Where Did That Extra PCR Band Come From?
6 min read · Updated October 30, 2026
You have a band and it is the wrong size. The question that can be answered from the sequence is not how much product each site makes — that needs a mispriming extension penalty the literature supports only to an order of magnitude — but which pairs of real priming sites on this template would give a product that size.
That list is finite, and ranking it correctly turns on one feature of a priming site.
First: is it the product you designed?
The cheapest explanation is that the band is the amplicon. Primers at positions 201 and 922 of a plasmid give a designed product of 722 bp, so a band measured at 722 bp is the thing you wanted, and the report says so rather than hunting for an exotic explanation.
That sounds trivial and is not: gel estimates are rough, and a band read as 700 against an expected 722 is frequently the same band. Check the designed product against the size window before anything else.
The 3' end decides whether a site can extend at all
A polymerase extends from a primer's 3' terminus. A mismatch there blocks extension far more reliably than mismatches anywhere else in the primer, which is the basis of allele-specific PCR and the reason a site's 3' anchor — how many of its terminal bases match without interruption — is the right thing to rank candidates by.
Worked on a template carrying a deliberate second forward-priming site: with two mismatches at the site's 5' end and an intact 3' terminus, the pair gives a 523 bp product with a 20-base 3' anchor, and it is a real explanation for a 523 bp band.
Add one more mismatch, at the 3'-terminal base, and the same pair still fits the size — the coordinates are unchanged — but the anchor drops to 0 and the report says every candidate has a mismatched 3'-terminal base, so these are size coincidences rather than explanations.
Both candidates fit the gel equally well. Only one of them can extend.
One primer can make both ends
A band does not have to come from one forward and one reverse primer. A primer that binds both strands at two positions, in the right orientations and at the right distance, gives a product with the same primer at each end — and that is a common cause of a band nobody can account for from the designed pair.
An enumeration that only considers forward-plus-reverse pairs misses these entirely, so it is worth confirming that the search covers both primers against both strands.
When nothing on the template fits
Ask for an explanation of a 1,200 bp band from the same primers and template and the honest answer is that there is none: nothing makes a product in the 1,080–1,320 bp window with up to four mismatches per primer.
That is a finding rather than a failure, and it points somewhere specific — a primer-dimer (short products, tens of base pairs), carryover of another template or genomic DNA, or a size read further off than the tolerance allows. The next move is to check the primers against each other rather than to keep searching the template.
The tolerance is a percentage rather than a fixed number of bases, because ±10% of 200 bp and ±10% of 5 kb are not the same reading precision, and a gel's precision scales with size.
What this does not cover
- Off-target sites outside the template you supplied. A genomic background needs a specificity screen against that genome, not a traceback on a plasmid.
- Yield. No share of the band is assigned to any candidate: the band is the input, not the output.
- Primer-dimers, which are a property of the two primers rather than of the template and are found by checking them against each other.
- Secondary structure in the template, which can suppress the intended product and leave a mispriming product looking dominant.
Frequently asked questions
How do I work out what an unexpected PCR band is?
Enumerate the pairs of priming sites on your template that would give a product of that size, allowing a few mismatches, then rank them by how much of each primer's 3' end matches without interruption. A pair that fits the size but has a mismatched 3'-terminal base is a coincidence rather than an explanation.
Why rank mispriming sites by the 3' end rather than by total mismatches?
Because the polymerase extends from the 3' terminus. Mismatches near the 5' end of a primer cost binding energy and still extend; a mismatched terminal base reliably does not. A site with two 5' mismatches and an intact 3' end is a far better explanation than one with a single mismatch at its last base.
Can one primer produce both ends of a product?
Yes, and it is a common cause of unexplained bands. If a primer binds both strands at two positions in the right orientations and at the right distance, the product carries that primer at both ends. Any search that only pairs the forward primer with the reverse one will not find it.
Nothing on my template explains the band. What now?
Look outside the template. The usual causes are a primer-dimer, which is short and comes from the primers rather than the template; carryover of another template or genomic DNA; or a size read further off than the search window allowed. Widening the mismatch allowance finds more explanations and more coincidences, so check the primers against each other first.
Related references
Related tools
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?
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