Pyrimidine
Nucleotides can be divided into two catergories, purines and pyrimidines. Pyrimidines are named so as they are derived from the six membered ring pyrimidine.[1] Examples of pyrimidines are cytosine, thymine and uracil (which is only present in RNA). These bond via hydrogen bonds with there complementary purine bases when in DNA's double helix form. They also exist in RNA, where thymine is replaced by uracil.
These bases are only refered to as nucleotides when bonded to one or more phosphate groups by diester bonds, when separate they are known as nucleosides.[2]