanine (G), and cytosine (C) . These bases fall into two categories: purines and pyrimidines. Adenine and guanine are purines, while thymine and cytosine are pyrimidines. The DNA bases pair to their mirrored base in two ways: A-T or C-G. Adenine pairs with thymine, and cytosine pairs with guanine. These paired bases between the DNA strands bond by hydrogen bonds. There are two hydrogen bonds between A-T and three between C-G. The sequence of bases in a portion of a DNA molecule, called a gene, carries the instructions needed to assemble a protein.