Mutations are local changes in the DNA content, caused by inexact replication. These errors in replication occur often due to radiation and other environmental conditions. There are various kinds of mutations. When the wrong base is incorporated instead of the true copy, we say that a substitution has occurred. A substitution may or may not alter the protein sequence, depending on the place it occurs (see the genetic code below). Insertion and deletion are the addition and removal of one or more bases, respectively. Substitutions, as well as insertions or deletions of a single base are called point mutations. In contrast, a rearrangement is a change in the order of complete segments along a chromosome.
Mutations are important to us for several reasons. They are responsible for inherited disorders and other diseases such as cancer, that involve alterations in the genes. At the same time, mutations are the source of phenotypic variation on which natural selection acts, creating species and changing them. For example, the human and the mice genomes are very similar. The major difference between them is the internal order of DNA segments.
When one observes sequences which differ from one another because of mutations, one often wishes to identify these mutations. A complex problem that arises is: