In 1958, scientists during the experiment established the semi-conservative principle of DNA replication. The object of the experiment

In 1958, scientists during the experiment established the semi-conservative principle of DNA replication. The object of the experiment was the bacterium E. coli. Bacteria for a long time in a medium with 14N, then 15N. Which method? what isotopes in the new DNA strand?

1) The method of tagged atoms was used
2) New DNA molecules contained one strand with isotope 14N (original) and one strand with isotope 15N (new)

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