2.22 Nitrogenous Bases in the Nucleotide

Nitrogenous bases are the key information-carrying components of nucleotides, which are the building blocks of nucleic acids like DNA and RNA. They are classified into two main groups based on their chemical structure.

Classification of Nitrogenous Bases

There are two major classes of nitrogenous bases, distinguished by their ring structure:

Purines

These are larger, double-ring structures.

• Adenine (A)
• Guanine (G)

Pyrimidines

These are smaller, single-ring structures.

• Cytosine (C)
• Thymine (T) - found exclusively in DNA.
• Uracil (U) - found exclusively in RNA, replacing Thymine.

Components of Nucleotides

A nucleotide is composed of three parts: a nitrogenous base, a pentose sugar (either deoxyribose in DNA or ribose in RNA), and one to three phosphate groups. The nitrogenous base is attached to the C-1 of the pentose sugar.

Nucleotides that are precursors for DNA or RNA synthesis initially have three phosphate groups (e.g., ATP, dGTP). During polymerization into a nucleic acid chain, two of the terminal phosphate groups are cleaved off, providing energy for the reaction.

Nucleosides vs. Nucleotides

The terminology for these molecules changes depending on the components present. A nucleoside consists of a base plus a sugar. A nucleotide consists of a base plus a sugar plus phosphate group(s).

The following table details the different types of nucleosides and nucleotides for both RNA and DNA.

Biological Significance

The specific sequence of these nitrogenous bases in DNA and RNA determines the genetic information of all living organisms, dictating everything from protein synthesis to hereditary traits. This process is central to Cell Division→ and the overall function of Prokaryotic and Eukaryotic Cells→.