Final answer:
Frameshift mutations are caused by the insertion or deletion of one or more nucleotides, which shifts the reading frame of the DNA sequence and leads to nonfunctional proteins.
Step-by-step explanation:
A frameshift mutation is caused by the insertion or deletion of one or more nucleotides, which changes the reading frame of the base sequence. When the number of nucleotides inserted or deleted is not a multiple of three, a frameshift occurs. This results in the ribosome reading the mRNA incorrectly, leading to a change in every amino acid following the mutation and potential inclusion of a premature stop codon. Proteins produced from genes with frameshift mutations are usually nonfunctional.