Nucleotides are the units on which nucleic acids are built. They are made up of a sugar molecule (deoxyribose in DNA, ribose in RNA), a phosphate group, and a nitrogen-containing base. The sugar molecule is bound to both the phosphate group and the base. Four types of nucleotides can be found in DNA according to the base that is bound to the sugar molecule. The four bases are A (adenine), T (thymine), C (cytosine) and G (guanine). In RNA thymine is replaced by uracil (U).

Nucleotides are composed of a pentose ribose or deoxyribose with a phosphate and base attached to it. There are 4 unique nucleotides in DNA, all containing a six atom ring with 2 nitrogen atoms and 4 carbon atoms. Cytosine and Thymine are pyrimidines with a double bonded oxygen to one carbon atom and double bonded carbon atoms in the ring. Cytosine contains a nitrogen bonded to the top carbon atom which is double bonded to a nitrogen atom. Thymine contains a second double bonded oxygen to the top carbon and a methyl group on one of the double bonded carbon atoms. Adenine and Guanine are purines with a second 5 atom ring with 3 carbons and 2 nitrogens that shares the double bond between the carbons with the first ring. Adenine contains one nitrogen group bonded to a carbon in the 6 member ring. Guanine contains two nitrogens bonded to two carbons in the 6 member ring. Uracil is a pyrimidine that replaces thymine in RNA. It has two double bonded oxygen atoms to two carbons in the 6 member ring.

Figure 1. DNA Nucleotides