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Because of the limiting value of the rate of DNA replication, rapidly dividing E.coli use multiple replication forks [1][2]. Thus, DNA replication of one generation has already begun in the previous generation.

To me, this poses a problem in understanding the results of the Meselson and Stahl experiment[3]:

Text book representation of Meselson and Stahl Experiment

Source Biology, Textbook for Class XII National Council of Educational Research and Training (https://ncert.nic.in), NCERT Publications (version: November 2021 Agrahayana 1943) https://ncert.nic.in/textbook/pdf/lebo105.pdf

Shouldn’t the DNA from generation I have somewhat more of 15N DNA, as its replication started when the E.coli was still in the 15N medium? Is that the case or is there another mechanism?

  1. J. Mol. Biol. (1968) 31, 519–540

  2. The EMBO Journal (2007) 26, 4514–4522

  3. Proc. Nat. Acad. Sci. (1958) 44, 671–682

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It is sometimes considered that the result was an artifact for a reason similar to that which you mention. However, it happened to be correct in its conceptual outcome, and was not rejected by subsequent better-controlled experimental tests. This is pretty standard for experimental science. The result would not have been so widely believed if it were not that everyone else's data agreed.

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  • $\begingroup$ Interesting the paper you reference. However it is not clear to me what you mean by "everyone else's data agreed"? "Subsequent results confirmed or were consistent with"? $\endgroup$
    – David
    Mar 19 at 15:08
  • $\begingroup$ @David yes, more or less that. If we are getting into the philosophy of science, we could say that their results were paradigmatic, or that the results were subjected to confirmation by various other experiments. But I think that going into epistemology of science is probably more than we require to say that the scientific community is more likely to accept results that are consistent with everyone's own private data. $\endgroup$ Mar 19 at 16:23
  • $\begingroup$ What evidence is there that such private data existed? $\endgroup$
    – David
    Mar 19 at 17:07
  • $\begingroup$ I voted for this answer on the basis that it linked to an article (actually a book review) that suggests that the result is an artifact of shearing when the DNA was forced through a syringe before ultracentrifugation. However I disagree with the social/historical remarks. 1. There was no reason to disbelieve the results on the basis of multiple replication forks because this was 1958 and nothing was known about them and certainly not the rate of DNA replication. 2. They were certainly not consistent with conservative replication. 3. They fitted with the Watson/Crick model. $\endgroup$
    – David
    Mar 20 at 15:42

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