What Distinguishes Purines from Pyrimidines in DNA and RNA?
The nucleotide bases in DNA and RNA include nitrogenous bases in the form of purines and pyrimidines. Purine bases are adenine and guanine having two carbon-nitrogen rings. On the other hand, pyrimidine bases such as cytosine and thymine have one carbon-nitrogen ring.
Structure of Purine and Pyrimidine
1. Purine
The structure of purine is largely heterocyclic with the aromatic compound comprising four nitrogen atoms. Two carbon rings are also present. These rings are made up of a fusion of imidazole rings and pyrimidine.
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2. Pyrimidine
Pyrimidine is heterocyclic in nature with the aromatic compound only consisting of one carbon ring and two nitrogen atoms.
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This inherent structure of the bases leads to purine and pyrimidine differences.
Nucleobases
Purine composes two out of four nucleobases both in deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) –
Adenine
Guanine
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Pyrimidines consist of the remaining bases in DNA and RNA –
Uracil
Cytosine
Thymine
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Function of Purine and Pyrimidine Bases
1. Purine Catabolism
Owing to the end-product of purine catabolism being uric acid, it has a significant role to play in our body. The urate oxidase enzyme is not present in the human body, and from uric acid, urate is created. The formation of uric acid takes place in the liver and is subsequently discharged with urine through the kidney.
In the human body, monosodium salt and un-dissociated uric acid are the least soluble. Such nature usually does not cause any problem in the human body unless urine has a very high acidic content. The concentration of urate eventually causes the development of gout.
2. Pyrimidine Catabolism
Pyrimidine catabolism leads to the end-product of carbon dioxide, ammonia and beta-amino acids. The beta-amino acid is mostly excreted, otherwise it is incorporated into muscle dipeptides.
Purine vs Pyrimidine
The Differences between Purine and Pyrimidine are the following -
Bonding between Purines and Pyrimidines
Purines pyrimidines are two distinct nitrogenous nucleotide bases. They form the five nucleobases presenting DNA and RNA. Pyrimidine bases are basically heterocyclic compounds that consist of a single carbon ring having two nitrogen atoms and purine bases consist of two carbon things with a greater number of nitrogen atoms. Purines always bond with pyrimidines via hydrogen bond and each bond follows Watson Crick base-pairing rules.
Presence in DNA and RNA
Purines and pyrimidines are heterocyclic aromatic compounds having two and three molecules respectively. They serve as a major component in the production of DNA and RNA. Also, they are useful for the energy production of cells.
In DNA:
DNA is a material that carries information about how any living being will look and function. The major backbone of the production of DNA is the presence of purines and pyrimidines. Purine has two types: adenine and guanine. Pyrimidines have three cytosine uracil and thymine. But In DNA only four types of nitrogen bases are used which are adenine, guanine, cytosine and thymine. Adenine pairs with thymine via two hydrogen bonds whereas guanine pairs with cytosine via 3 hydrogen bonds.
In RNA:
RNA is an important molecule found in the blood cells and it is necessary for living beings. The major backbone of the production of RNA is purines and pyrimidines. Purine has two types and pyrimidine has three types of molecules. But in RNA only four types of molecules are present. RNA is produced with nitrogen bases as adenine, guanine, cytosine and uracil. The difference between them is that they contain sugar fibres whereas DNA contains the different sugar deoxyribose.
Now that you have a basic understanding of the bases as well as the difference between pyrimidine and purine, challenge yourself by solving the following!
Test Yourself
1. Which of the following are nucleotides?
(a) Nitrogen bases + Pentose sugar + Phosphate
(b) Nitrogen bases + Pentose sugar
(c) Purine bases
(d) None of the above
2. _____ is a purine base.
(a) Uracil
(b) Thymine
(c) Cytosine
(d) Adenosine
Solutions:
1.(a) Nitrogen bases + Pentose sugar + Phosphate
2.(d) Adenosine
Did You Know?
The identification of pyrimidine compounds took a long time. Even though its isolation took place somewhere between 1837 and 1864, the recognition of its structures did not come through till 1868.
Quick Summary
Purines and pyrimidines are bases introduction of DNA and RNA.
Purine and pyrimidine have families of Nitrogen bases that make up nucleic acids.
Purine has two rings and pyrimidines have one ring.
DNA consists of purines adenine and guanine and pyrimidines cytosine and thymine.
RNA consists of curing adenine and guanine and pyrimidines cytosine and uracil.
Adenine pairs with thymine by two hydrogen bonds.
Guanine pairs with cytosine via three hydrogen bonds.
Purines are bigger and pyrimidines are smaller.
Purine has four nitrogen atoms and pyrimidines have two nitrogen atoms.
The major difference between purines and pyrimidines is their structure.
Bonding between them happens due to the difference in their size structure.
FAQs on Purines vs Pyrimidines: Core Differences for Students
1. What is the primary structural difference between purines and pyrimidines?
The primary structural difference lies in their chemical composition. Purines possess a two-ringed structure, consisting of a six-membered pyrimidine ring fused to a five-membered imidazole ring. In contrast, pyrimidines are smaller and have only a single six-membered ring. This size and shape difference is fundamental to how they pair up within the DNA double helix.
2. What are the specific examples of purine and pyrimidine bases in nucleic acids?
The nitrogenous bases found in nucleic acids (DNA and RNA) are classified as follows:
- Purines: Adenine (A) and Guanine (G). Both are present in DNA and RNA.
- Pyrimidines: Cytosine (C), Thymine (T), and Uracil (U). Cytosine is found in both DNA and RNA. However, Thymine is exclusively found in DNA, while Uracil is found only in RNA.
3. Why must a purine always pair with a pyrimidine in the DNA double helix?
A purine must always pair with a pyrimidine to maintain the constant diameter of the DNA double helix. This is known as complementary base pairing. If two purines were to pair, the structure would be too wide. If two pyrimidines paired, it would be too narrow. Pairing a larger, two-ring purine with a smaller, single-ring pyrimidine ensures a consistent width, which is essential for the stability of the DNA molecule. This pairing is further stabilized by specific hydrogen bonds: two between Adenine (A) and Thymine (T), and three between Guanine (G) and Cytosine (C).
4. How does the pyrimidine composition differ between a DNA and an RNA molecule?
Both DNA and RNA contain the purines Adenine (A) and Guanine (G), as well as the pyrimidine Cytosine (C). The key difference is in the second pyrimidine base. DNA molecules contain Thymine (T) as the pyrimidine that pairs with Adenine. In RNA molecules, Uracil (U) serves this role instead, pairing with Adenine. Therefore, the presence of Thymine indicates DNA, while the presence of Uracil indicates RNA.
5. Beyond forming DNA and RNA, what are other important functions of purines and pyrimidines?
While their most famous role is as the building blocks of nucleic acids, purines and pyrimidines have other vital functions. Purine derivatives, such as Adenosine Triphosphate (ATP), are the primary energy currency of the cell. They also form parts of essential coenzymes like NAD+ and FAD, which are crucial for metabolic reactions and cellular respiration.
6. If a DNA sample is found to contain 30% Adenine, what are the percentages of the other three bases?
Based on the principle of complementary base pairing (Chargaff's rules), the percentage of Adenine (A) equals the percentage of Thymine (T). Therefore, the sample contains 30% Thymine. Together, A and T make up 60% of the DNA. The remaining 40% consists of Guanine (G) and Cytosine (C). Since G always pairs with C, their amounts are also equal. Thus, the remaining 40% is split evenly between them, meaning the sample contains 20% Guanine and 20% Cytosine.
