In chemistry, a polymer is a chemical compound formed by a sequence of repeated smaller units called monomers. Deoxyribonucleic acid (DNA) consists of paired polymers made up of four different monomers. The monomers in DNA consist of a deoxyribose sugar molecule, a phosphate group and a nitrogen base ring. It is the nitrogen base ring that differentiates the four otherwise identical monomers and thus the monomers in DNA are usually called by their respective nitrogen bases.
Adenine is one of the two purine bases found in DNA monomers. In an adenine-containing monomer, the adenine ring forms a bond with the 1' carbon atom of a deoxyribose molecule and the phosphate group binds to the 5' carbon atom. Adenine monomers form base pairs together with thymine, meaning the adenine from one DNA strand will form hydrogen bonds only with a thymine monomer on the companion strand. Adenine cannot bond with other adenine base pairs or with cytosine or guanosine pairs because the molecules would not fit together in such a way that hydrogen bonds could form.
Guanine is the other purine base found in DNA. As with adenine, a guanine-containing monomer will contain a guanine ring bonded to the 1' carbon atom of deoxyribose with a phosphate group bound to the 5' carbon of the sugar. Guanine monomers form base pairs with cytosine monomers on the other DNA strand. As purines, both guanine and adenine contain a six-atom ring of carbon and nitrogen atoms bound to a five-atom ring.
Cytosine is a pyrimidine base found in DNA monomers. Pyrimidines consist of a single ring of six carbon and nitrogen atoms rather than a double ring, making pyrimidines much smaller than purines in molecular size. In base pairing, cytosine from one DNA strand forms hydrogen bonds with guanine monomers on the other strand. An interesting fact about cytosine is that it tends to be unstable and can spontaneously transform to uracil if the conditions are right.
Thymine is the second pyrimidine base found in DNA and it forms bonds with adenine on the second DNA strand. Thymine is the one nitrogen base found in DNA but not in ribonucleic acid, or RNA. In RNA, thymine is replaced by uracil, another pyrimidine that can also form bonds with adenine. The only difference in structure between thymine and uracil is that thymine contains a methyl group or a carbon and three hydrogen atoms, bonded to the 5' carbon atom. In uracil, the 5' carbon atom bonds only a single hydrogen atom.