Southern blot concepts for DNA

A Southern blot asks which restriction fragment of DNA carries a sequence your probe can recognise. You cut the DNA, separate the fragments by size, denature them so the strands come apart, transfer those strands to a membrane, hybridise a labeled probe, and wash at a stringency that matches the question you wrote down. This page explains that chain of ideas. It does not give a labeling recipe, a radioactivity table, or a clinical protocol. The gel step is agarose gel electrophoresis for DNA. A western blot is the protein cousin and a different molecule, described in western blot from gel to membrane.

The photograph is the gel half of the story: DNA bands glowing on an ultraviolet box under an orange safety shield. The membrane and the probe come after that picture. Enzymes, membranes and ladders are catalogue classes in the molecular biology catalogue. The fragment sizes and the probe type belong on the quote request.

Who the method still serves

Use the concept when the question is genomic context. Which fragment, after this enzyme, holds the sequence? Is a transgene present as a band of the size the map predicts? Does a related organism light the same probe under stricter washing? PCR is the better tool when you only need a short amplicon between two primers. Sequencing is the better tool when you need the bases. A Southern band is a hybridisation event at a length. It is not a base call and not a copy-number assay unless the whole method was built and controlled for that purpose, which this note does not provide.

The expected fragment length comes from the sequence and the cut sites. NCBI Nucleotide is a public place to retrieve a recorded sequence. NCBI BLAST is a public place to ask what else a probe sequence resembles. A probe that hits a gene family will produce a family blot if the wash is permissive. Know that before you call a second band contamination.

The physical idea

Restriction enzymes cut double-stranded DNA at defined sites, so a genome becomes a set of fragments. Agarose electrophoresis lines those fragments up by length, with a ladder on the same gel. The percentage follows the fragment window, as the agarose guide describes. At this point the DNA is still double-stranded. A photograph under the orange shield shows that the digest and the separation happened. It does not show that a probe will bind.

Denaturation opens the helix, classically with alkali, so each fragment becomes strands stuck in place, then transferred. Transfer moves those strands from the gel onto a membrane, often a positively charged nylon sheet for nucleic acids. Capillary transfer is the historical class. Vacuum and electrophoretic transfer are other classes. The direction and the contact matter. A bubble is a hole in the pattern, just as it is on a protein blot, and the chemistry is not the same chemistry.

The probe is a labeled nucleic acid complementary to the sequence you care about. During hybridisation it finds strands that match it well enough under those salt and temperature conditions. Washes then remove probe that bound badly. What remains is what the detection step will show. Radioactive probes and non-radioactive probes are detection classes, not different questions. The question lives in the sequence of the probe and in the stringency of the wash.

Reagent and equipment classes

You need genomic DNA that is actually high molecular weight if the fragments are long, a restriction enzyme used as its card describes, agarose and buffer, a ladder that brackets the expected fragment, a denaturation method your institution already runs, a membrane meant for nucleic acid, a probe, a hybridisation system, and a detection path matched to the label. Incomplete digestion invents extra bands and looks like a second locus. The enzyme card, not a blog, sets time and units.

Probe labels fall into two regulatory classes. Radioactive labels, commonly a phosphorus-32 class, sit under the institution's radiation rules: who may order isotope, how it is shielded, how waste is held, and how exposure is monitored. This article gives no activity and no recipe. Non-radioactive labels, such as digoxigenin, biotin, or a chemiluminescent enzyme class, sit under chemical rules and the detection kit's own note. Biotin probes collide with blocking reagents that themselves contain biotin. Choose the block with that in mind. Formamide appears in some hybridisation buffers to lower the temperature. It is a hazardous solvent. Follow the institutional SOP. Do not treat a published buffer table as permission.

The molecular weight ladder on the gel has to survive as a size reference you can align to the blot, by a lane you can see after transfer or by a mark you made from the gel photograph. A blot with no size reference is a smear of hybridisation, not a fragment length. The transfer membrane is not optional packaging. Its charge and its pore are part of whether long fragments left the gel and stayed.

A conceptual workflow and the stringency branch

Confirm the digest on the gel in the photograph's kind of imager, shield in place, ladder visible. Denature. Transfer. Fix the DNA to the sheet by the method that membrane uses, often a crosslink step. Hybridise with a probe whose sequence you can defend. Wash. Detect. At the wash, branch on purpose.

High stringency, meaning a hotter wash and lower salt within the method you are following, keeps close matches. Use it when one locus should light and extra bands would confuse the claim. Low stringency keeps relatives. Use it when a family or a cross-species question is the point, and expect more than one band. Write the wash you used next to the image. A later reader cannot see stringency in the band.

If the digest gel was already a smear, stop. A probe on sheared DNA produces a smear, and you will call it a deletion or a rearrangement. If the gel was fine and the blot is blank, the failure is denaturation, transfer, the probe, or the detection, in that order of suspicion. If every lane lights including a sample that should lack the sequence, stringency is too low or the probe is not specific. BLAST the probe again before you rewrite the genome.

Safety, radiation and research limits

Ultraviolet light on the gel in the photograph requires the orange shield. Ethidium bromide and alternative stains follow the bottle. Restriction enzymes are ordinary cold reagents. The probe is not ordinary if it is radioactive. Institutional radiation rules decide whether that class is allowed in the building at all. Non-radioactive labels still have safety data. The WHO Laboratory biosafety manual, 4th edition is background for biological risk of the DNA's source organism. It does not licence isotope work, and it does not turn the blot into a diagnostic test.

A band at the mapped size, absent from a negative DNA, under a stated stringency, supports hybridisation to a fragment of about that length. It does not count copies, prove a mutation-free insert, or identify a patient.

Ovens, humidity and a wash you did not choose

Hybridisation is often an overnight step in an oven or a tube. A power cut changes the temperature and therefore the stringency without changing your notes, unless you notice. Do not interpret a blot whose oven stopped. Humidity and a poorly sealed bag dry the edge of the membrane. That edge looks like a lane that failed to transfer. Seal the bag, keep the sheet wetted with the hybridisation solution the method names, and discard a blot that dried halfway. In a hot building, a "room temperature" wash is warmer than the protocol assumed and is a higher stringency than you wrote down. Measure the wash temperature. Do not guess it from the season.

What to put in an enquiry

State the organism, the expected fragment sizes, the enzyme class, whether the probe will be radioactive or a named non-radioactive class, and the membrane type. Say that radiation rules are an institutional matter you have already settled, or that you need a non-radioactive class because they are not settled. The nucleic acid analysis pathway is the route from digest to a sequence-level check when a band is not enough. Ask whether a quotation is possible for membrane, ladder and enzyme. Do not ask a catalogue to approve a diagnostic Southern or to supply isotope outside your institution's rules.