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Figure 2. Pulsed-eld Gel Electrophoresis

In pulsed-field gel electrophoresis, the chromosomal DNA from bacterial isolates is treated with a restriction enzyme that cuts the DNA at random, infrequent locations, yielding pieces of DNA. In this example, the restriction enzyme is Sma I. Whenever Sma I encounters a CCCGGG sequence in the DNA, it makes a cut.

  • e mixture of different size pieces of DNA are then

embedded in a porous matrix—an agarose gel. e matrix is subjected to an electric current that switches direction according to a predetermined pattern. Since the DNA pieces are negatively charged they move through the matrix toward the positively charged end.

  • e shorter pieces move further because they can get

through the pores more easily. e longer pieces move more slowly because they migrate through the pores less easily. is process is referred to as pulsed-field gel electrophoresis. e result is a pattern of DNA bands on the gel, like you see in the figure, with shorter pieces of DNA represented by bands near the bottom of the gel and longer pieces of DNA represented by bands near the top of the gel. e DNA patterns of different isolates can be compared to determine their genetic relatedness.

“Dr. Collins and the Case of the Mysterious Infection” by Lemons & Huber

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