Detection of specific sequences among DNA fragments separated by gel electrophoresis. Southern EM. J Mol Biol. 1975 Nov 5;98(3):503-17. doi: 10.1016/s0022-2836(75)80083-0.

OTHER VIDEOS YOU MIGHT LIKE:
• The discovery of tRNA: a fundamental of protein synthesis -    • The discovery of tRNA: a fundamental ...  
• Phenylketonuria (PKU) and the Guthrie test -    • Phenylketonuria (PKU) and the Guthrie...  
• An a-MAIZE-ing story about family, corn and sweet, sweet evolution -    • An a-MAIZE-ing story about family, co...  

DNA Electrophoresis is a lab staple. Your sample is digested with a restriction endonuclease, placed in a gel along with a buffer, a binding pigment and then a current is passed through, creating bands by fragment size. But while you can use a DNA ladder to infer the genetic information present, the exact DNA present in the gel can be a mystery.

When restriction endonucleases were first discovered the selection was limited, and what we possessed digested the DNA too finely to read the results of higher order animals. One method of identifying DNA at the time was hybridisation. DNA was first denatured using SSC buffer, then hybridized to a radiolabeled DNA or RNA probe. The more accurate the match, the more stable the new molecule. The hybridised DNA could then be placed on a piece of x-ray film and left to develop, creating an autoradiograph. This technique could be applied to electrophoreised gels by carefully slicing the gel, eluting the DNA and then hybridizing it on a filter. But the process lost the context electrophoresis could provide. What if you could hybridize the DNA right there on the gel? That didn’t work, the DNA simply leached out of the gels during hybridization. The DNA had to be fixed in place somehow – but how? Capillary action! Edwin Southern showed that it’s possible to ‘blot’ the DNA onto a piece of nitrocellulose, allowing it to be hybridized while keeping the bands intact, replicating their original positions on the gel. This technique is still widely used in labs today, doing everything from identifying the mutated genes in tumours, IDing bacteria, and spotting viruses in early stages of infection. It still has advantages over PCR, and variants of the technique that don’t use radioactive labelling have made it more accessible than ever.

Creator: Lukas Keune

References:
Dubnau D, Smith I, Morell P, Marmur J. Gene conservation in Bacillus species. I. Conserved genetic and nucleic acid base sequence homologies. Proc Natl Acad Sci U S A. 1965;54(2):491-498. John HA, Birnstiel ML, Jones KW. RNA-DNA hybrids at the cytological level. Nature. 1969;223(5206):582-587.
Mellars G, Gomez K. Mutation detection by Southern blotting. Methods Mol Biol. 2011;688:281-291.
Otrakji CL, Voigt W, Amador A, Nadji M, Gregorios JB. Malignant angioendotheliomatosis - a true lymphoma: a case of intravascular malignant lymphomatosis studied by Southern blot hybridization analysis. Hum Pathol. 1988;19(4):475-478.
Southern E. Gel electrophoresis of restriction fragments. Methods Enzymol. 1979;68:152-176.
Southern EM. Detection of specific sequences among DNA fragments separated by gel electrophoresis. J Mol Biol. 1975;98(3):503-517.
Southern EM. Blotting at 25. Trends Biochem Sci. 2000;25(12):585-588.