In molecular biology, transversion refers to the substitution of a purine for a pyrimidine or vice versa. It can only be reversed by a spontaneous reversion. Because this type of mutation changes the chemical structure dramatically, the consequences of this change tend to be more drastic than those of transitions. This is because the wobble position of the DNA, which to a large extent is responsible for the degeneracy of the code, is more tolerant of transition than a transversion, such that a transition mutation is more likely to lead to the same amino acid being encoded for. Transversions can be caused by ionizing radiation and alkylating agents.