As biology unit 1 21/5/2014 panic thread

I can type out summarized points if you can be more specific about what you need,problem is that I don't have a camera I can use at the moment :C

I have a question, in the messelson and Stahl experiment, why is it that when we grow n15 bacteria in medium with normal n15 nucleotides for TWO generations, why does it only give light and hybrid DNA and not Heavy???.?.
Can anyone properly explain this whole experiment please
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Meselson and Stahl's experiment first involved growing E.coli bacteria on a medium with only ¹⁵N.

I am going to guess that you understand why we are using nitrogen isotopes? Anyway, this is so they can grow and reproduce by making DNA, proteins, etc.

Growing the bacteria on only ¹⁵N meant that all of their DNA was dense/ heavy.

After they allowed the bacteria to grow for a few generations, they moved these bacteria to a medium with ¹⁴N.

After allowing them to grow on this medium for a generation, they tested the DNA and saw that it was hybrid DNA. This is because the DNA molecules of the bacteria now had 1 heavy strand and 1 light strand.

The bacteria were then allowed to reproduce for another generation. The DNA was then tested and shown to be hybrid and light. Half of the DNA molecules had all light strands, and half had one heavy and one light strand.

This is experiment was carried out to confirm that DNA replicates semiconservatively.This is when the DNA unzips and each strand acts as a template for new nucleotides to pair up with its bases.

This experiment disproved the other ideas of how DNA replicates (conservative and dispersive).

You can do some further research on the internet if you're interested in knowing more details to help get the ideas into your head! .. I often find that extra reading on the net can help! Youtube videos have been a life saver to me too!

I hope that explanation is okay and not too waffley haha.. Good luck!