DNA sequences are not immutable. A mutation is any permanent change in the nucleotide sequence of the genome. Mutations can be as small as a single base substitution or as large as the gain or loss of entire chromosomes.

They arise from replication errors, DNA damage, or mobile element activity. Most mutations are neutral or mildly deleterious; a few are beneficial and fuel evolution. In a medical context, mutations that disrupt gene function cause genetic disease or contribute to cancer.

Cells possess elaborate repair systems that correct many lesions before they become permanent. Germline mutations are transmitted to offspring; somatic mutations affect only the individual. Sequencing technologies detect mutations with high sensitivity, enabling diagnosis and research into disease mechanisms.

• Permanent change in DNA sequence
• Ranges from single bases to large rearrangements
• Can be neutral, deleterious, or beneficial
• Source of genetic variation and disease