The class of molecules known as purines are all derivatives of the heterocyclic compound purine, which is actually never found in nature. Guanine, shown in the image, is a purine molecule modified by an amino group and a ketone oxygen. The standard purines used in high-energy bonding and DNA/RNA synthesis are guanine and adenine.
Pyrimidines are molecules that are derivatives of pyrimidine. Like purine, pyrimidine is a heterocyclic molecule that is not found in nature. Cytosine, shown in the image, is similar to guanine in that it is a pyrimidine modified with an amino group and a ketone oxygen.
There are two main cellular functions for purines and pyrimidines. First, the purines adenine and guanine and the pyrimidines cytosine, thymine and uracil are all utilized for the production of DNA and RNA. These nitrogenous bases are synthesized linked to a phosphorylated ribose sugar residue, and these nucleoside monophosphates are incorporated into growing strands of new DNA or RNA during replication or transcription. The second function of pyrimidines and purines is short-term energy storage. The most common form of energy in all cells is adenosine triphosphate, or ATP. Release of the third phosphate to produce adenosine diphosphate, or ADP, is an extremely favorable reaction and can drive reactions requiring energy input. Guanine triphosphate and guanine diphosphate are utilized by certain enzymes and receptors as an on/off switch, while cytosine triphosphate and uridine triphosphate are both used in the production of biomolecules.
Purines and pyrimidines used by the cell for nucleotide synthesis (adenine, cytosine, guanine, thymine and uracil) have numerous hydrogen-bonding atoms, such as nitrogen, oxygen and hydrogen. These molecules are designed in a way that cytosine and guanine form three hydrogen bonds, while adenine and either thymine in DNA or uracil in RNA form two hydrogen bonds. During DNA replication, polymerases form A-T and C-G pairs with extremely minute error rates due to their hydrogen-bonding efficiencies. Improper base pairing is detected rapidly due to the inherent instability of incorrect pairs.
Nucleotide triphosphates are common ingredients for various standard laboratory procedures. Polymerase chain reaction (PCR) requires the input of a cocktail of NTPs for the amplification of DNA. ATP can be added to a mixture to drive a desired unfavorable reaction.