There are five different nucleotides within genetic material: adenine, thymine, cytosine, guanine, and uracil.
These nucleotides can be classified as either purines or pyrimidines. Purines contain one ring, whereas pyrimidines contain two. Adenine and guanine are purines. Cytosine, thymine, uracil are the pyrimidines. A purine must always pair with a pyrimidine within a base pairing.
The possible nucleotides found in deoxyribose nucleic acid (DNA) are adenine, thymine, cytosine, guanine, and uracil. Adenine (A) and thymine (T) is one of the purine/pyrimidine base pairs that occur in DNA. Guanine (G) and cytosine (C) is the other DNA purine/pyrimidine base pair.
The nucleotides that are in RNA are: adenine, uracil, cytosine and guanine. Cytosine and guanine continue to pair in RNA. However, uracil replaces thymine in RNA. Thus, adenine pairs with uracil in RNA.