If a total-RNA tube reads high ng/µL and A₂₆₀/A₂₈₀ near 2.0, yet RT-qPCR Cq values stall above 35, the spectrophotometer is not lying about absorbance — it is silent about length. Broken RNA still absorbs at 260 nm and can even look more concentrated once bases unstack. Integrity is read from 28S and 18S rRNA on a simple RNase-free agarose gel. This page is a go/no-go for molecular biology labs that own a gel box but not always a Bioanalyzer.
This is a laboratory decision guide, not a kit shopping list and not a diagnostic protocol. All guidance below is for research-use nucleic acid quality control. UV ratios are a different checkpoint; see Why an RNA A260/A280 of 1.8 is not a purity pass.
Updated 3 September 2026.
Why a high A₂₆₀ can be degraded RNA
A microvolume spectrophotometer reports light lost at 260 nm. Under the Beer–Lambert relation that number tracks aromatic bases in the path, whether those bases sit in intact transcripts or in short fragments. The usual conversion (A₂₆₀ = 1.0 ≈ 40 µg/mL RNA, 1 cm path) is a mass estimate, not a count of full-length molecules (Desjardins and Conklin, 2010).
Hydrolysis of the phosphodiester backbone does not destroy the bases. Unstacking them can raise A₂₆₀ (hyperchromicity), so a degraded prep may print a higher “concentration” than the same mass of intact RNA. Residual genomic DNA, protein, solvents, and plant secondary metabolites absorb in the same window. A pretty 260/280 therefore does not license skipping a size check before reverse transcription.
MIQE treats spectrophotometric purity and integrity as separate checkpoints (Bustin et al., 2009). Integrity changes qPCR results in a measurable way (Fleige and Pfaffl, 2006; Vermeulen et al., 2011). If the assay cares about transcript length, read the gel — or a RIN — before you swap reverse transcriptase lots.
What you actually see on the gel (it is rRNA, not mRNA)
Total cellular RNA is not an equimolar mix of coding sequences.
| RNA class | Typical mass fraction | Eukaryotic size | What the agarose lane shows |
|---|---|---|---|
| rRNA | ~75–85% | 28S (~4.7–5.0 kb), 18S (~1.9 kb), plus 5.8S/5S | Two dominant bands; these are the integrity markers |
| tRNA | ~15% | 73–93 nt | Usually lost at the dye front |
| mRNA | ~1–5% | a continuum (~0.5 kb to >10 kb) | Faint haze along the lane, never a single band |
| small RNAs | <1% | 20–300 nt | Below a routine ethidium/GelRed visual limit |
Individual mRNAs never reach the mass needed for a discrete band. The two bright eukaryotic bands are the 80S ribosomal RNAs. Prokaryotic markers are 23S (~2.9 kb) and 16S (~1.5 kb).
In an undamaged eukaryotic cell the two ribosomal subunits are equimolar. 28S is about 2.5× longer than 18S, so it binds more intercalating dye. Visual or densitometric brightness of 28S:18S ≈ 1.5–2.5:1 (often cited as ~2:1) is the working intact pattern. That ratio is an empirical bench rule, not a Bioanalyzer RIN (Schroeder et al., 2006).
Figure 1. Intact eukaryotic total RNA on ordinary agarose: two dominant rRNA bands, 28S brighter than 18S (about 1.5–2.5:1). mRNA is not a separate band.
28S fades first. A smear is not a death sentence.
Longer chains have more phosphodiester bonds, so 28S is statistically hit before 18S. The visual sequence is reproducible:
- Early. 28S dims; ratio drops below ~1.5:1; 18S is still sharp.
- Moderate. Ratio inverts (<1:1). Treat as widespread cleavage.
- Advanced. 28S gone, 18S a faint outline, compact low-molecular-weight junk below ~500 nt. Full-length templates for RT-qPCR are not there.
A faint continuous haze in a properly loaded lane (about 200–500 ng total RNA) is often the mRNA continuum plus pre-rRNA, not proof of degradation. Underloading (<100 ng) or clipping the background in the imager erases that haze and leaves lonely rRNA bands. Overloading (>1 µg) paints the whole lane. Judge the rRNA ratio against that background, not the smear alone.
Fuzzy or smiling bands with a still-normal 28S:18S and no low-MW pile-up are usually electrophoresis, not nuclease:
- field stronger than ~6–7 V/cm (heat)
- exhausted running buffer
- salt or ethanol in the eluate
- camera out of focus or sensor clipped
Degradation moves mass down the lane and inverts the ratio. Ugly-but-2:1 is a running problem.
Figure 2. Partial vs severe degradation: 28S fades first; ratio inversion; smear is not by itself a death sentence.
Ordinary agarose is enough for a go/no-go
Formaldehyde–MOPS or glyoxal gels melt RNA secondary structure so migration tracks length. That is the right tool for Northern sizing. It is the wrong default for daily QC: formaldehyde is toxic and slow.
For “are 28S and 18S still there?” a 1.0–1.2% non-denaturing agarose gel in 1× TAE or TBE answers in about 30 minutes (Masek et al., 2005). Secondary structure blocks nucleotide-level sizing against a DNA ladder; it does not hide the 4.7 kb vs 1.9 kb mass gap.
Optional: a bleach gel (0.5–1.0% v/v household sodium hypochlorite in molten TAE) denatures RNA and kills trace RNase without a fume hood (Aranda et al., 2012). Capillary systems (RIN/RQI) remain the numeric standard when a core facility demands a cutoff.
Same agarose, loading dye, and nucleic-acid stain class as a DNA gel may be used. The whole chain — water, tips, tank, comb, buffer — must be RNase-free.
Shared DNA tanks are the usual failure. A four-step wash is ordinary lab practice, not a secret formula: detergent or 0.1% SDS → deionized water → nuclease-free water → 70–80% ethanol, air-dry. Overnight peroxide or NaOH soaks are textbook, not required for a 30-minute QC gel. Run 5–7 V/cm for 20–35 minutes; heat from a long, hot run will degrade RNA in the gel.
Bright wells are junk, not “very concentrated RNA”
RNA is highly charged and enters 1% agarose readily. Fluorescence stuck in the well, sometimes trailing into the top of the lane, is almost never “the RNA was too concentrated.” It is material that cannot enter the pores:
- high-molecular-weight genomic DNA (overloaded columns, harsh lysis)
- nucleoprotein that still has protein on it
- polysaccharide networks and polyphenol cross-links, especially in plant lysates
Those same contaminants scatter UV and inflate A₂₆₀. Loading that eluate into reverse transcriptase is how “high yield, dead Cq” happens.
Figure 3. Bright wells vs RNA: protein / polysaccharide / gDNA stuck in the well, common in plant lysates — not “high concentration.”
When the methods gap is plant lysate — one kit
Woody perennials, fibrous roots, mature leaves, and starch-rich organs pack polyphenols and polysaccharides. Once compartments break, quinones cross-link RNA and acidic polysaccharides co-precipitate with it. The gel then shows a glowing well; the pedestal still prints a high ng/µL. Plant qPCR papers have been warning that RNA quality — not only UV — has to be assessed in that matrix (Die and Román, 2012; Gambino et al., 2008).
A silica kit built for simple mammalian lysates often leaves that secondary-metabolite load in the eluate. Clearing it is a chemistry problem (PVP/PVPP for polyphenols; CTAB or LiCl-class steps for polysaccharides), not a branding problem.
The Plus Complex Plant RNA Mini Kit (Cat# CYRN53) is a phenol-free silica workflow with a dedicated genomic-DNA elimination column, written for difficult plant tissues (the manual names Dendrobium, Salvia, starch-rich seeds, and several fruits). For particularly complex plants the same manual allows PVP40 in lysis buffer CLB at 2% final. That is chemistry aimed at secondary metabolites — not a 28S:18S certificate. The manual’s typical OD260/OD280 of 2.0–2.2 is a UV window on that protocol, not a gel ratio, not a lot release, and not a number to copy onto other Changyu RNA kits. Store and ship those buffers at room temperature; 4 °C or −20 °C precipitates them. After extraction, the gel is still the integrity check: clear wells and a 28S band brighter than 18S, or re-extract. See the RNA and DNA extraction kit list for the rest of that family.
What a slab gel cannot do
Non-denaturing agarose is qualitative to semi-quantitative.
- It does not replace a RIN/RQI when a repository wants a number (Schroeder et al., 2006). A common core cutoff is RIN ≥ 7; a phone photo of a slab gel is not that metadata.
- rRNA is an indirect mRNA proxy. Severe nuclease hits both; targeted mRNA decay can still leave 28S/18S standing.
- Intact rRNA bands say nothing about phenol, guanidinium, ethanol, or humic fragments that inhibit RT and polymerase. Those need a dilution series or a spike-in (Taylor et al., 2019) — not another gel.
This protocol is for basic research and laboratory investigation. It is not designed, validated, or approved for clinical diagnostic workflows, patient-sample evaluation, disease management, or in vitro diagnostic (IVD) procedures.
Decision matrix before reverse transcription
Pair the pedestal printout with a 1% agarose lane before cDNA synthesis.
| Spectrophotometer | 1% agarose | Assessment | Decision |
|---|---|---|---|
| High A₂₆₀, A₂₆₀/A₂₈₀ ≈ 2.0, A₂₆₀/A₂₃₀ ≥ 2.0 | Sharp 28S and 18S; ratio ≈ 1.5–2.5:1; wells clear | Intact total RNA | Proceed to RT-qPCR. |
| High A₂₆₀, A₂₆₀/A₂₈₀ ≈ 1.9–2.1 | Dim 28S, dominant 18S, ratio < 1 | Cleavage; hyperchromicity may inflate yield | Do not reverse-transcribe. Re-isolate; inspect harvest. |
| High A₂₆₀, A₂₆₀/A₂₈₀ ≈ 2.0–2.2 | No rRNA bands; compact smear <500 nt | Advanced degradation | Discard. Full-length template is gone. |
| Elevated A₂₆₀, messy ratios | Bright well; little migration | gDNA / protein / polysaccharide plug | Do not proceed. DNase and/or a chemistry built for that matrix. |
| High A₂₆₀, A₂₆₀/A₂₈₀ ≈ 2.0, A₂₆₀/A₂₃₀ < 1.0 | Sharp 2:1 rRNA; clear wells | Intact RNA plus salt/solvent | Precipitate or wash, then reverse-transcribe. |
Before you replace reverse transcriptase, redesign primers, or blame the cycler, spend 30 minutes on an RNase-free 1% agarose gel. 28S brighter than 18S, wells empty, UV treated as optical density not integrity — that is still the cheapest integrity check in gene-expression research.
Images: captions only. Publisher may add unlabeled Changyu-run gels later; do not invent them.
Common questions
Can a NanoDrop reading replace an RNA agarose gel? No. A₂₆₀ counts bases, not length. Degraded RNA still absorbs, and hyperchromicity can raise the printed yield. Integrity is a 28S:18S (or RIN) measurement. MIQE keeps purity and integrity as separate checks.
Does a smear on the RNA gel mean the sample is degraded? Not by itself. A faint haze is often the mRNA continuum. Degradation is ratio inversion and a low-molecular-weight pile-up. Fuzzy bands with a still-normal 28S:18S are usually running or imaging artifacts.
Why is the well glowing if concentration is high? RNA enters 1% agarose. Well fluorescence is gDNA, protein, or polysaccharide — common in plant lysates — not proof of high RNA mass. Do not reverse-transcribe that eluate.
Bottom line
A spectrophotometer reports how much UV the tube absorbs. A 1% agarose gel reports whether 28S is still brighter than 18S and whether the well is empty. Those are different questions. Ordinary non-denaturing agarose is enough for a research-lab go/no-go; formaldehyde gels are for Northern sizing; RIN is for numeric cutoffs. Smear is not a death sentence. Ratio inversion is. Bright wells are junk. Read the gel before you blame the enzyme.
Research use only. Not for diagnostic, therapeutic, or clinical use.
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References
- Bustin, S. A., Benes, V., Garson, J. A., Hellemans, J., Huggett, J., Kubista, M., Mueller, R., Nolan, T., Pfaffl, M. W., Shipley, G. L., Vandesompele, J., & Wittwer, C. T. (2009). The MIQE guidelines: Minimum information for publication of quantitative real-time PCR experiments. Clinical Chemistry, 55(4), 611–622. https://doi.org/10.1373/clinchem.2008.112797
- Schroeder, A., Mueller, O., Stocker, S., Salowsky, R., Leiber, M., Gassmann, M., Lightfoot, S., Menzel, W., Granzow, M., & Ragg, T. (2006). The RIN: an RNA integrity number for assigning integrity values to RNA measurements. BMC Molecular Biology, 7, 3. https://doi.org/10.1186/1471-2199-7-3
- Aranda, P. S., LaJoie, D. M., & Jorcyk, C. L. (2012). Bleach gel: a simple agarose gel for analyzing RNA quality. Electrophoresis, 33(2), 366–369. https://doi.org/10.1002/elps.201100335
- Masek, T., Vopalensky, V., Suchomelova, P., & Pospisek, M. (2005). Denaturing RNA electrophoresis in TAE agarose gels. Analytical Biochemistry, 336(1), 46–50. https://doi.org/10.1016/j.ab.2004.09.010
- Vermeulen, J., De Preter, K., Lefever, S., Nuytens, J., De Vloed, F., Derveaux, S., Hellemans, J., Speleman, F., & Vandesompele, J. (2011). Measurable impact of RNA quality on gene expression results from quantitative PCR. Nucleic Acids Research, 39(9), e63. https://doi.org/10.1093/nar/gkr065
- Fleige, S., & Pfaffl, M. W. (2006). RNA integrity and the effect on the real-time qRT-PCR performance. Molecular Aspects of Medicine, 27(2–3), 126–139. https://doi.org/10.1016/j.mam.2005.12.003
- Taylor, S. C., Nadeau, K., Abbasi, M., Lachance, C., Nguyen, M., & Fenrich, J. (2019). The ultimate qPCR experiment: producing publication quality, reproducible data the first time. Trends in Biotechnology, 37(7), 761–774. https://doi.org/10.1016/j.tibtech.2018.12.002
- Die, J. V., & Román, B. (2012). RNA quality assessment: a view from plant qPCR studies. Journal of Experimental Botany, 63(17), 6069–6077. https://doi.org/10.1093/jxb/ers276
- Desjardins, P., & Conklin, D. (2010). NanoDrop microvolume quantitation of nucleic acids. Journal of Visualized Experiments, (45), e2565. https://doi.org/10.3791/2565
- Gambino, G., Perrone, I., & Gribaudo, I. (2008). A rapid and effective method for RNA extraction from different tissues of grapevine and other woody plants. Phytochemical Analysis, 19(6), 520–525. https://doi.org/10.1002/pca.1078
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