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[Biochemistry](https://durgical.com/en/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

- [HIV infection and AIDS](https://durgical.com/en/conditions/hiv-infection-and-aids/)
- [Fragile X syndrome](https://durgical.com/en/conditions/fragile-x-syndrome/)

## Related

- [ELISA (enzyme-linked immunosorbent assay)](https://durgical.com/en/biochemistry/topics/elisa/)
- [Polymerase chain reaction (PCR)](https://durgical.com/en/biochemistry/topics/polymerase-chain-reaction/)
- [DNA microarrays](https://durgical.com/en/biochemistry/topics/dna-microarrays/)
- [Trinucleotide repeat expansion disorders](https://durgical.com/en/biochemistry/topics/trinucleotide-repeat-expansion-disorders/)
- [Recombinant DNA and cloning](https://durgical.com/en/biochemistry/topics/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.

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Educational information, not medical advice. For any symptom, see a health professional.

Sources: [Southern Blot](https://www.genome.gov/genetics-glossary/Southern-Blot) (National Human Genome Research Institute (NHGRI), 2026); [Northern Blot](https://www.genome.gov/genetics-glossary/Northern-Blot) (National Human Genome Research Institute (NHGRI), 2026); [Western Blot](https://www.genome.gov/genetics-glossary/Western-Blot) (National Human Genome Research Institute (NHGRI), 2026); [Lyme Disease Tests: MedlinePlus Medical Test](https://medlineplus.gov/lab-tests/lyme-disease-tests/) (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).

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