The bacterium Escherichia coli divides once every 50 minutes at ${36^ \circ }C$. 

E. coli were grown on a medium containing only heavy nitrogen, ${}^{15}N$, until all of the bacterial DNA contained heavy nitrogen. 

Some of the bacteria were moved from a heavy nitrogen medium and cultured in a medium with only light nitrogen, ${}^{14}N$ (0 minutes). 

These bacteria continued to reproduce and samples were extracted and centrifuged at regular intervals.

Hybrid DNA contains both 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.  

Which proportion of the DNA of the sample taken at 150 minutes will be at the upper position?