A nucleotide is the basic subunit of DNA and RNA. There are only 5 distinct nucleotides typically appearing in both molecules, making for unique sequences in a polymer formed by phosphodiester bonds between successive nucleotides. This sequence endows DNA and RNA with the capability of encoding the information essential for life as we know it.
The most well known of the building blocks of nucleic acid, adenosine is probably best identified by its derivative, ATP (Adenosine Triphosphate). This molecule is the principal product of cellular respiration, and acts as a catalyst and substrate for countless chemical reactions throughout the cell. However, its progenitor, adenosine, plays a much more foundational role, providing one of the basic components of the very mechanism that drives creation of proteins, which act as the principal mediators of the cell's structure, signaling pathways, gene expression, respiration, defense, and general homeostasis.
Less well known is guanosine, which also forms a molecule used as a substrate for a subset of reactions in the cell: guanosine triphosphate is used by G-proteins, which are the intracellular effectors of signaling cascades mediated by membrane-bound GPCRs (G-protein coupled receptors).