In an induced mutation, factors outside the cell - so-called mutagens - cause the mutations.
For example, chemical mutagens, such as so-called base analogues, can be incorrectly incorporated into the base sequence of the DNA, leading to point mutations. Certain chemical substances cause biochemical changes in the bases; if this creates a "gap" in the DNA strand, it can then be filled by an incorrect base.
Physical mutagens are, for example, UV light or X-rays. Here, bases can fuse with each other and be damaged, so that the structure and sequence of the DNA are changed. Special viruses can also induce gene mutations.