In chemistry, the term heterocycle, or heterocyclic compound, refers to cyclic aromatic compounds, and their alkyl derivatives, that contain nitrogen, oxygen or sulfur atoms within the ringed structure. These contrast with arenes, which only contain carbon atoms in the ring structure. Heterocycles play a large role in the biochemistry of living organisms and in organic chemistry. For example, DNA consists of the heterocyclic purine bases, adenine and guanine, and the heterocyclic pyrimidine bases thymidine and cytodine. Biological sugar molecules, including ribose, glucose and many others, are also heterocyclic chemicals. Other important heterocycles include pyridine, quinoline, indole, imidizole, thiophene, oxazole, thiazole and triazole. The presence of the heteroatoms in heterocycles imparts polarity to the chemicals, as well as acidic or basic properties not present in the parent arene analog.