Carbohydrates are multifunctional compounds that contain several hydroxyl units as well as a ketone, aldehyde, or a carboxyl unit. Carbohydrates are important in mammalian biology, as well as in the biology of plants and insects. Glycosides are molecules in which a sugar is bound to another substructure via a glycosidic bond. Nucleosides are the structural subunit of nucleic acids such as DNA and RNA. Nucleosides have a ribose unit and a pyrimidine or a purine nucleobase. Glycoses are chiral aldehyde-alcohol or chiral ketone-alcohol carbohydrates that are usually monosaccharides. Pentose and hexose derivatives form cyclic hemiacaetals. An amino sugar is a molecule in which a hydroxyl group has been replaced with an amine group.