Purine vs. Pyrimidine

What's the Difference?

Purine and pyrimidine are two types of nitrogenous bases found in DNA and RNA molecules. They both play a crucial role in the structure and function of nucleic acids. However, there are some key differences between the two. Purine bases, such as adenine and guanine, have a double-ring structure, while pyrimidine bases, such as cytosine, thymine, and uracil, have a single-ring structure. Another difference is that purines always pair with pyrimidines in DNA and RNA strands through hydrogen bonding. Adenine pairs with thymine in DNA or uracil in RNA, while guanine pairs with cytosine. Overall, purines and pyrimidines are essential components of nucleic acids, contributing to the genetic information and functioning of living organisms.


Chemical StructureDouble-ring structureSingle-ring structure
Number of Rings21
Number of Nitrogen Atoms42
Base PairsAdenine (A) pairs with Thymine (T)Cytosine (C) pairs with Guanine (G)
Presence in DNAYesYes
Presence in RNAYesYes
AbbreviationA, GC, T, U

Further Detail


Purines and pyrimidines are two types of nitrogenous bases that play a crucial role in the structure and function of nucleic acids, such as DNA and RNA. These bases are the building blocks of the genetic code and are responsible for carrying and transmitting genetic information. While both purines and pyrimidines are essential for life, they differ in their chemical structure, function, and occurrence in nucleic acids.

Chemical Structure

Purines are heterocyclic aromatic compounds consisting of a pyrimidine ring fused with an imidazole ring. The two main purine bases found in nucleic acids are adenine (A) and guanine (G). Adenine contains an amino group (-NH2) attached to the carbon atom at position 6 of the purine ring, while guanine has an amino group at position 2 and a carbonyl group (=O) at position 6.

On the other hand, pyrimidines are also heterocyclic aromatic compounds but have a single six-membered ring structure. The three pyrimidine bases found in nucleic acids are cytosine (C), thymine (T) in DNA, and uracil (U) in RNA. Cytosine and thymine have a keto group (=O) at position 2, while uracil lacks the methyl group present in thymine.

Function in Nucleic Acids

Purines and pyrimidines have distinct roles in the structure and function of nucleic acids. Purines pair with pyrimidines through hydrogen bonding to form the complementary base pairs that stabilize the double helix structure of DNA. Adenine (A) always pairs with thymine (T) through two hydrogen bonds, while guanine (G) pairs with cytosine (C) through three hydrogen bonds.

Pyrimidines, on the other hand, are involved in the recognition and binding of specific molecules, such as proteins and enzymes, during various cellular processes. They also participate in the regulation of gene expression and play a role in the synthesis of proteins through the process of translation.

Occurrence in Nucleic Acids

Purines and pyrimidines are present in different proportions in DNA and RNA. In DNA, adenine (A) and guanine (G) are the purine bases, while cytosine (C) and thymine (T) are the pyrimidine bases. The ratio of purines to pyrimidines in DNA is approximately 1:1. In RNA, however, uracil (U) replaces thymine (T) as a pyrimidine base, resulting in a higher proportion of pyrimidines compared to purines.

Biological Significance

Purines and pyrimidines are essential for various biological processes and have significant implications for human health. Mutations or alterations in the structure or function of these bases can lead to genetic disorders and diseases. For example, certain genetic mutations in the purine metabolism pathway can result in conditions like Lesch-Nyhan syndrome or gout.

Pyrimidine analogs, such as cytarabine and 5-fluorouracil, are widely used in cancer chemotherapy due to their ability to inhibit DNA and RNA synthesis, thereby preventing the proliferation of cancer cells. Purine analogs, such as azathioprine and mercaptopurine, are also used as immunosuppressive drugs in organ transplantation and autoimmune diseases.


In summary, purines and pyrimidines are fundamental components of nucleic acids and play vital roles in the storage and transmission of genetic information. While purines consist of a fused pyrimidine and imidazole ring structure, pyrimidines have a single six-membered ring. Purines pair with pyrimidines through hydrogen bonding, forming the complementary base pairs that stabilize the DNA double helix. The occurrence of purines and pyrimidines differs in DNA and RNA, with uracil replacing thymine in RNA. Understanding the attributes of purines and pyrimidines is crucial for unraveling the complexities of genetics and developing targeted therapies for various diseases.

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