The natural mutation rate for cells is about 10-6, meaning that a mutation may occur spontaneously once every one million times a DNA molecule is replicated.  In terms of spontaneous mutations in base pairs, the rate is about 10-10 (one in ten billion) and in genes along a chromosome, there are normally about 10-7 (one in 10 million).  Mutagenic agents increase the rate at which such mutations occur.

Most mutations are harmful to the cell.  If these result in the death of the cell they are called lethal mutations.

Beneficial mutations give the cell characteristics others of its type do not have, allowing it to survive in changing environments or make use of nutrients others of its type cannot.  The accumulation of beneficial mutations results in the evolution of new species.