Mutations can be spontaneous or induced

It is useful to distinguish between mutations that are spontaneous or induced, based on their causes. Spontaneous mutations are permanent changes in the genetic material that occur without any outside influence. The movement of transposons is an example of spontaneous mutation. Spontaneous mutations can also occur because cellular processes are imperfect, and may occur by several mechanisms:

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Figure 15.4 Spontaneous and Induced Mutations (A) All four nitrogenous bases in DNA exist in both a prevalent (common) form and a rare form. When a base spontaneously forms its rare tautomer, it can pair with a different base. (B) Mutagens such as nitrous acid (HNO2) can induce changes in the bases. (C) The results of both spontaneous and induced mutations are permanent changes in the DNA sequence following replication.

Induced mutations occur when some agent from outside the cell—a mutagen—causes a permanent change in DNA. As we mentioned above, retroviruses can function as mutagens. In addition, certain chemicals and radiation can cause mutations: