A frameshift mutation is a change in DNA that alters how a cell reads a protein-coding sequence. It usually happens when one or more DNA bases are inserted or deleted, shifting the way the sequence is divided into groups of three. Because those groups determine which amino acids are added to a growing protein, a frameshift can change much of the protein from the mutation onward.
The consequences can be substantial, but they are not automatically harmful. Much depends on where the mutation occurs, how much DNA is inserted or deleted, and whether the affected gene is essential for normal cell function.
How DNA encodes proteins
DNA stores genetic information in a sequence of four chemical bases: adenine (A), thymine (T), cytosine (C), and guanine (G). In protein-coding genes, cells ultimately use this information to determine the sequence of amino acids in a protein.
The genetic code is read in units of three bases called codons. Each codon specifies an amino acid or provides a signal to stop protein production.
For example, a simplified sequence might be read as:
THE CAT ATE THE RAT
If the sequence is instead treated as a continuous series of letters, removing one letter near the beginning changes every three-letter grouping that follows:
THE CTA TET HER AT…
The example is only an illustration of the reading-frame principle. In real DNA, the “letters” are bases, and the codons are interpreted by the cellular machinery that produces proteins.
The key point is that the genetic code is read three bases at a time. Adding or removing a number of bases that is not a multiple of three can therefore change the reading frame.
What makes a mutation a frameshift?
A frameshift generally results from the insertion or deletion of one or more bases when the total number added or removed is not divisible by three.
Suppose a coding sequence contains:
ABC DEF GHI JKL
If one base is inserted near the beginning:
ABX CDE FGH IJK L…
The original grouping has been shifted. Every codon downstream of the insertion may now be interpreted differently.
The same principle applies to a deletion. Removing a single base can cause the codons after the deletion to be regrouped.
By contrast, adding or removing three bases—or six, nine, and so on—does not shift the reading frame. Such a mutation can still matter because it adds or removes one or more amino acids, but it is called an in-frame insertion or deletion, not a frameshift mutation.
Why a frameshift can have large effects
A frameshift can affect a protein in two related ways.
First, it can change the amino acid sequence from the mutation onward. The protein may therefore contain a long stretch of incorrect amino acids rather than the sequence specified by the original gene.
Second, the altered reading frame may encounter a premature stop codon. This can cause protein production to end much earlier than it normally would, producing a shortened protein.
A protein’s function depends on its amino acid sequence and on how that sequence folds into a three-dimensional structure. Changing a substantial portion of the sequence can prevent the protein from folding correctly or from carrying out its normal biological role.
In some cases, cells also recognize messenger RNA containing a premature stop signal and destroy it through a quality-control process called nonsense-mediated mRNA decay. When this happens, little or no abnormal protein may be produced.
The result is that a frameshift can effectively reduce or eliminate the normal activity of a gene.
Where the frameshift occurs matters
Not every frameshift has the same consequences.
A frameshift near the beginning of a protein-coding region has the potential to alter a large portion of the resulting protein. A frameshift closer to the end affects fewer downstream codons, although even a small change near the end can disrupt an important functional region.
The particular gene also matters. Losing the function of a gene that is essential for cell division, DNA repair, development, or another critical process can have very different consequences from changing a gene whose activity is less essential or whose function can be compensated for by other proteins.
Even within the same gene, the exact location of a mutation can influence whether the resulting protein retains any useful activity.
Frameshift mutations can occur naturally
DNA is not perfectly protected from change. Mutations can arise when DNA is copied, repaired, or exposed to certain sources of damage.
Small insertions and deletions can occur when DNA replication machinery makes an error. Repetitive stretches of DNA can be particularly prone to certain insertion and deletion errors because the strands can temporarily misalign during replication.
Cells have DNA-repair systems that detect and correct many such errors. When repair fails to correct a change, however, the alteration can become a permanent mutation in that cell’s descendants.
A mutation can also be inherited if it occurs in a germ cell or in the cells that give rise to eggs or sperm. Mutations that arise in other cells are generally somatic mutations and are not passed from parent to child, although they can be important in diseases such as cancer.
Not every frameshift causes disease
The phrase “frameshift mutation” describes the molecular change, not its medical outcome.
Some frameshift variants have little or no observable effect. This can happen if the affected sequence does not have an important function, if the mutation occurs in a region that does not ultimately affect the protein in a meaningful way, or if the organism has other biological mechanisms that compensate for the change.
Other frameshifts clearly disrupt gene function and can contribute to inherited disorders or other diseases.
Whether a particular variant is harmful therefore requires more information than simply knowing that it is a frameshift. Scientists and clinicians consider the affected gene, the location of the variant, the resulting protein or RNA, biological evidence, and the individual’s genetic and clinical context.
Frameshift mutations and cancer
Frameshift mutations can also occur in tumor cells. If a mutation disrupts a gene involved in controlling cell growth, repairing DNA, or maintaining normal cellular behavior, it may contribute to cancer development.
Tumors can accumulate many genetic changes as cells divide. Some changes help cancer cells survive or multiply, while others are incidental consequences of the genomic instability that can accompany cancer.
Frameshift mutations are therefore important in cancer genetics, both as possible contributors to disease and as clues about which cellular pathways have been disrupted.
How frameshifts differ from other mutations
A frameshift is one type of insertion or deletion mutation. It is useful to distinguish it from other common changes in DNA.
A substitution replaces one base with another. Depending on the codon, this may leave the amino acid unchanged, change one amino acid, or create a premature stop signal.
An in-frame insertion or deletion adds or removes a number of bases divisible by three. The reading frame remains intact, although the protein gains or loses one or more amino acids.
A frameshift insertion or deletion changes the reading frame because the number of bases added or removed is not divisible by three. The altered grouping can affect every downstream codon until a stop signal is reached.
| Mutation type | What happens to the reading frame? | Typical effect |
|---|---|---|
| Base substitution | Usually unchanged | May have no effect or change one amino acid or a stop signal |
| In-frame insertion/deletion | Remains intact | Adds or removes amino acids |
| Frameshift insertion/deletion | Changes | Alters downstream codons and may introduce a premature stop |
These categories describe the molecular event. They do not, by themselves, determine whether a mutation is harmless, harmful, or beneficial.
Why “one base” can make such a difference
The seriousness of a frameshift comes from the structure of the genetic code. Because codons contain three bases, the insertion or deletion of a single base changes the grouping of the entire sequence that follows.
That is fundamentally different from simply changing one base within one codon. A single-base substitution may affect only one amino acid. A one-base insertion or deletion can potentially alter every subsequent amino acid in the protein.
This is why a seemingly tiny change in DNA can produce a much larger molecular effect.
At the same time, the effect is not necessarily catastrophic. The mutation’s position, the gene involved, the resulting RNA and protein, and the biological importance of that gene all determine what happens next. Understanding frameshift mutations therefore requires looking beyond the number of bases changed to how that change affects the information encoded by the gene.
