Gene mutation

A nucleotide in a DNA molecule is replaced by another nucleotide that has a different base.

For example the base sequence AUGGCUACCUGG, but due to substitution AUGGAUACCUGG, this now means that the aminoo acid sequence may change. 

the original base sequence forms the polypeptide - MetAlaThrTrp 

The new base sequense forms the polypeptide- MetAspThrTrp

When a substitution occurs it only changes one amino acid and not the rest of the polypeptide so the protein may still be functional as it will not completely change the tertiary structure of the protein.

Due to the degenerate code when substitution occurs the same polypeptide may still form.

When a nucleotide is lost from the normal DNA sequence.

A loss of a nucleotide causes a frame shift to everyother nucleotide after it in the sequence, causing all triplets in the sequence to be read differently.

Example- AUGUUUCACGAU , MetPheHisAsp

After deletion occurs- AUGUUCACGAU, MetPheThr- 

This completely changes the tertiary structure of the polypeptide and therefore becomes non-functional