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Sorting DNA by Size: Gel Electrophoresis

DNA fragments can be studied using a technique called gel electrophoresis. Gel electrophoresis separates nucleic acids or proteins according to their size and charge. As shown in step O of Figure 13-3, the DNA samples are cut with a restriction enzyme. The DNA is then placed in wells made on a thick gel, as shown in step ©. An electric current runs through the gel for a given period of time. Negatively charged DNA fragments migrate toward the positively charged end of the gel. Smaller DNA fragments migrate faster and farther than longer fragments, and this separates the fragments by size. In step ©, the DNA is transfered to a nylon membrane and radioactive probes are added. The probes bind to complementary DNA. In step ©, an X-ray film is exposed to the radiolabeled membrane. The resulting pattern of bands is called a DNA fingerprint.

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