A classic example of a transition caused by the chemical conversion of one base into another is provided by the effects of nitrous acid. Figure 4.19 shows that nitrous acid per forms an oxidative deamination that converts cytosine into uracil. In the replication cycle following the transition, the U pairs with an A, instead of with the G with which the original C would have paired. So the C.G pair is replaced by a T.A pair when the A pairs with the T in the next replication cycle. Nitrous acid also deaminates adenine, causing the reverse transition from A.T to G.C.