The bacterium Escherichia coli splits once every 50 minutes at $36^\circ\text{C}$. E. coli were cultured on a medium that had only heavy nitrogen, $^{15}\text{N}$, until every part of the bacterial DNA contained heavy nitrogen (0 minutes). Some of the bacteria were transferred from the heavy nitrogen medium and then grown in a medium containing only light nitrogen, $^{14}\text{N}$. A sample of bacteria was taken after each of three generations. The DNA was removed and spun in a centrifuge. Hybrid DNA includes both heavy and light nitrogen. The diagram indicates the possible band positions (upper, middle and lower) of the DNA. The real band positions in the first two samples are shown. What fraction of the DNA in the sample collected at 150 minutes will be in the upper position?
- A25%
- B50%
- C75%
- D100%