Frameshift Mutation & Indels
A frameshift mutation is a genetic alteration caused by an insertion or deletion of nucleotides in numbers not divisible by three, permanently altering the reading frame downstream.
Overview & Biological Context
Because the translational machinery decodes mRNA in non-overlapping triplets of nucleotides, any insertion or deletion (indel) that is not a multiple of 3 (e.g. +1, +2, -1, -2 nucleotides) displaces the reading frame. Downstream of the mutation, every single codon is misread into an incorrect amino acid, almost invariably encountering a premature stop codon within 20 to 50 residues, leading to loss-of-function truncated proteins.
Molecular Mechanism & Decoding
DNA polymerase replication slippage at repetitive mononucleotide runs (e.g., AAAAAA or GGGGGG) is the primary cause of spontaneous frameshift indels. Chemical mutagens such as ethidium bromide or proflavine intercalate between stacked base pairs, physically distorting the double helix and causing the replication machinery to insert or delete extra bases.
Normal vs. +1 Nucleotide Insertion Frameshift Mutation
Key Takeaways & Rules
An indel of exactly 3 or 6 nucleotides adds or removes amino acids without shifting the downstream reading frame (in-frame indel).
Transcripts with frameshift-induced premature stop codons are recognized and degraded by cellular NMD surveillance.
Targeted frameshift mutations are the primary mechanism by which Cas9 creates gene knockouts in research.
Clinical Significance & Laboratory Applications
Frameshifts are prominent drivers of hereditary cancer syndromes and genetic disorders. For example, single-base deletions in BRCA1 (e.g., 185delAG) or BRCA2 trigger premature stop codons, predisposing patients to breast and ovarian cancer. In CRISPR/Cas9 genome editing, non-homologous end joining (NHEJ) repairs double-strand breaks by introducing stochastic frameshift indels, which researchers harness to knock out target genes.
Related Online Analysis Tools
Frequently Asked Questions
Is a frameshift mutation worse than a missense mutation?
Yes, in most cases. A missense mutation alters only a single amino acid while leaving the rest of the protein intact. A frameshift mutation scrambles all downstream amino acids and almost always terminates the polypeptide prematurely, typically causing complete loss of protein function.
Can two frameshift mutations cancel each other out?
Yes. If a +1 insertion is followed shortly downstream by a -1 deletion, the original reading frame is restored. Only the short interval between the two indels will contain mutated amino acids, potentially preserving the overall folding and activity of the protein.