Purines make up half the bases used in DNA construction. The two notable purines are adenine and guanine, which are usually abbreviated "A" and "G", respectively. These bases pair correspondingly with their matching pyrimidines, thymine and cytosine. Thymine is abbreviated "T", while cytosine is abbreviated "C". These bases pair with each other in this order: adenine to thymine, and cytosine to guanine. This is called complementary base pairing. Together, these four bases are the foundation for the composition of DNA, which is the genetic blueprint for the identity of every cell in a persons body. DNA is contained within the cells nucleus, which is a large, spherical organelle within the center of the cell.