What type of mutation occurs when the number of nucleotides inserted or deleted is not a multiple of three?

A frameshift mutation occurs when nucleotides are inserted into or deleted from a DNA sequence, and the total number of nucleotides affected is not a multiple of three. This shift alters the reading frame of the genetic code, changing how the ribosomes read the sequence during translation. As a result, the amino acids encoded downstream of the mutation can become completely different from what they were originally. This can lead to significant changes in the resulting protein, often resulting in a nonfunctional protein.

In contrast, point mutations involve a change in a single nucleotide that may only affect one amino acid or none at all (silent mutations), but they do not shift the reading frame. Missense mutations are a specific type of point mutation leading to the incorporation of a different amino acid. Only frameshift mutations lead to alterations in the reading frame due to an inappropriate number of nucleotides being inserted or deleted.