Biochemistry · Topic
Southern, Northern and Western blots !
Blots separate molecules by size in a gel, move them to a membrane and detect them with a probe: DNA (Southern), RNA (Northern) or protein (Western).
Updated 2026-11-01
Key facts
- Southern: DNA, with a labelled DNA probe. Northern: RNA, with a DNA or RNA probe. Western: proteins, with antibodies.
- Mnemonic 'SNoW DRoP': Southern-DNA, Northern-RNA, Western-Protein.
- Common steps: electrophoresis → transfer to a membrane (nitrocellulose or nylon) → blocking → probe or antibody → detection.
- In a Western blot proteins are denatured with SDS and separated by molecular weight (SDS-PAGE).
- Southern blots detect deletions, large repeat expansions and restriction polymorphisms (RFLP).
In detail
- Southern: DNA is cut with restriction enzymes, separated on agarose, denatured to single strands and transferred; the probe hybridises by complementarity.
- Northern: measures the size and amount of an mRNA in a tissue; today RT-qPCR has largely replaced it.
- Western: after SDS-PAGE, proteins move to the membrane by electric current; a primary antibody recognises them and a labelled secondary gives the signal.
- Southwestern blot: detects proteins that bind a DNA sequence (transcription factors).
- Electrophoresis separates by charge and size: nucleic acids (negative) run towards the positive pole and small fragments travel further.
- Also called
- Blotting, Immunoblot
Clinical notes
- For decades the Western blot was the confirmatory HIV test; differentiation immunoassays and viral RNA tests are now preferred.
- It is used for confirmation in Lyme disease and some parasitic infections.
- Southern blotting is still useful to size large CGG expansions and their methylation in fragile X syndrome.
Related conditions
Related
- ELISA (enzyme-linked immunosorbent assay)
- Polymerase chain reaction (PCR)
- DNA microarrays
- Trinucleotide repeat expansion disorders
- Recombinant DNA and cloning
Test yourself
Think of the answer, then tap the question to see it.
Which blot uses antibodies to detect proteins?
The Western blot, in which proteins are separated by SDS-PAGE and recognised with antibodies.
Why is DNA denatured before transfer in a Southern blot?
To get single strands that the probe can hybridise to by complementarity.
Which technique shows a large CGG expansion in fragile X?
The Southern blot, which also shows whether the FMR1 promoter is methylated.
Educational information, not medical advice. For any symptom, see a health professional.
Sources: Southern Blot (National Human Genome Research Institute (NHGRI), 2026); Northern Blot (National Human Genome Research Institute (NHGRI), 2026); Western Blot (National Human Genome Research Institute (NHGRI), 2026); Lyme Disease Tests: MedlinePlus Medical Test (MedlinePlus, US National Library of Medicine, 2026); Textbook of Biochemistry for Medical Students, 6th ed. (Jaypee Brothers Medical Publishers (Vasudevan DM, Sreekumari S, Vaidyanathan K), 2011).
Spotted a mistake?We are always improving for the community. Tell us what is wrong and our team will review it.Report a mistake
Representative model, for educational purposes only.