Structural and Functional Differences Between Purine and Pyrimidine Bases
What are Purines and Pyrimidines?
The purines and pyrimidines are the building blocks of DNA and RNA that form heterocyclic, aromatic compounds as well as belong from two distinct nitrogenous bases. They have many similarities with the chemical anatomy of the organic compound pyridine (C5H5N) and are also closely related to benzene (C6H6) since here: a nitrogen atom replaces one Carbon atom. Purines and Pyrimidines also serve as the basis for several other chemical compounds like caffeine, thiamine, theobromine, etc. The Purines consist of compounds like adenine and guanine, while the pyrimidines comprise of popular compounds like cytosine and thymine.
Pyrimidines primarily have four carbon atoms and two nitrogen atoms, giving it the shape of a ring, as the Nitrogen atoms take the 1st and 3rd place in the ring. The pyrimidines are easily distinguishable like uracil, uric acid, and barbiturates due to such a distinct structure. The pyrimidines are instrumental in the signalling functions of cells, storing energy in the form of phosphates and enzyme regulation, and creating starch and protein.
Purines, on the other hand, consist of pyrimidines and imidazole rings (also known as a five atom ring with two non-consecutive nitrogen atoms). These have a two-ringed composition with nine atoms overall - five-carbon and four nitrogen atoms. Purines are found in a surplus amount in meat, fishes, and grains, and many other food items like starch and proteins. Being a crucial part of the DNA and RNA structure, they have similar functionalities as Pyrimidines.
Structure of Purine and Pyrimidine
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The purines and pyrimidines both contain active molecules like the ones present in drugs and vitamins. The purines have a melting point of 214 °C (487K), and the pyrimidines have a melting point of 20-22°C (room temperature). The purine's molar mass is 120.11 g mol-1, and for pyrimidines, the molar mass is 80.088 g mol-1. They are each synthesized in the lab via the Traube Purine Synthesis method and Biginelli Reaction, respectively.
These compounds contain hydrogen bonding between each other and, therefore, link both the strands present in the DNA double helix structure and make parallel structures between DNA and RNA. In the case of DNA, the purine adenine bond formation takes place with the pyrimidine thymine, while the purine guanine forms a bond with the pyrimidine cytosine. For RNA, the adenine bonds with uracil and guanine need to bond with cytosine. Therefore, to establish DNA or RNA, equal proportions of purines and pyrimidines is a pre-requisite.
Since for DNA and RNA, several other configurations can occur, including that of the methylated pyrimidines, such structures are called 'wobble pairings' as exceptions to the Watson-Crick pairs found in the purine and pyrimidine bases.
Difference Between Purine and Pyrimidine
Purine Catabolism
Purine yields uric acid as the final product in the human body. Simultaneously, other mammals have enzymes like the urate oxidase that form more soluble allantoin as the final product. The guanine nucleotides get hydrolyzed to that of the nucleoside guanosine and are then introduced to phosphorolysis. Since human nucleotidases aren't hyperactive, the AMP is further deaminase to IMP, which is then degraded to yield hypoxanthine.
Conversion of Base to Uric Acid
The adenine and guanine nucleotides have the common intermediate known as xanthine and form xanthine oxidase. When it occurs in the liver, the guanine is deaminated to release ammonia that is carried as glutamine. The xanthine oxidase is present in large amounts in the liver and intestines.
Pyrimidine Catabolism
The Pyrimidines are the final products of the catabolism between the beta-amino acids and the ammonia and carbon dioxide. The pyrimidines that are synthesized from the nucleic acids, with the help of nucleotidases and the pyrimidine nucleoside phosphorylase, form the four-amino group of cytosine and five-methylcytosine. It releases ammonia and carbon dioxide.
Since the purines and pyrimidines are heterocyclic, they can come together to form several nitrogenous bases. However, since purines are made up of two rings instead of one, they have a heavier molecular weight than that of others. The circular ring structure plays its role in the melting points and solubility of these compounds. The aforementioned ways represent how these molecules are synthesized and broken down differently by the body in different places, as the purines are manufactured in the liver and the pyrimidines in the tissues.
FAQs on Difference Between Purines and Pyrimidines Explained
1. What is the difference between purines and pyrimidines?
The main difference between purines and pyrimidines is that purines have a double-ring structure while pyrimidines have a single-ring structure.
- Purines: Two fused nitrogen-containing rings; larger molecules (e.g., adenine and guanine).
- Pyrimidines: One six-membered nitrogen-containing ring; smaller molecules (e.g., cytosine, thymine, and uracil).
- Both are nitrogenous bases found in DNA and RNA and are essential for genetic information storage.
2. What are purines in DNA and RNA?
Purines are nitrogenous bases with a double-ring structure found in DNA and RNA.
- The two purines are adenine (A) and guanine (G).
- They are components of nucleotides, which form nucleic acids.
- In base pairing, adenine pairs with thymine (in DNA) or uracil (in RNA), and guanine pairs with cytosine.
3. What are pyrimidines in DNA and RNA?
Pyrimidines are nitrogenous bases with a single-ring structure present in DNA and RNA.
- In DNA: cytosine (C) and thymine (T).
- In RNA: cytosine (C) and uracil (U).
- They pair with purines during DNA replication and transcription.
4. Why do purines pair with pyrimidines in DNA?
Purines pair with pyrimidines in DNA to maintain a uniform double-helix width and stable structure.
- A double-ring purine pairs with a single-ring pyrimidine to keep the helix diameter constant.
- Adenine forms two hydrogen bonds with thymine.
- Guanine forms three hydrogen bonds with cytosine.
- This complementary base pairing ensures accurate DNA replication.
5. What are the structural differences between purines and pyrimidines?
The structural difference between purines and pyrimidines lies in the number and arrangement of nitrogen-containing rings.
- Purines: Two fused rings (a six-membered ring joined to a five-membered ring).
- Pyrimidines: One six-membered ring.
- Purines are larger and heavier than pyrimidines.
- Both contain nitrogen atoms that participate in hydrogen bonding.
6. What is the function of purines and pyrimidines?
The primary function of purines and pyrimidines is to store and transmit genetic information as components of DNA and RNA.
- They form the nitrogenous base part of nucleotides.
- Their specific sequence encodes genetic instructions.
- Adenine also forms part of energy molecules like ATP (adenosine triphosphate).
7. How many purines and pyrimidines are found in DNA?
DNA contains two purines and two pyrimidines.
- Purines: Adenine (A) and Guanine (G).
- Pyrimidines: Cytosine (C) and Thymine (T).
- According to Chargaff’s rule, the amount of adenine equals thymine, and guanine equals cytosine.
8. Is uracil a purine or pyrimidine?
Uracil is a pyrimidine found in RNA.
- It has a single-ring structure characteristic of pyrimidines.
- In RNA, uracil replaces thymine.
- Uracil pairs with adenine through two hydrogen bonds.
9. How are purines and pyrimidines synthesized in cells?
Purines and pyrimidines are synthesized through specific metabolic pathways in cells called de novo nucleotide synthesis pathways.
- Purine synthesis: The ring is built step-by-step onto a ribose sugar to form inosine monophosphate (IMP).
- Pyrimidine synthesis: The ring is formed first and then attached to ribose phosphate.
- Cells can also recycle bases through the salvage pathway.
10. What is an easy way to remember purines and pyrimidines?
An easy way to remember purines and pyrimidines is to use simple memory tricks based on their names and structures.
- Purines: "Pure As Gold" → Adenine (A) and Guanine (G).
- Pyrimidines: "Cut the Py" → Cytosine (C), Uracil (U), Thymine (T).
- Purines are larger (double-ring), while pyrimidines are smaller (single-ring).
