MidnighttRose
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Need help with a worksheet question given by teacher - 'explain the results for the second generation of the meselson and stahl experiment'; there's a diagram of test tubes, the second-gen one is the one with a line at N14 (lighter) and N15 (higher).
I answered it wrong apparently
my answer:
"batch c shows the results after 3 generations, according to the semi-conservative model of DNA replication this means there's been a 50% split as half is now heavy nitrogen and half is lighter nitrogen. After the first generation, it's 100% heavy, then in the second, the introduction of lighter nitrogen means it's now got one strand of lighter nitrogen and one heavy. so has a medium-density, whereas in the 3rd gen its been split completely 50% so half the strands are heavy and half are light, hence the different band positionings"

My teachers comment was that I didnt even answer the question. I'm quite confused as to what I was meant to have written - need to rewrite the real answer to give in. help??
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Travv
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The 2nd generation does not have a heavy molecule, only an intermediate and light one. The light molecule exists because the N14 strand of the intermediate DNA acts as a template for a new molecule, because the N14 medium is being used the new molecule will consist of two strands of N14 and therefore be lighter.

This result proves that DNA replication must be semi-conservative, as the N14 strand must act as a template, otherwise the light molecule wouldn’t exist. We’ve already seen that the N15 strand acts as a template (hence the existence of the intermediate molecule, which has one strand of light and one of heavy DNA). Because both strands act as a template and the new molecule consists of one strand of original, and one strand of new DNA, DNA replication must be semi conservative.
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