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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 their 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].

Pyrimidine refers to the structure of the molecule whereby it is a 'six-membered pyrimidine ring'. Examples of molecules which possess this structure are the nitrogenous bases Thymine, Cytosine and Uracil [3].


  1. Alberts, Molecular Biology of the Cell 5th Edition
  2. Stryer, Biochemistry 6th Edition
  3. Alberts,B., Johnson, A., Lewis,J., Raff,M., Roberts,K., Walter, P.,(2007) Molecular biology of the cell. 5th ed. New York: Garland Science
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