Chapter 14: Gene Mutation, DNA Repair, and Transposition

Mutations are defined as alterations in nucleotide sequences and are classified by their molecular nature—point mutations affecting single base pairs, frameshift mutations disrupting the reading frame through insertions or deletions, and chromosomal rearrangements affecting large DNA segments. The functional consequences of mutations vary considerably, producing loss-of-function alleles that reduce or eliminate protein activity, gain-of-function alleles that enhance or alter normal function, and dominant-negative mutations that interfere with wild-type protein. Mutations arise through spontaneous mechanisms including replication errors during DNA synthesis, tautomeric shifts that cause mispairing between bases, and spontaneous base damage, or through exposure to mutagens such as ultraviolet radiation, ionizing radiation, and chemical compounds including alkylating agents and intercalating molecules. The cell maintains genome stability through multiple DNA repair pathways: mismatch repair corrects replication errors, base excision repair removes damaged bases, nucleotide excision repair eliminates larger lesions, and double-strand break repair restores broken DNA molecules. Deficiencies in these repair systems lead to disease, as exemplified by xeroderma pigmentosum, where nucleotide excision repair failure causes extreme photosensitivity and cancer predisposition. The Ames test provides a rapid bacterial assay for identifying mutagenic chemicals. Transposable elements represent a major class of mobile DNA that can relocate within genomes through DNA-based mechanisms or RNA-mediated retrotransposition, causing mutations, altering gene regulation, and contributing to evolutionary diversity. Historical work with maize transposons and Drosophila copia elements, combined with modern evidence from human pathogenic insertions, demonstrates how transposition drives both genetic instability and genome evolution.