Genetics & Molecular · 1975
Southern blot: DNA detection by gel transfer and hybridization
Southern blotting (detection of specific sequences among DNA fragments separated by gel electrophoresis)
In 1975, molecular biologists could cut genomic DNA into fragments with restriction enzymes and separate those fragments by size on an agarose gel. What they could not do was identify which band among hundreds contained a specific sequence of interest. Edwin Southern, working at the University of Edinburgh, solved that problem with a deceptively simple approach: transfer the fragments from the gel onto a membrane that could then be probed with a labeled DNA strand complementary to the target sequence.
The mechanics of the Southern blot relied on capillary action. A stack of absorbent paper drew buffer upward through the gel, carrying the DNA fragments with it and depositing them on a nitrocellulose membrane placed directly above. The membrane was then incubated with a radiolabeled probe that hybridized only to the complementary sequence. Autoradiography revealed the position of the target band. Southern published the method in the Journal of Molecular Biology in 1975, and the protocol was reproducible enough that labs around the world adopted it quickly.
Before Southern's technique, there was no practical way to ask a simple question: does this patient's genomic DNA contain a particular mutation or deletion? The blot made that question answerable. Within a few years, researchers at UCSF used it to detect the sickle cell mutation prenatally by probing for restriction fragment length differences flanking the beta-globin gene. Thalassemia diagnosis followed, and the technique became the basis of the first molecular prenatal diagnoses.
The name prompted a tradition of naming analogous techniques geographically. Northern blotting, which applies the same transfer-and-hybridize logic to RNA, and Western blotting, which does so for proteins after SDS-PAGE separation, were named by analogy. Neither technique was developed by anyone named Northern or Western. The naming convention has since extended to further methods, all of which owe their conceptual structure to Southern's original paper.
Southern's technique also underpinned early forensic DNA analysis before PCR made smaller samples tractable. It remained the clinical standard for detecting major gene rearrangements, deletions, and copy number changes through the 1980s and into the 1990s. PCR and, later, next-generation sequencing displaced it for most clinical applications, but Southern blotting still has a role in confirming large structural variants and expanded repeat sequences that short-read sequencing cannot reliably characterize.
Key People
- Edwin Southern — British molecular biologist at Edinburgh who developed the gel-transfer hybridization method.
- Yuet Wai Kan — UCSF hematologist who first applied Southern blotting to prenatal diagnosis of sickle cell disease.
- Alec Jeffreys — British geneticist who extended restriction fragment analysis toward DNA fingerprinting in the 1980s.
J Mol Biol. 1975
Related landmarks
- 1977 · Sanger DNA Sequencing (dideoxy chain-termination method) (Genetics & Molecular)
- 1978 · Recombinant Human Insulin (synthesis of the gene) (Genetics & Molecular)
- 1985 · Polymerase Chain Reaction (PCR) (Genetics & Molecular)
- 1990 · First Approved Human Gene Therapy (ADA-SCID, Ashanthi DeSilva) (Genetics & Molecular)