Escherichia coli, a bacterium, divides once every 50 minutes at $36\,^{\circ}\text{C}$. E. coli were cultured in a medium that contained only heavy nitrogen, $^{15}\text{N}$, until every bacterial DNA molecule contained heavy nitrogen. Some of the bacteria were transferred from the heavy nitrogen medium to a medium containing only light nitrogen, $^{14}\text{N}$ (0 minutes). These bacteria then kept reproducing, and samples were taken out and centrifuged at regular times. Hybrid DNA has both heavy and light nitrogen. The diagram displays the possible band positions of DNA at the upper, middle and lower levels. The actual band positions in the first two samples are shown. What proportion of the DNA in the sample taken at 150 minutes will be at the upper position?
- A25%
- B50%
- C75%
- D100%