What constitutes a nucleoside?

A nucleoside is defined as a molecular structure that consists of two key components: a nitrogenous base and a sugar molecule. The nitrogenous base can be either a purine (such as adenine or guanine) or a pyrimidine (such as cytosine, thymine, or uracil). The sugar component is typically a five-carbon sugar called ribose in ribonucleotides (which form RNA) and deoxyribose in deoxyribonucleotides (which form DNA).

This combination of a nitrogenous base and sugar forms the basic unit of nucleic acids, which are essential for storing and transmitting genetic information in living organisms. When a phosphate group is added to a nucleoside, it becomes a nucleotide, which is an important building block of DNA and RNA, but a nucleoside itself lacks this phosphate group. Hence, the correct identification of a nucleoside hinges on recognizing its structure as comprising a nitrogen base and a sugar.