Band Traceback — Find Where an Unexpected PCR Band Came From
Work backwards from the size you measured: which pairs of priming sites on your template could make a product that big, and can their 3' ends actually extend?
🌐 Nothing you paste is logged or stored — every tool is also callable via REST & MCP, and in bulk from the batch tools
You have a band and it is the wrong size. The useful question is not “how much product will each site make” — that needs a mispriming extension penalty the literature only supports to an order of magnitude — but the one you can actually answer: which pairs of real priming sites on this template give a product of the size on the gel? This searches both primers against both strands, allowing the mismatches you specify, keeps every site pair whose product falls inside your size window, and ranks them by the 3' ANCHOR: how many of each primer's 3'-terminal bases match without interruption. That end is where the polymerase extends from, so an interrupted 3' terminus is the one thing that reliably stops a mispriming event — a site that fits the size but has a mismatched last base is a coincidence, and the tool says so rather than offering it as an answer. It also checks whether the band is simply the product you designed, and when nothing on the template fits it says that plainly and points at what does explain a band from nowhere: a primer-dimer, carryover of another template, or a size read further off than you allowed.
Working on a whole construct rather than one template and one band? Open SeqStudio — the full editor, with live feature annotation, plasmid maps, restriction and primer panels, undo/redo, multi-document tabs and GenBank / SnapGene import and export.
How to use the Band Traceback tool
- 1Paste the template you amplified and both primers, 5'→3'.
- 2Enter the band size you measured off the gel, and widen the tolerance if the read was rough.
- 3Raise the mismatches per primer if nothing is found at first — higher finds more explanations and more coincidences.
- 4Read the candidates best-anchored first: each gives the product's coordinates, which primer bound at each end, the mismatches, and the 3' anchor that decides whether it can extend.
Frequently asked questions
Why rank by the 3' end rather than by predicted yield?
Because the 3' end is the part that is solidly established and needs no fitted constants: a polymerase extends from the primer's 3' terminus, so an interrupted terminus stops the event almost regardless of how well the rest of the primer binds. Predicted yield would need a per-mismatch extension penalty, and the published values for that span two to four orders of magnitude — a number built on them would look precise and mean very little. So candidates are ORDERED by how much of each 3' end matches, and none is assigned a share of the band.
How is this different from In-silico PCR?
In-silico PCR runs forwards: given primers, here are the products. This runs backwards: given a product size you have already measured, here are the site pairs that could account for it. The difference in practice is the mismatch tolerance and the filtering — you are deliberately looking for the imperfect sites a clean in-silico run does not report, and only those consistent with the band in front of you.
It found nothing. What does that mean?
That the band is not coming from these primers on this template, which is itself a useful answer. The usual causes are a primer-dimer (a strong band at tens of base pairs, formed between the primers rather than on the template — check that with the Oligo Analyzer or Cross-dimer Checker), carryover of genomic DNA or another plasmid in the prep, or a size read further off than the tolerance you allowed. Widening the tolerance and raising the mismatch limit are the two things to try before concluding.
Can one primer make both ends of a product?
Yes, and it is a common cause of a band nobody designed. Wherever the template carries the reverse complement of a primer, that primer can prime the other strand, so a single primer can bracket a product on its own. Those pairings are searched and each candidate names which primer bound at each end, so a “forward + forward” product is visible as one rather than being reported as a normal pair.
Why is the size window a percentage?
Because that is how a gel reads. Ten per cent of 200 bp is twenty bases and ten per cent of 5 kb is five hundred, and a fixed window would be far too tight at one end of the range and far too loose at the other. You can override it with a fixed number of bases when you have a reason to.
Does it consider off-target sites in the genome?
No — only the template you paste. That is the honest boundary: a band from a paralogous locus or from genomic carryover is not on the template, so it cannot be found here, and the result says so when nothing fits. Use the Primer Designer's specificity screen for a genome-wide look.
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