point mutation

A point mutation is the smallest possible spelling error in DNA: a change at a single position, affecting just one nucleotide. If the genome is a long sentence, a point mutation is one letter typed wrong while every other letter stays put.

Point mutations come in a few flavours. A base may be swapped for another, called a substitution; a single base may be inserted; or a single base may be deleted. Substitutions are the most common kind and are further classified by their effect on the protein the gene encodes — they may leave the amino acid unchanged, change it, or introduce a stop signal.

Although a point mutation touches only one letter, its consequences are not always small. In sickle-cell disease a single substitution in the beta-globin gene changes one amino acid and reshapes red blood cells. Yet the very same kind of change elsewhere in the genome may have no effect at all, which is why context matters as much as size.