Introduction
Purines and pyrimidines are the building blocks/ nitrogenous bases that makeup DNA and RNA. The sugar-phosphate chain forms the backbone of a DNA strand. Nitrogenous bases form as attachments to these chains; if the number of rings that make up the base is two, it is a purine. On the other hand, it is a pyrimidine if the base has only one ring. To perform several cellular activities efficiently, a cell needs purines and pyrimidines in similar quantities.
German chemist Emil Fischer coined the term ‘Purine’ in 1898. Purines inhibit the enzymes (catalysts for a bio-chemical reaction) required for purine production and activate the enzymes necessary for pyrimidine formation. Similarly, pyrimidines inhibit enzymes needed for pyrimidine creation and activate those essential for purine production. Therefore, at any given time, the purines and pyrimidines in a cell will be approximately equal. A cell’s DNA sequence alters if the enzymes that are responsible for purine production break down.
The bonding of purines and pyrimidines is essential for the efficient functioning of DNA and RNA. According to Chargaff’s rule, the ratio of purines and pyrimidines should be 1:1. Moreover, the same levels of adenine and thymine, as well as guanine and cytosine should be present in an organism’s DNA. Therefore, in DNA, it is possible to calculate the percentage of each complementary pair and the bases that form it.
Purines Vs. Pyrimidines
Purines are nitrogen bases with a double-ringed structure consisting of five carbon and four nitrogen atoms. Pyrimidines are nitrogen bases with a single-ring structure composed of four carbon and two nitrogen atoms. The elements of purine are adenine and guanine in DNA and RNA, whereas the elements of pyrimidine differ in DNA and RNA (though cytosine is common, DNA has thymine, and RNA has uracil).
Difference Between Purines And Pyrimidines In Tabular Form
| Parameters Of Comparison | Purines | Pyrimidines |
| Structure | Purines have a double-ring structure. The double rings are a fusion of a pyrimidine ring and an imidazole ring. | Pyrimidines are single-ringed structures. |
| Types | Adenine and Guanine are the purines present in DNA and RNA. | Cytosine and thymine are the pyrimidines present in DNA. Uracil is found in RNA instead of thymine. |
| Base pairing | Adenine pairs with thymine in DNA with two hydrogen bonds. | Cytosine pairs with guanine in DNA with three hydrogen bonds. |
| Size | Purines are larger than pyrimidines because of their double-ringed structure. | Pyrimidines are smaller than purines. |
| Nitrogen atoms | Purines have four nitrogen atoms. | Pyrimidines have two nitrogen atoms. |
| Carbon atoms | Purines have five carbon atoms. | Pyrimidines possess four carbon atoms. |
| Biosynthesis takes place in | The liver is the primary synthesizer/producer of purines in the human body. | Varieties of tissues produce pyrimidines. |
What Are Purines?
Purines are heterocyclic (compounds with atoms of at least two different elements) organic compounds. They are the most common nitrogen-containing heterocycles. Adenine and Guanine form a hydrogen bond with pyrimidines like cytosine, thymine, or uracil (complementary base pairing). These hydrogen bonds are crucial for stabilizing the double helix structure formed by DNA strands. Hydrogen bonds can be broken/pulled apart easily; therefore, the two strands of DNA form into two single-stranded DNA molecules when melted. To remember the purines in DNA, the mnemonic ‘Pure As Gold’ (purines are adenine and guanine) is used. Some other well-known purines are xanthine, hypoxanthine, caffeine, and isoguanine.
Purines may function as neurotransmitters [molecules that send signals that affect another cell across a synapse (a junction/gap between two nerve cells)]. Apart from DNA and RNA, purines are significant components in ATP, GTP, and NADH. Purines are not found in nature but can be created through organic synthesis (intentional construction of organic compounds). Emil Fischer used Uric acid to start a set of reactions that resulted in purine. The uric acid was reacted with PCI5, HI, PH4I, and finally zinc dust to synthesize purine.
Moreover, high levels of purines are present in meat and meat products (especially liver and kidney), anchovies, sardines, sweetbreads, yeast, gravy, and so on. Moderate levels of purine are found in pork, fish, green peas, lentils, mushrooms, wheat bran, and so on. Plants generally have low levels of purine; however, some plants and algae like legumes and spirulina have high purine concentrations.
All organisms belonging to the three domains (eukaryotes, bacteria, and archaea) can biosynthesize purines. High consumption of meat and seafood is directly proportional to the increased risk of gout. However, consuming a moderate amount of purine-rich vegetables does not result in an increased risk of gout.
Purines Present In DNA And RNA
Purine metabolism results in the formation of adenine and guanine. These two purines are found in DNA and RNA.
Adenine
Albrecht Kossel used the term ‘adenine’ in reference to the pancreas from which it was extracted in 1885. It is derived from the nucleotide inosine monophosphate. Adenine can be formed using the Formadide method or its modified form. Formamide is heated under 120 degree Celsius within a sealed flask for approximately 5 hours. Acid catalysts like phosphorus oxychloride or phosphorus pentachloride and sunlight are used to increase the intensity of the reaction. After the solution cools down, water is added to the flask containing formamide and adenine (the result of the reaction).
The water-formamide-adenine solution is filtered using activated charcoal. The water and formamide molecules pass through the filter, as they are small. The adenine molecules are too large to filter through and so get attached to the charcoal. Aqua ammonia is used to separate the activated charcoal and adenine. The solution containing water, ammonia, and adenine is left to dry. Adenine crystallizes into pure white powder after the loss of ammonia.
Earlier, adenine was referred to as Vitamin B4. However, as the body synthesizes it, and it is not necessary to acquire it through diet, it is no longer considered a part of Vitamin B. Nevertheless, niacin and riboflavin (types of Vitamin B) bind with adenine to form NAD and FAD. Adenine pairs with thymine in DNA and uracil in RNA.
Guanine
The chemical formula of guanine is C5H5N5O. A German chemist Julius Bodo Unger first isolated it in 1844; however, it was Emil Fischer who determined its structure and proved that uric acid can be converted into guanine. It pairs with cytosine in DNA and RNA. Guanine can be synthesized using the Fischer-Tropsch method. An equimolar (same number of atoms) gas mixture of carbon dioxide, hydrogen, and NH3 is heated to 700 degrees Celsius for 15 – 24 minutes and left to cool. The mixture is reheated to 100 – 200 degrees Celsius for 16 – 44 hours using an alumina catalyst. This process results in the formation of guanine and uracil.
Guanine is present in the iridocytes (specialized skin cells that produce color) of fish. Also, it is found in reflective deposits of the eyes of some reptiles such as crocodiles and deep-sea fish (fish that live in darkness/water zone not lit by the sun’s rays) such as lantern fish. Mr. Jaquin extracted crystalline guanine from the Alburnus alburnus’ (a fish) scale in 1656. The extracted substance was mistaken as pearl essence.
Crystalized guanine is indispensable in the cosmetic industry. It is added to shampoos, nail polish, eye shadows, metallic paints, and simulated pearls to create a shimmering iridescent (color seems to change with the change in angle) effect. Mica (silicate minerals), faux pearls, and aluminum or bronze particles are used as alternatives to guanine. Spiders and scorpions convert ammonia into guanine, as they can excrete it with minimal water loss.
What are Pyrimidines?
The term pyrimidine was first proposed by Pinner in 1884. Pyrimidines, like purines, are heterocyclic organic compounds. However, they have only one ring. The pyrimidines in DNA and RNA are remembered using the mnemonic ‘CUT the PY’ (Cytosine, Uracil, Thymine – Pyrimidines). They aid in cell signaling and energy storage in DNA or RNA molecule. The basic rule of purine and pyrimidine pairing is that adenine pairs with thymine, whereas guanine pairs with cytosine (Watson-Crick base pairing rule). Sometimes, purines may pair with pyrimidines in contrast to the traditional complimentary base pairing. This phenomenon is known as wobble pairing.
Chargaff’s rule makes it pretty simple to calculate the percentage of the four nucleobases present in a DNA molecule. As the ratio of purines and pyrimidines is equal, the other three nucleobases can be calculated if the value or percentage of one nucleobase is known. For example, the number of adenine in DNA is equal to the number of thymine in it. The sum of this base pair is equal to the sum of the other base pair. Therefore, the percentage of guanine or cytosine is calculated by dividing the sum by two. However, in some rare cases, uracil is found in DNA instead of thymine. In that case, the rule does not work.
Hydrogen bonding between a pyrimidine and another pyrimidine is not possible, as the distance between the molecules is too wide. Similarly, purine-purine pairing is not good, as the molecules will be too close to each other; such proximity leads to overlap repulsion. Hydrogen bonds weaken, and the DNA strands supported by the base pairing will separate when the temperature reaches the melting point (20 – 22 degree Celsius). The melting point largely depends on the molecules’ lengths and the GC (guanine-cytosine) content. The higher the GC content, the higher the melting point, and vice versa.
Pyrimidines In DNA And RNA
Two of the four nucleobases present in DNA and RNA are pyrimidines. Cytosine is present in both nucleic acids, whereas DNA has thymine, and RNA has uracil.
Cytosine
Albrecht Kossel discovered cytosine in 1894. He proposed a structure in 1903 and synthesized it in the same year. A cytosine pairs with guanine in DNA and RNA. However, it is an unstable element and may change into uracil (another pyrimidine) through catabolism/spontaneous deamination. NASA found cytosine in meteorites on October 2021 when it used novel means of extraction so as to not damage the nucleotides present in the meteorites. It is believed that cytosine did not persist in meteorites due to deamination into uracil. The chemical formula of cytosine is C4H5N3O.
Thymine
Albrecht Kossel isolated thymine in 1893 from the thymus glands (the reason this pyrimidine was named thymine). Excess or deficient thymine in DNA leads to mutation. The mutations occur predominantly when thymine pairs with adenine in DNA. Methylisothiourea was reacted with ethyl formyl propionate, and hydrolysis was done on the resulting intermediate to obtain thymine. Many other easy preparation methods have also been developed throughout the years. The chemical formula of thymine is C5H6N2O2. Rarely, thymine occurs in RNA due to the breakdown of uracil.
Uracil
Uracil binds/pairs with adenine in RNA. Demethylated uracil results in thymine. A German chemist Robert Behrend coined the term ‘uracil’ when he discovered it while attempting to synthesize derivatives of uric acid. However, Albert Ascoli originally discovered it in the 1900s. Uracil has the ability to absorb light. Uracil is used to determine contamination in tomatoes; the presence of uracil in tomatoes indicates that lactic acid bacteria have contaminated them. Uracil derivatives are used to manufacture pesticides and weed killers. The chemical formula of uracil is C4H4N2O2.
Main Difference Between Purines And Pyrimidines In Points
- Purines have two carbon-nitrogen rings, whereas pyrimidines have only one carbon-nitrogen ring.
- Purine is larger than pyrimidine, as it is a fusion of pyrimidine and imidazole rings and contains five carbon atoms and four nitrogen atoms. A pyrimidine contains four carbon and two nitrogen atoms.
- Uric acid is the resulting product of purine catabolism (molecule breakdown), whereas ammonia and carbon dioxide are derived at the end of pyrimidine catabolism.
- A purine’s melting point is 214 degree Celsius. On the other hand, a pyrimidine’s melting point is 20 – 22 degree Celsius.
- The chemical formula of a purine is C5H4N4, whereas a pyrimidine’s chemical formula is C4H4N2.
Conclusion
The most significant difference between purines and pyrimidines is their structure, size, and presence in DNA and RNA. Other than that, they are similar to each other, as they aid in energy storage, protein synthesis, cell signaling, and so on. These nitrogenous bases provided structural support to DNA and RNA. If these bases melt (separate) or pair wrongly, they would not be able to offer the necessary support.
