Strand slippage is a replication error where a newly made DNA strand briefly detaches from its template and reattaches in the wrong position, causing bases to be added or lost. It happens most often in repetitive stretches of DNA, where identical sequences make it easy for the strands to misalign. This seemingly small copying mistake is responsible for a range of consequences, from harmless genetic variation to serious diseases like Huntington’s and certain cancers.
How Strand Slippage Happens
During normal DNA replication, a protein called DNA polymerase moves along one strand (the template) while building a matching new strand base by base. The process is usually smooth, but repetitive sequences create a vulnerability. When the polymerase hits a stretch where the same short pattern repeats many times, it can pause and detach from the DNA entirely.
Once the polymerase falls off, the freshly built end of the new strand separates from the template briefly, a natural process sometimes called “breathing.” In non-repetitive DNA, there’s only one place where the new strand can reattach because the sequence is unique. But in a repetitive region, multiple positions look identical. The new strand can slide forward or backward and lock onto a different copy of the repeat, thinking it’s back where it started. When the polymerase returns and resumes copying, the misalignment is baked in.
Researchers studying this process in detail found that slippage involves three distinct steps: the polymerase pauses within a repeated sequence, it dissociates from the DNA, and then the tip of the new strand re-anneals at the wrong repeat. That final re-annealing step appears to be the bottleneck. If the new strand can’t find and lock onto a misaligned repeat quickly, the polymerase may simply return to the correct position and no error occurs.
Why Direction Matters: Insertions vs. Deletions
The direction of slippage determines whether the final DNA ends up with extra bases or missing ones. If the new strand loops out (slips backward relative to the template), a segment of the template gets copied twice, producing an insertion. If the template strand loops out instead (the new strand slips forward, skipping a section), bases are lost, producing a deletion.
In lab experiments, most polymerases show a preference for producing deletions rather than insertions. This bias toward contractions has been observed across several types of DNA polymerase. Single-base deletions are especially common: when a template base unstacks (flips out of its normal position), it changes which base is presented at the polymerase’s active site, increasing the chance that one nucleotide simply gets skipped.
These insertions and deletions, collectively called “indels,” can be devastating if they occur in a gene. Because proteins are encoded in groups of three DNA bases, adding or removing one or two bases shifts the entire reading frame downstream. Every amino acid after the error is wrong, and the resulting protein is usually nonfunctional.
Repetitive DNA Is the Weak Spot
Not all DNA is equally vulnerable to slippage. The risk scales with two factors: the number of repeats and the length of the repetitive tract. A stretch of DNA containing eight consecutive thymine bases is far more prone to slippage than a stretch with only three. Longer alleles with higher repeat counts are measurably less stable than shorter ones.
Microsatellites, short repeating sequences scattered throughout the genome (patterns like CA-CA-CA or ATT-ATT-ATT), are particularly susceptible. These sequences are found in every human genome and are normally harmless. But their repetitive nature makes them hotspots for slippage. In one set of experiments using a polymerase without proofreading ability, tracts with just three repeats showed a 160-fold increase in mutations compared to non-repetitive DNA. Error rates climbed further as repeat length increased, consistent with the idea that longer repeats give the new strand more wrong positions to land on.
Polymerase processivity also plays a role. Processivity describes how far a polymerase can travel along the template before falling off. Low-processivity polymerases dissociate more frequently, creating more opportunities for the new strand to misalign. In experiments with T7 DNA polymerase stripped of its processivity-enhancing partner protein, the error rate for frameshift mutations in repeated sequences jumped 46-fold.
How Cells Catch and Fix Slippage Errors
Cells have a dedicated cleanup system for slippage mistakes called mismatch repair (MMR). After replication, MMR proteins scan newly copied DNA for errors, including the small loops that form when an insertion or deletion has occurred. The system is remarkably effective: an estimated 99.8% of single-base insertion/deletion mismatches are corrected by MMR before they become permanent mutations. By comparison, MMR catches about 97% of simple base-pair mismatches (where one letter is swapped for another).
The polymerase itself also has a first line of defense. Many replicative polymerases carry a built-in proofreading function that can detect and remove a misplaced base immediately after it’s added. Proofreading is less effective against slippage-type errors than against simple substitutions, but it still helps. In experiments with T7 polymerase, eight-repeat tracts were about seven times more stable when the enzyme retained its proofreading ability.
When Repair Fails: Cancer and Microsatellite Instability
When the mismatch repair system itself is broken, slippage errors accumulate rapidly across the genome. This condition is called microsatellite instability (MSI), and it’s a hallmark of certain cancers, particularly colorectal cancer.
MSI arises through two main routes. In Lynch syndrome, an inherited condition accounting for roughly 3% of all colorectal cancers, a person is born with a mutation in one copy of an MMR gene. When the second copy is lost through a later mutation or silencing event, the repair system fails entirely. The other route is non-inherited: the MMR gene MLH1 can be shut down by chemical modifications to its promoter region, a process called methylation. Both paths lead to the same outcome, a flood of uncorrected slippage errors that drive tumor development.
Colorectal cancers with MSI tend to appear in the right side of the colon, are often poorly differentiated or produce mucus, and typically contain high numbers of immune cells infiltrating the tumor. That last feature turns out to be clinically important: the abnormal proteins generated by frameshift mutations act as flags for the immune system, which may explain why MSI-high cancers often respond well to immunotherapy.
Trinucleotide Repeat Diseases
Some of the most dramatic consequences of strand slippage involve trinucleotide repeats, three-letter DNA sequences that expand from one generation to the next. More than 30 neuromuscular and neurodegenerative diseases are caused by these expansions, including Huntington’s disease, fragile X syndrome, and myotonic dystrophy (the most common form of adult-onset muscular dystrophy).
In these disorders, a trinucleotide repeat that is normally present in moderate copy number gradually grows longer through successive rounds of slippage. Once the repeat crosses a critical length threshold, it disrupts the gene’s function. The expanded repeats can also form unusual DNA structures, like hairpins and loops, that further promote slippage during replication. This creates a vicious cycle: longer repeats are less stable, so they expand faster, a phenomenon called “anticipation” that explains why these diseases often worsen across generations.
Slipped-strand DNA structures were first hypothesized to exist in 1958. Decades later, researchers confirmed their presence in patient tissues from people with myotonic dystrophy, validating the long-standing theory that out-of-register mispairing of repeat units on complementary strands drives these expansions in living cells, not just in test tubes.
How Microsatellite Instability Is Detected
Because strand slippage leaves a measurable footprint in repetitive DNA, clinicians can test tumor samples for microsatellite instability to guide cancer treatment decisions. The gold standard is a PCR-based test that examines five specific microsatellite markers in both tumor and normal tissue from the same patient. If two or more markers show a size shift of 3 or more base pairs, the tumor is classified as MSI-high. A single shifted marker means MSI-low, and no shifts means the microsatellites are stable.
An alternative approach uses immunohistochemistry to check whether the four key MMR proteins are present in tumor cells. Loss of any one protein suggests the repair system is broken. For patients under 65, male patients, those with right-sided colon cancer, or those with poorly differentiated tumors, current guidelines recommend using both methods together for the most reliable result.