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ANALYTICAL CURRENTS DNA sequencing by RNA polymerase 20 nm
merase molecule, while 2.5 µM A 2.5 µM G the other was attached 1 mM C,G,U 1 mM A,C,U to the distal end of a DNA strand. As the 1 µM C 2.5 µM U RNA polymerase moved 1 mM A,G,U 1 mM A,C,G along the length of the 10 s 10 s DNA to transfer information from DNA to Limited amounts of a given nucleotide cause the RNA polyRNA, the changes in merase to pause as it moves along the length of a single DNA displacement of the two molecule. (Adapted with permission. Copyright 2006 American polystyrene beads were Association for the Advancement of Science.) measured with angleaf repeated the assay four times so that strom-level precision. each nucleotide type was limited in When the investigators carried out amount—the order of the pauses comthe assay in a buffer in which one of piled from each of the assays allowed the four nucleotides was present in them to infer the DNA sequence. They very low concentrations, the RNA could identify 30 out of 32 bases in a polymerase was forced to pause at stretch of DNA in
