Nucleosides are the precursor molecules for the monomers (nucleotides) that make up the nucleic acids DNA and RNA. They consist of glycoslyamines made by attaching an organic nucleobase (like adenine, cytosine, guanine, uracil or thymine) to a five-carbon (pentose) ribose sugar ring for RNA or to a deoxyribose ring for DNA. They are converted into nucleotides by the addition of a phosphate group in the process of phosphorylation. They can also be formed by partial hydrolysis of a nucleic acid.
Nucleotides are single monomer units for DNA and RNA molecules consisting of a base nucleoside, adenine, thymine, guanine or cytosine, a ribose or deoxyribose sugar and a phosphate group. Adenine, guanine and cytosine occur in both RNA and DNA. Thymine occurs only in DNA and uracil only in RNA. They are abbreviated A, G, C and T in DNA and A, G, C and U in RNA. Thousands of nucleotides linked together make up a DNA or RNA molecule. The base sequences in DNA and RNA contain the genetic information of a cell. The sequence of the bases within the nucleic acid determines which proteins will be made. The sequence of a DNA or RNA molecule is usually described as the sequence of its bases, e.g. AAAAGTTCGTCTAGGTC. Three nucleotides in a gene form a codon that encodes a single amino acid in a protein sequence. Ribose-containing nucleosides include ribonucleoside monophosphate (NMP), ribonucleoside diphosphate (NDP), and ribonucleoside triphosphate (NTP). When the nucleoside contains the sugar deoxyribose, the nucleotides are called deoxyribonucleoside mono-, di-, or tri-phosphates (dNMP, dNDP, or dNTP. . They also have a variety of physiologic and metabolic functions. For example, nucleotides have important roles in cellular energy transport and transformations (notably ATP and NAD+/NADH), and in enzyme regulation where they are found as individual molecules.