In molecular biology, transversion refers to the substitution of a purine for a pyrimidine or vice versa. Molecular biology is the study of Biology at a molecular level Purine ( 1) is a heterocyclic Aromatic Organic compound, consisting of a Pyrimidine ring fused to an Imidazole ring Pyrimidine is a Heterocyclic Aromatic Organic compound similar to Benzene and Pyridine, containing two Nitrogen Atoms It can only be reverted by a spontaneous reversion. Because this type of mutation changes the chemical structure dramatically, the consequences of this change tend to be more severe and less common than that of transitions. Not to be confused with the evolutionary concept of a Transitional fossil. Transversions can be caused by ionizing radiation and alkylating agents.