As DNA analysis methods were being developed, it was common to separate DNA fragments of different sizes on agarose gels, as described in the preceding section. Many times, though, scientists wanted to do further testing steps that just would not work in gels. A biochemist named Edwin Southern came up with a method to solve this problem in 1975.
Southern’s method employs a membrane made of nitrocellulose or nylon, which is laid onto the gel after the DNA fragments have been separated. The gel and membrane are then placed into a buffer solution (a solution that resists change in pH), and some absorbent material is placed on top of the membrane. The absorbent material draws the solution upward, through the gel and through the membrane, but the DNA fragments in the gel are too big to go through the membrane, so they stick to it. Because the membrane is the same size as the gel, it retains the original position and orientation of the DNA fragments in the gel.
Once the DNA fragments are on the membrane, they are stable and cannot easily be removed. Furthermore, the membrane is tough and resilient, allowing scientists to do further tests on the DNA right on the membrane. This process of transferring DNA fragments from a gel to a membrane is called “Southern blotting,” and the resulting membrane is often just called a “blot.” This procedure was an important part of the earliest kind of forensic DNA typing, and it is still used regularly in research labs.
