JOHN MCLEAN / SCIENCE PHOTO LIBRARY JOHN MCLEAN / SCIENCE PHOTO LIBRARY
DNA sequencing. Hands comparing the sequences of five DNA (deoxyribonucleic acid) autoradiograms. Each sequence is composed of four columns of irregularly-spaced bands of DNA. Each sequence was formed by fragmenting a radioactively-tagged DNA sample and separating it using an electric current (electrophoresis). An autoradiogram of the resulting bands was then made using radiographic film. The bands show the position of the base pairs on the DNA samples. Since every person has a unique sequence of base pairs, an autoradiogram of their DNA can be used to identify them. Here a test sequence (far left) is being compared with four sequences from known sources.
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