Which type of mutation changes one DNA base into another?

A base substitution changes one DNA base into another.

A substitution is a type of point mutation involving one nucleotide position. Depending on the affected codon and the genetic code, it can be silent, missense, or nonsense. A silent substitution leaves the encoded amino acid unchanged; a missense substitution changes an amino acid; and a nonsense substitution creates a stop codon.

Substitutions can arise from copying errors, chemical damage, radiation, or imperfect DNA repair. They may have no detectable effect, alter a protein, or contribute to disease, depending on where they occur and how the change affects gene function.

A substitution is different from an insertion or deletion, which adds or removes DNA. Insertions and deletions can cause a frameshift when their length is not a multiple of three, changing the reading frame of downstream codons.