April 2010
Written by Sigrid Auweter

Nucleotides contain three structural units: a base, a sugar, and one or more phosphate groups.

There are five different bases found in nucleotides: uracil (U), thymine (T), cytosine (C), adenosine (A) and guanine (G). U, T and C are collectively called pyrimidines; A and G are referred to as purines.

Nucleotides can be differentiated by their sugar. Ribonucleotides, the building blocks of RNA, contain a sugar called ribose, while deoxyribonucleotides, the building blocks of DNA, contain deoxyribose.

 Chemical structure of nucleotides. (click on image to enlarge)


Nucleotides can be linked to one another through phosphodiester bonds. A chain of ribonucleotides is called a ribonucleic acid, or RNA. A chain of deoxyribonucleotides is dubbed deoxyribonucleic acid, or DNA. DNA contains the bases A, G, C, and T, while RNA contains U instead of T.

In the cell, nucleotide chains can form base pairs with complementary stretches of DNA or RNA.  Base pairs assemble between A and T, A and U, and G and C. A complementary strand of a nucleic acid chain is an opposite facing strand that contains a sequence of bases that can perfectly pair with the first strand.

 Two complementary strands of DNA. (click on image to enlarge)

DNA is almost always found together with a perfectly complementary strand, i.e., DNA is usually double-stranded. Double-stranded DNA twists into a helical structure, forming the famous double helix. RNA, on the other hand, exists in the cell mainly as a single-stranded molecule, which can, however, fold back onto itself if there are complementary stretches.

 The DNA double helix.
(click on image to enlarge)

 An RNA molecule with complementary stretches. (click on image to enlarge)