A nitrogenous base is a chemical compound that forms the building blocks of nucleotides, which are then the building blocks of nucleic acids like DNA and RNA. Each nucleotide can only fit together within a DNA double helix in one of two ways. Adenine pairs with thymine, and guanine pairs with cytosine. These pairings form the basis for the genetic code in which information is encoded and stored.
Each of the two complementary strands of a DNA molecule are joined by hydrogen bonds between the nitrogenous bases in the center of the molecule. These bases are divided into two groups called purines and pyrimidines: The two-ringed purines adenine and guanine pair with the single-ringed pyrimidines thymine and cytosine. This pairing is called complementary base-pairing. The pair of adenine with thymine and guanine with cytosine forms the famous double-helix shape that is so characteristic of DNA.
The hydrogen bonding between adenine and thymine, or cytosine and guanine is critical because it determines the width of the DNA helix. This pairing also ensures that the DNA helix has a constant length, so that the nitrogenous bases can fit side by side along the DNA molecule.
Nitrogenous bases are also important for other functions of the cell, such as storing and transmitting genetic information. They also serve as the foundation for adenine triphosphate, a molecule that is the cell's universal energy transfer molecule. This molecule provides energy to protein molecules, and it is based on the nitrogenous base adenine.