Native versus denaturing gels

Native and denaturing gels answer different questions, and the trouble starts when a notebook treats them as two exposures of the same fact. A native gel lets the molecule keep its shape and much of its own charge. A denaturing gel tries to erase shape so that length dominates. This page is a troubleshooting path for that mix-up, especially for a protein band that seems to disappear or jump when a reducing agent is added. DNA gels in the ordinary agarose sense are usually native. The method background is agarose gel electrophoresis for DNA. Protein lanes read as SDS-PAGE are a different habit, set out in reading a protein gel.

This is research troubleshooting, not a diagnostic electrophoresis. Matrices and ladders are sourced as classes from the molecular biology catalogue, with the gel type named on the quote request.

Pick the gel from the question

Use a denaturing system when the sentence you want to write is about size. For protein, that is usually SDS-PAGE: detergent, a charge coat, a reducing agent of the dithiothreitol or beta-mercaptoethanol class when disulfides should be opened, and heat when the sample buffer calls for it. The number you get is apparent mass against a protein ladder. For RNA, a size claim usually needs a denaturing gel so hairpins do not masquerade as length. For ordinary double-stranded DNA, native agarose is the size tool, because the helix is already a similar shape from fragment to fragment and the charge follows the backbone.

Use a native system when the sentence is about a complex, a shape, or an activity. Oligomers that only exist folded, a mobility shift from a bound partner, or an enzyme stain that requires the protein still work, all die in SDS. Blue-native methods are a class aimed at membrane complexes. They are still not SDS ladders. Read them with the rules of that class.

The photograph at the top is a native DNA agarose gel on an ultraviolet box, seen through an orange safety shield. It is not a picture of a denaturing protein slab and not a picture of an RNA formaldehyde gel. Keep that distinction when the same imager is used for every tray in the room.

What actually differs in the lane

In a native protein gel the polypeptide keeps charge patches and a hydrodynamic shape. Two proteins of equal mass can separate widely. One protein can split into forms that differ by a cofactor or a partner. The ladder, if you use a native marker set, is only a rough guide, and an SDS ladder run beside a native sample is decorative. Do not quote kilodaltons from it.

In a denaturing protein gel the intent is the opposite. SDS overwhelms charge, reduction opens disulfides, and the mesh reports length as apparent mass. UniProt still supplies the sequence mass you hoped to see. The gel is allowed to disagree for the reasons the reading guide lists: modifications, tags, incomplete unfolding.

DNA on the agarose gel in the photograph migrates largely by length because the native helix is regular. Shape returns as a problem for circles. Supercoiled plasmid is not a linear ladder fragment of the same sequence. RNA is the nucleic acid that most often needs denaturation for a size reading. Secondary structure is stable enough to change mobility on a native gel, so a "short" band can be a folded longer transcript. Denaturing RNA systems, formaldehyde-agarose among them, are chosen for that reason and bring their own chemical hazards. Follow the system your institution already runs.

A band that shifts or vanishes when you reduce it

Treat this as a position problem before a degradation story. Pour, or buy, one gel. Load matched samples that differ only by the reducing agent. Include a ladder that covers both the high position you saw and the subunit mass you expect. Heat both tubes the same way if heat is part of the denaturing prep, so you do not confound reducer with temperature.

If the high band fades and a new band appears near the expected subunit mass, the first species was disulfide-linked. The protein did not vanish. It changed address. If the new band is faint, the subunit may have run into a crowded lysate region, or part of the material may have fallen below the stain's comfort. Load less of the non-reduced sample and a bit more of the reduced sample only after you have admitted you are changing load, not chemistry.

If the band is gone and nothing new appears inside the ladder, widen the search. The subunit may be small enough to sit on the dye front of that percentage. A higher-percentage gel is the next slab, not a longer run that walks the subunit off the bottom. The subunit may also have aggregated and stayed in the well. Look at the well before you discard the lane. An epitope that depended on a disulfide will vanish on a western blot even when the polypeptide is still on the membrane. That is an antibody fact. A total-protein stain of the gel, or of the blot, separates "the protein left" from "the antibody stopped recognising it."

If both lanes look the same, the reducer may never have worked. Old reducing agent oxidises, especially in a warm bottle that was opened repeatedly. A non-reduced control you trust, a protein you know shifts, is the function check. A harsher smell is not a titre.

Complexes that "disappear" because you changed format

A band on a native gel can be several chains. Move that sample into SDS and the complex is no longer the object in the lane. Several new bands, or one subunit band, are the denaturing answer. Calling the native band lost is a category error. If activity was the point, assay the native gel or the fractions, and keep the denaturing gel as the chain inventory. If the western blot is the point, remember that a typical blot is denaturing. It will not preserve the complex the native gel just showed you.

DNA has a smaller version of the same trap. A native agarose band of uncut plasmid shifts when you linearise it. The sequence length did not change. The shape did. RNA that looked short on a native gel can look longer once it is denatured and run against an RNA ladder treated the same way. Do not size that native RNA band with a DNA ladder and call it a transcript length.

Safety and the limits of either gel

Denaturing protein work still uses acrylamide monomer if you cast. That is a neurotoxin and an institutional SOP, not a troubleshooting shortcut. Reducing agents and SDS are chemical hazards. Formaldehyde or other denaturants in an RNA gel are chemical hazards of their own class. Ultraviolet viewing of the DNA gel in the photograph requires the orange shield. Ethidium bromide and alternative stains follow the bottle's safety data.

Neither format is a clinical test. Biosafety follows the organism or the lysate, not the choice of detergent. The WHO Laboratory biosafety manual, 4th edition is background for that institutional decision.

Heat that quietly denatures a native run

A native gel that runs hot is only partly native. Joule heating in a warm room unfolds some proteins and changes DNA or RNA structure mid-lane, so the band smears or shifts without anyone adding SDS. Lower the field, cool the run if your setup allows it, and do not interpret a smiled native lane as a new isoform. The same room oxidises reducing agent left on the bench, which creates the opposite bug: a denaturing lane that never reduced. Cap the tube, follow the laboratory's expiry habit for that bottle, and do not start a comparison you cannot finish if the power is likely to drop. A gel that sat without a field diffuses, and a shift you measure afterwards is not the shift the reducer produced.

What to put in an enquiry

Say whether you need a native protein system, an SDS denaturing system, ordinary DNA agarose, or a denaturing RNA gel. Name the mass or length window and whether a reducing-agent comparison is part of the experiment. Ask for ladders that match that chemistry. An SDS marker will not rescue a native gel. The nucleic acid analysis pathway covers the nucleic-acid side of the bench. Ask whether a quotation is possible, and ask the supplier to confirm the gel class in writing rather than shipping a denaturing cassette against a native request.