Purines and Pyrimidines are the most important building blocks of DNA and RNA. They are the nitrogenous bases that make up the nucleic acids.
Purines are made up of a six-membered and a five-membered nitrogen-containing ring, which are fused together.
- Adenine—6-amino purine
- Guanine—2-amino-6-oxy urine
- Hypoxanthine—6-oxy purine
- Xanthine—2,6-dioxy purine
Adenine and Guanine are found both in DNA and RNA. As Hypoxanthine and Xanthine are synthesized, they are not included in nucleic acids.
Pyrimidines have only a six-membered nitrogen-containing ring.
- Uracil—2,4dioxy pyrimidine
- Thymine—2,4-dioxy-5-methyl pyrimidine
- Cytosine—2-oxy-4-amino pyrimidine
- Orotic acid—2,4-dioxy-6-carboxy pyrimidine
Cytosine is present in both DNA and RNA. Uracil is found only in RNA and sometimes tRNA contain some Thymine along with Uracil. Thymine is normally found in DNA.
Every DNA strand has a backbone, made up of a sugar-phosphate chain. A nitrogenous base, composed of carbon and nitrogen rings, is attached to each one of these sugars. The number of rings of the attached base determines whether the base is a purine (two rings) or a pyrimidine (one ring).
To hold the two strands together, a hydrogen bond is formed by the purines on one strand of DNA with the corresponding pyrimidine available on the opposite DNA strand, and vice versa. This is called base pairing.
This is the most important function of purines and pyrimidines, within the DNA molecules. This hydrogen bonding is not as strong as a covalent bond, therefore, this base-pairing easily separate to allow transcription and replication.
One strand of DNA is always an exact complement of the other as far as purines and pyrimidines go. The reason for this is, purines always bind with pyrimidines, and this is called complementary pairing. Within a DNA molecule the ratio of these two will always be constant. This phenomenon is called Chargaff’s Rule, named after Irwin Chargaff, who noticed this for the first time.
It is always constant which purines pair with which pyrimidines, as is the number of hydrogen bonds between them:
- Adenine pairs with Thymine with two hydrogen bonds.
- Guanine pairs with Cytosine with three hydrogen bonds.
There is a way to remember this pairing. The letters made up of straight lines (A and T) are paired together, whereas the letters made up of curves (G and C) go together.
In a DNA molecule, the number of adenines is always equal to the number of Thymines.
The same is the case with Guanines and Cytosines. Therefore, if the percentage of one nitrogen base is known, within a DNA molecule, then the percentages of each of the other three can be figured out very easily. The complementary pair will have the same percentage, and the other two bases will each be the sum of the first pair subtracted from 100% and divided by two.
Purines vs. Pyrimidines
In order to identify the main differences between purines and pyrimidines,remember the three Ss’: structure, size and source. The very basics are in the table below:
|Structure||Double carbon-nitrogen ring with four nitrogen atoms||Single carbon-nitrogen ring with two nitrogen atoms|
|Source||Adenine and Guanine inn both DNA and RNA||Cytosine in both DNA and RNA Uracil only in RNA Thymine only in DNA|
The most important difference between the two is in their structures.
The purines have a two-ringed structure consisting of a nine-membered molecule with four nitrogen atoms, as you can see in the figures below.
Chemical Structure of Adenine
Structure of Guanine
The pyrimidines have only one single ring, which has just six members and two nitrogen atoms.
Purines are bigger than pyrimidines as they are pyrimidines fused with a second ring. This size difference is one of the reasons that complementary pairing occurs. If the purines bonded together, they would be so big that pyrimidines would not be able to reach other pyrimidines or purines on the other side. Likewise, if the pyrimidines bonded together, there would not be enough space for the purines. This will make the DNA strand very weak as the spaces between the two strands will be very large.