A famous experiment, Matthew Meselson and Franklin Stahl grew E. coli cells in a medium containing only the "heavy" isotope or 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 semi conservative replication, which was proposed by Watson and Crick. 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 NH_4CI as a nitrogen source, what percent of daughter molecules are composed of hybrid DNA after 3 generations? 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.

1. 25% percent of daughter molecules are composed of hybrid DNA after 3 generations

2. 0% of daughter molecules would be composed of hybrid DNA if DNA exhibited conservative replication

Explanation:

1. According to semi conservative model of replication, the two DNA strands separate during replication and each gives rise to a new DNA strand. So in a newly synthesised DNA molecule one strand is old and one stand is new.

Here, the medium contained 15N so all DNA molecules had 15N in beginning. Then they were transferred to 14N medium. After first round of replication all the molecules will be 15N14N hybrid since 14N will be used now for replication. After second round of replication 50% of molecules will be 14N14N and other 50% will be hybrid. After third round of replication, 75% molecules will be 14N14N and 25% will be hybrid.

2. According to conservative model of replication, parental DNA strands stay together after replication so a newly synthesised DNA molecule will have both new strands. In this case, a hybrid will never be formed since the 15N15N strand will always remain together and all the subsequent DNA molecules will have 14N14N composition.