A frameshift mutation is a change in DNA caused by the insertion or deletion of nucleotides that shifts the way a cell reads a gene. When the change occurs in a protein-coding region and the number of inserted or deleted nucleotides is not a multiple of three, it can alter every codon after the mutation. The result may be a severely altered protein or no functional protein at all.
Frameshift mutations are therefore often more disruptive than small DNA changes that leave the reading frame intact. But their effects are not always severe. The consequences depend on where the mutation occurs, how much DNA is inserted or deleted, and whether the altered gene is essential for normal cell function.
How DNA is read to make a protein
Genes contain DNA sequences that can provide instructions for making proteins. During protein synthesis, a cell ultimately reads a protein-coding sequence in groups of three nucleotides called codons. Each codon corresponds to an amino acid or serves as a signal to stop protein production.
For example, consider a simplified sequence:
THE CAT ATE THE RAT
The grouping matters. If one character is removed:
THC ATA TET HER AT...
the grouping changes from that point onward. DNA does not literally use words or letters in this way, but the principle is similar: changing the grouping can change how the entire downstream sequence is interpreted.
A frameshift mutation occurs when nucleotides are inserted or deleted in a number that is not divisible by three. Adding or removing three nucleotides, for example, can add or remove one amino acid without shifting the downstream reading frame. Adding or removing one or two nucleotides shifts the frame.
What makes a mutation a “frameshift”?
The genetic code is read three nucleotides at a time. Suppose a coding sequence contains:
ATG-AAA-GGC-CTT-...
Now imagine that one nucleotide is inserted near the beginning:
ATG-CAA-AGG-CCT-T...
The codons after the insertion have been regrouped. Consequently, the amino acids specified downstream can be completely different.
The same basic effect occurs with a deletion. If one nucleotide is removed, the remaining nucleotides are regrouped into new codons from that point onward.
This is the defining feature of a frameshift: the reading frame has moved.
By contrast, an insertion or deletion of three nucleotides does not shift the frame. It can still affect the resulting protein by adding or removing an amino acid, but the downstream codons retain their original grouping.
Why frameshifts can change an entire protein
A frameshift near the beginning of a protein-coding sequence can alter a large portion of the protein. The new reading frame may specify a very different series of amino acids until the cell encounters a stop codon.
That stop codon may appear earlier than it normally would. When this happens, the protein can be prematurely shortened. A shortened protein may lack regions required for folding, stability, localization, or biological activity.
In some cases, the altered sequence produces a protein that is unstable and rapidly broken down. In others, the mutation can prevent a useful protein from being produced altogether.
The impact is especially significant when the affected gene normally produces a protein that performs an essential role. Losing that function can disrupt cellular processes and, depending on the gene and the organism, contribute to disease.
Where the mutation occurs matters
Not every frameshift has the same consequence. A frameshift in a protein-coding region can be very disruptive, but its effect depends strongly on its location.
A frameshift near the beginning of a coding sequence has the potential to change much of the protein. A frameshift closer to the end may affect fewer amino acids, although even a small change can eliminate an important functional region.
Mutations outside protein-coding regions can have different consequences. Some DNA sequences regulate when and where genes are expressed, while others are transcribed into functional RNA rather than translated into proteins. A small insertion or deletion in one of these regions is not automatically a frameshift, because the term specifically describes a shift in the reading frame of a coding sequence.
How frameshift mutations arise
Insertions and deletions can occur through several mechanisms. DNA replication is not perfectly error-free, and repetitive DNA sequences can make it easier for the copying machinery to slip or misalign. DNA damage and imperfect repair can also produce insertions or deletions.
Frameshift mutations can be inherited if they occur in a germ cell or in a cell lineage that contributes to reproduction. They can also arise after conception, during the lifetime of an individual. A mutation that develops in a particular tissue may affect only some cells rather than the entire body.
Mutations can also be introduced or selected during the development of cancer. If an insertion or deletion disrupts a gene involved in controlling cell growth or DNA repair, it may contribute to the accumulation of additional abnormalities.
Frameshift mutations and genetic disease
A frameshift can cause disease when it substantially reduces or eliminates the normal function of an important gene. This is one reason genetic testing sometimes identifies insertions or deletions as potentially disease-causing variants.
The relationship is not automatic, however. A DNA change must be interpreted in its biological context. The same general type of mutation can have very different consequences depending on the gene, the exact location of the change, and how the resulting RNA or protein is handled by the cell.
Some genes can tolerate loss of one copy, while others cannot. In some cases, a person can carry a damaging variant without developing disease because another copy of the gene provides sufficient function. In other situations, loss of gene function can have a major effect.
What happens to the RNA after a frameshift?
The consequences of a frameshift can begin before a protein is ever made. Cells have quality-control mechanisms that monitor messenger RNA, or mRNA, the molecule that carries protein-making instructions from DNA.
A frameshift that creates a premature stop signal can cause the affected mRNA to be recognized as abnormal. One important mechanism, called nonsense-mediated mRNA decay, can reduce the amount of that abnormal message available for protein production.
This means a frameshift mutation can sometimes result in very little abnormal protein rather than simply producing a full-length protein with an altered section.
However, the details depend on the mutation and the gene. Not every premature stop triggers the same response, and some altered messages can still produce abnormal proteins.
Frameshift versus other small mutations
Frameshift mutations are one type of small insertion or deletion, often abbreviated indel. They are different from substitutions, in which one nucleotide is replaced by another.
A substitution can be:
- Silent, meaning it does not change the encoded amino acid.
- Missense, meaning it changes one amino acid to another.
- Nonsense, meaning it changes a codon into a stop signal.
An insertion or deletion involving a number of nucleotides divisible by three is often called in-frame because it preserves the downstream reading frame. It may add or remove one or more amino acids without changing all subsequent codons.
A non-multiple-of-three insertion or deletion causes a frameshift and can alter the downstream sequence extensively.
This distinction helps explain why the simple size of a DNA mutation does not necessarily predict its biological importance. A one-nucleotide deletion can be more disruptive than a three-nucleotide deletion because the first can change the reading frame while the second does not.
Can a frameshift ever have little effect?
Yes. The term describes what happens to the reading frame, not the severity of the biological outcome.
A frameshift in a region that has little or no functional importance may have a limited effect. Some mutations occur in parts of genes that are not critical for the resulting protein’s function. Others may occur in genetic contexts where the altered sequence is not translated into a functional protein.
Even within a coding region, the outcome depends on the exact position and the cell’s response to the altered RNA. Scientists therefore do not determine whether a variant is harmful solely by seeing the word “frameshift” in a genetic test report.
The important question is what that particular change does to the particular gene and its biological function.
Why the reading frame is so important
The seriousness of a frameshift comes from the structure of the genetic code itself. Because codons are read in groups of three, inserting or deleting one or two nucleotides changes the grouping of all subsequent nucleotides.
That can simultaneously change many amino acids and introduce an early stop signal. The resulting protein may be radically different, truncated, unstable, or absent.
Inserting or deleting three nucleotides avoids that wholesale regrouping. The difference between three and two nucleotides may therefore be biologically much larger than the numerical difference suggests.
Frameshift mutations are serious when they disrupt a gene’s ability to produce a functional product. Understanding the reading frame makes clear why a seemingly tiny change in DNA can sometimes have effects that extend through much of a protein—and why the precise location and context of the mutation ultimately determine what happens.
