The mutation described, where a single nucleotide is changed from an A to a G during DNA replication, is called a point mutation. More specifically, this particular type of point mutation is known as a substitution mutation. In this case, the nucleotide adenine (A) is replaced by the nucleotide guanine (G). Substitution mutations can have different effects on the resulting protein or gene function, depending on where the mutation occurs within the gene sequence and the specific amino acid encoded by that codon.