Nucleosides are polymers that have just a base and a .

Chemically altered nucleosides derived from canonical ribo-or deoxyribonucleoside-derivatives of adenosine, cytosine, guanosine, and uridine or thymidine are found in all types of nucleic acids, DNA and RNA. They are particularly abundant in noncoding RNAs, such as transfer RNAs and ribosomal RNA of higher organisms. By increasing the structural diversity of nucleic acids, modified nucleosides play important roles in gene expression and in regulating many aspects of RNA functions. They also contribute to nucleic acid stability and to protection of genetic materials against virus aggression. In this chapter we present a historical overview of the discovery, occurrence, and diversity of the many naturally occurring modified nucleosides that are present in both DNA and RNA of diverse organisms. We also briefly describe the different enzymes that accomplish these nucleic acid ‘decorations’. More information about the structure, function, biosynthesis and evolutionary aspects of selected modified nucleosides in DNA and RNA and their corresponding modification enzymes can be found elsewhere in this volume.

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