Mutation is a change in the original DNA sequence of a gene. Mutations come in different forms such as substitution of a nucleotide with another one, deletion of single or a group of nucleotides, insertion of one or more unnecessary nucleotides and so on. Mutations could potentially change the correct codes of the DNA and consequently result in a defective protein structure.
A prevalent form of a mutation is substitution of a nucleotide by another one.
Normal DNA sequence: Intermediate step: Completed mutation:
ATAGGCTAGAATCC ATAAGCTAGAATCC ATAAGCTAGAATCC
TATCCGATCTTAGG TATCCGATCTTAGG TATTCGATCTTAGG
Substitution of an A for G in the original DNA sequence is a mutation and is completed by subsequent substitution of its complement from a C to a T.