In the Meselson and Stahl method why were culture samples taken after each replication cycle?
The Meselson-Stahl experiment was designed to prove that DNA replication was semiconservative. By growing bacteria in different isotopes of nitrogen, they were able to create DNA with different weights (N-15 heavier than N-14). By doing this they could then transfer a bacteria grown in N-15 to a medium containing only N-14 and determine how is DNA synthesized (DNA contains nitrogen, so after one round of replication all the new DNA will only contain N-14, lighter, and the only DNA will still contain N-15, heavier). By taking cultures after each replication cycle, they were able to determine what was the composition of the new DNA at every cycle.
What they found was that after one replication the DNA went from all N-15 to 50% N-15 and 50% N-14. In the next generation they got two kinds of DNA in equal amounts: one was 100% N-14 and the other was again 50% N-14 and 50% N-15. After yet another round of replication they found the same: two kinds of DNA (the same 100% N-14 and the other was again 50% N-14 and 50% N-15). Interestingly this time the amount of "50% N-14 and 50% N-15" DNA was less than the 100% DNA type. In essence, DNA was being synthesized in a semiconservative manner, using the N-15 DNA as template and adding a new N-14 copy to the N-15 copy at every round, and in the processes creating N-14 copies of the DNA which then would be used a templates themselves. The best way to understand this is to look at an image:
Hope this helped
Thanks a lot even
Thanks a lot !!!
It really gives a clear picture of the experiment.
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Thanks Ivan it was really a good explanation indeed. I want to ask from which book we can study more about this and from where you have taken this nice picture?
The image I posted came from a Google search. Here is the link.
As for a book that goes over this experiment, I think that most basic genetics texts should explain this experiment pretty well. I can't think of any one in particular that would be better than others.