The bacterium Escherichia coli divides once every 50 minutes at 36 °C. E. coli were grown on a medium containing only heavy nitrogen, 15N, until all of the bacterial DNA contained heavy nitrogen (0 minutes). Some of the bacteria were moved from a heavy nitrogen medium and cultured in a medium with only light nitrogen, 14N. Some bacteria were collected after each of three generations. Their DNA was extracted and centrifuged. Hybrid DNA contains heavy and light nitrogen. The diagram shows the possible positions (upper, middle and lower) of the bands of DNA. The actual positions of bands in the first two samples are shown. [Diagram showing test tubes with DNA bands at 0, 50, 100, 150 minutes] Which proportion of the DNA of the sample taken at 150 minutes will be at the upper position?

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