Most confusing RNA gel patterns have a simple explanation: small RNAs running at the bottom, genomic DNA near the well, secondary structure on native gels, or a DNA ladder that does not size RNA correctly. Only a few patterns actually mean your RNA is degraded. This guide goes pattern by pattern. For the reference picture of intact total RNA, start with 28S:18S rRNA ratio.
Pattern 1: haze or band at the very bottom
Normal. The 5.8S and 5S rRNAs and tRNAs (roughly 70–160 nucleotides) co-migrate as a faint low band. It becomes a problem only when it is bright and connected to a smear that runs upward — then it is a signature of degradation (see what degraded RNA looks like).
Pattern 2: a band or bright material at or near the well
Usually genomic DNA carried over from extraction. It will not affect a gel check but will give false signal in RT-qPCR unless removed with DNase and confirmed with a no-RT control. Very bright wells can also be insoluble material or overloaded sample.
Pattern 3: 28S and 18S at the “wrong” size
On a native gel, rRNA folds and migrates differently from linear double-stranded DNA, so reading RNA against a DNA ladder gives misleading sizes — 28S and 18S often appear larger or smaller than their true lengths. True sizes (human: 28S ≈ 5 kb, 18S ≈ 1.9 kb) are only measured on a denaturing gel with an RNA ladder. The ratio and sharpness of the two bands matter more than their apparent position.
Pattern 4: only one rRNA band
In vertebrate samples, losing 28S first is an early sign of degradation. In many insects and some other invertebrates, however, the 28S rRNA carries a natural “hidden break” and co-migrates with 18S, so a single band is normal. See the special case in our 28S:18S guide.
Pattern 5: fainter bands than expected
Band brightness reflects RNA mass. Fainter bands mean less RNA in the band — less loaded, losses during clean-up, or partial degradation. 28S normally looks about twice as bright as 18S because it carries more mass.
Pattern 6: extra bands in IVT mRNA
For in vitro transcribed RNA, two or three bands on a native gel are often conformers of the same molecule. Re-run on a denaturing gel: if they collapse into one band, the product is fine; if not, consider dsRNA by-products or truncated transcripts. More in mRNA on agarose gel.
Common questions
What are the smears at the bottom of an RNA gel?
A faint band or haze at the bottom is normal: it is the small RNAs (5.8S, 5S rRNA and tRNA) running together. A strong smear running up from the bottom toward 18S indicates degradation.
Why do my 28S and 18S bands run at about 7 and 3 kb instead of 5 and 2 kb?
On a non-denaturing gel, RNA secondary structure and a double-stranded DNA ladder make apparent sizes unreliable. Only a denaturing gel with an RNA ladder gives true sizes.
What are the true sizes of 28S and 18S rRNA?
In human cells, 18S rRNA is about 1.9 kb and 28S rRNA about 5 kb; 28S size varies between species.
Does a fainter band mean less RNA?
Band intensity tracks the mass of RNA in the band, so a fainter band can mean less loaded or partial degradation. Compare bands within the same lane rather than across lanes loaded differently.
Why does my IVT mRNA show two or three bands?
Common reasons are secondary-structure conformers on a native gel, double-stranded RNA by-products, or prematurely terminated transcripts. A denaturing gel distinguishes conformers from true size variants.
Need cleaner RNA to start with? Changyu Biotech supplies silica-column RNA extraction kits such as the CYRN28 Plus RNA Mini Kit and a full range of RNA/DNA extraction kits for research labs worldwide. Request a quote.