What type of mutation occurs when a single base is added or deleted from the DNA sequence?

A mutation that involves the addition or deletion of a single base pair in the DNA sequence is classified as a frameshift mutation. This type of mutation significantly alters the reading frame of the gene during translation, potentially leading to a completely different amino acid sequence downstream of the mutation. When a base is added or deleted, the entire sequence of codons changes from that point onward, which can result in a nonfunctional protein or one with altered functionality, depending on where the mutation occurs.

Frameshift mutations can have dramatic effects on an organism because they can disrupt the normal function of proteins, possibly leading to various diseases or phenotypic changes. The other types of mutations listed, such as missense, nonsense, and silent mutations, involve different mechanisms or effects on the protein formed. Missense mutations change one amino acid in the protein, nonsense mutations introduce a premature stop codon, and silent mutations result in a codon that still codes for the same amino acid, thus generally not affecting the protein's function at all.