Nitrogenous Bases - Definition and Structures

The 5 Main Nitrogen Bases

Although there are many nitrogenous bases, the five most important to know are the bases found in DNA and RNA, which are also used as energy carriers in biochemical reactions. These are adenine, guanine, cytosine, thymine, and uracil. Each base has what is known as a complementary base that it binds to exclusively to form DNA and RNA. The complementary bases form the basis for the genetic code.

Let's take a closer look at the individual bases...

Adenine

Adenine and guanine are purines. Adenine is often represented by the capital letter A. In DNA, its complementary base is thymine. The chemical formula of adenine is C​₅H₅N₅. In RNA, adenine forms bonds with uracil.

Adenine and the other bases bond with phosphate groups and either the sugar ribose or 2'-deoxyribose to form nucleotides. The nucleotide names are similar to the base names but have the "-osine" ending for purines (e.g., adenine forms adenosine triphosphate) and "-idine" ending for pyrimidines (e.g., cytosine forms cytidine triphosphate). Nucleotide names specify the number of phosphate groups bound to the molecule: monophosphate, diphosphate, and triphosphate. It is the nucleotides that act as building blocks of DNA and RNA. Hydrogen bonds form between the purine and complementary pyrimidine to form the double helix shape of DNA or act as catalysts in reactions.

Guanine

Guanine is a purine represented by the capital letter G. Its chemical formula is C₅H₅N₅O. In both DNA and RNA, guanine bonds with cytosine. The nucleotide formed by guanine is guanosine.

In the diet, purines are abundant in meat products, particularly from internal organs, such as liver, brains, and kidneys. A smaller amount of purines are found in plants, such as peas, beans, and lentils.

Thymine

Thymine is also known as 5-methyluracil. Thymine is a pyrimidine found in DNA, where it binds to adenine. The symbol for thymine is a capital letter T. Its chemical formula is C₅H₆N₂O₂. Its corresponding nucleotide is thymidine.

Cytosine

Cytosine is represented by the capital letter C. In DNA and RNA, it binds with guanine. Three hydrogen bonds form between cytosine and guanine in the Watson-Crick base pairing to form DNA. The chemical formula of cytosine is C4H4N2O2. The nucleotide formed by cytosine is cytidine.

Uracil

Uracil may be considered to be demethylated thymine. Uracil is represented by the capital letter U. Its chemical formula is C₄H₄N₂O₂. In nucleic acids, it is found in RNA bound to adenine. Uracil forms the nucleotide uridine.

There are many other nitrogenous bases found in nature, plus the molecules may be found incorporated into other compounds. For example, pyrimidine rings are found in thiamine (vitamin B1) and barbituates as well as in nucleotides. Pyrimidines are also found in some meteorites, although their origin is still unknown. Other purines found in nature include xanthine, theobromine, and caffeine.

Review Base Pairing

In DNA the base pairing is:

• A - T
• G - C

In RNA, uracil takes the place of thymine, so the base pairing is:

• A - U
• G - C

The nitrogenous bases are in the interior of the DNA double helix, with the sugars and phosphate portions of each nucleotide forming the backbone of the molecule. When a DNA helix splits, like to transcribe DNA, complementary bases attach to each exposed half so identical copies can be formed. When RNA acts as a template to make DNA, for translation, complementary bases are used to make the DNA molecule using the base sequence.

Because they are complementary to each other, cells require approximately equal amounts of purine and pyrimidines. In order to maintain a balance in a cell, production of both purines and pyrimidines is self-inhibiting. When one is formed, it inhibits the production of more of the same and activates production of its counterpart.