What term describes uridine when it is present only in RNA?

Uridine is specifically defined as a nucleoside that consists of the base uracil attached to a ribose sugar. In the context of RNA, when uridine is present, it plays a crucial role in the structure and function of the molecule. Unlike nucleotides, which include a phosphate group in addition to the sugar and base, uridine alone does not contain this phosphate group and is therefore classified as a nucleoside.

While nucleotides are the building blocks of nucleic acids and consist of a nucleoside combined with a phosphate group, uridine is specifically identified as a component of RNA when it exists solely as a nucleoside. The term does not apply to amino acids or base pairs, as these refer to completely different types of biological molecules. Amino acids are the building blocks of proteins, and base pairs refer to the pairs of nitrogenous bases (such as adenine-thymine or guanine-cytosine) that form the rungs of the DNA ladder or the complementing bases in RNA. Thus, describing uridine as a nucleoside accurately captures its role and chemical composition when present in RNA.