Over a lifetime DNA can undergo changes or mutations in the sequence of bases , A , C , G , and T , resulting in changes in the proteins that are made . Mutations can occur during DNA replication if errors are made and not corrected in time . Mutations can also occur as the result of exposure to environmental factors such as smoking and UV radiation . Mutations contribute to genetic variation within species and can be inherited , particularly if they have a positive effect . Usually , cells can recognize any potentially mutation-causing damage and repair it before it becomes a fixed mutation , however , mutations can sometimes disrupt normal gene activity and cause diseases like cancer . There are several types of genetic mutations :
Insertion – when a base is added to the sequence .
Deletion – when a base is deleted from the sequence .
Point mutation – a change in one base in the DNA sequence .
Substitution – when one or more bases in the sequence are replaced by the same number of bases ( for example , a cytosine substituted for an adenine ).
Inversion – when a segment of a chromosome is reversed end to end .