A nucleoside is a basic building block of nucleic acids, which are essential for storing and transmitting genetic information. It consists of two main components: a nitrogenous base and a sugar molecule. The nitrogenous base can be either a purine (like adenine or guanine) or a pyrimidine (like cytosine, thymine, or uracil), while the sugar is typically ribose in RNA and deoxyribose in DNA. Nucleosides play a crucial role in cellular processes, including energy transfer and signaling. When a phosphate group is added to a nucleoside, it becomes a nucleotide, which is the fundamental unit of DNA and RNA. Nucleosides are also important in the development of antiviral and anticancer drugs, as they can mimic natural nucleotides to disrupt viral replication or cancer cell growth.