In a famous experiment, Matthew Meselson and Franklin Stahl grew E. coli cells in a medium containing only the "heavy" isotope of nitrogen, 15N. These cells were then transferred to a medium with the "light" isotope of nitrogen, 14N. The results of the experiment supported the hypothesis of semiconservative replication, which was proposed by Watson and Crick.
a) Heavy DNA (15N DNA), hybrid DNA, and light DNA (14N DNA) can be separated by centrifugation. If cells containing 15N DNA are transferred to a medium with only 14N NH4CI as a nitrogen source, what percent of daughter molecules are composed of hybrid DNA after 3 generations?
b) Predict what percent of daughter molecules would be composed of hybrid DNA if DNA exhibited conservative replication -that is, if a daughter DNA molecule were composed of newly synthesized DNA only.