They added the chemical that triggered the first gene (corresponding to barcode A) for 24 hours, followed by the one for the second gene (corresponding to barcode B) for the next 24. “In theory we should have all the recording proteins turned on throughout the process, but only the RNA for signal A in the first half and signal B in the second half,” Bhattarai-Kline says.
When the scientists sequenced the E. Coli’s genomes, that’s exactly what they found: The DNA receipts for barcode A were integrated into the Crispr array first, followed by those of barcode B. To double-check their work, they reversed the conditions, adding the chemical for
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