In a famous experiment, Matthew Meselson and Franklin Stahl grew E. coli cells i

Question

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 (5N DNA), hybrid DNA, and light DNA (4N DNA) can be separated by centrifugation. If cells containing 15N DNA are transferred to a medium with only 4N NH4CI as a nitrogen source, what percent of daughter molecules are composed of hybrid DNA after 4 generations? Number 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 Number

Explanation / Answer

1. For every generation, the percentage of the original DNA reduces by half. So, for the first generation, if the heavy DNA becomes 50%, all the chromosomes would be hybrid. For the 2nd generation 50% of the DNA will be hybrid. Hence, going by that rate, by the 4th generation, 12.5% daughter molecules are composed of hybrid DNA.

2.Conservative replication would leave the two original template DNA strands together in a double helix and would produce a copy composed of two new strands containing all of the new DNA base pairs. So, there is no chance of DNA strands being hybridized. So, the percentage of hybrid DNA molecules is 0%.

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