A mutation is a change in the nucleotide base sequence on a DNA molecule.

The most common form is the point mutation, a change in a single nucleotide on on side or the other of the molecule.

Common point mutations include base pair insertion, deletion or substitution.

Purine for purine substitution (A to G or G to A)is called transition.  Purine to pyrimidine or vice-versa (A to C or T, G to C or T) is called a transversion.

Insertions and deletions can cause frameshift mutations that cause the sequence of amino acids in the final polypeptide product to be different, thus producing different proteins or nonfunctional polypeptides.

If the point mutation has no effect on the polypeptide, it is called a silent mutation.  This occurs due to the redundancy of the genetic code.

Inversion, duplication and transposition of multiple nucleotides are called gross (large-scale) mutations.

The above mutations can occur both in prokaryote and eukaryote cells.  Eukaryote cells can also undergo chromosomal mutations.  The most common chromosomal mutation is nondisjunction, a failure of chromosomes to separate during meiotic division.  One of the daughter cells form after this mutation will have one less chromosome that the other.  If these are human cells that mature into gametes and participate in fertilization, the resulting offspring will have a genetic disorder such as Down's or Turner's Syndrome.  In plants, nondisjunction can result in changes such as the oversized fruit of cultivated strawberries (natural strawberry fruits are small and bitter).