Nucleoside

A nucleoside is a unit made up of a deoxyribose sugar and a base bonded together. If the base is a purine, then it is attached to the sugar's C-1 by N-9 or if the base is a pyrimidine then it is attached to the sugar's C-1 by N-1.

There are four nucleoside units in RNA and they are known as adenosine, guanosine, cytidine and uridine. The four nucleoside units in DNA are called deoxyadenosine, deoxyguanosine, deoxycytidine and thymidine. When a nucloside has a phosphate group attached to the C5 of its deoxyribose sugar, it is then called a nucleotide, which is a monomer of nucleic acid