Purines and Pyrimidines

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Purines and pyrimidines are the two distinct nitrogenous nucleotide bases which form the five nucleobases present in DNA and RNA[1]. Pyrimidine bases are heterocyclic compounds which consist of a single carbon ring containing two nitrogen atoms, whereas purine bases consist of two carbon rings (a pyrimidine ring fused to an imidazole ring) containing a greater number of nitrogen atoms, so their structures provide distinct differences splitting nucleobases into two types. There are two types of Purines: Adenine and Guanine, and three types of Pyrimidines: Cytosine, Thymine (which is only used in DNA, as its substituted by Uracil in RNA during transcription) and Uracil (which is only present in types of RNA, such as mRNA and tRNA substituting Thymine)[2].

Base pairing

Purines always bond with pyrimidines via hydrogen bonds following the Chargaff rule in dsDNA, more specifically each bond follows Watson-Crick base pairing rules[3]. Therefore adenine specifically bonds to thymine forming two hydrogen bonds, whereas guanine forms three hydrogen bonds with Cytosine. During transcription and translation as thymine is substituted by uracil in RNA (mRNA and tRNA), uracil forms two hydrogen bonds with adenine so the pyrimidine-purine base pairing is maintained.


  1. Berg JM, Tymoczko JL, Gatto GJ Jr, Stryer L. Biochemistry. W. H. Freeman; 2015.
  2. Alberts B, Johnson A, Lewis J, Morgan D, Raff M, Roberts K, et al. Molecular Biology of the Cell. Garland Publishing; 2014.
  3. Griffiths AJF, Miller JH, Suzuki DT, Lewontin RC, Gelbart WM. Structure of DNA. 2000 [cited 2017 Dec 6]; Available from: https://www.ncbi.nlm.nih.gov/books/NBK21786/
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