AS Edexcel Biology Unit 1 June 12- 14th of May

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Meselon and Stahl used heavy and light strands of DNA to investigate whether DNA replication is semi conservative.

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They did this by using DNA from E.Coli bacteria that had been grown in a medium containing heavy isotope of nitrogen, 15N.

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All the nucleotides in the bacteria at the start of their experiment contained heaving nitrogen; this made DNA denser than normal.

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So for stage 1's test tube you will draw a band on the lower part of the test tube

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Then they moved the bacteria into a medium containing only normal 14N.

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This meant all the new nucleotides incorporated into the replicated DNA were “light” but the original DNA nucleotides were heavy.

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They allowed the bacteria to divide and their DNA to replicate once.

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They then extracted then centrifuged the DNA.

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Meselon and Stahl’s result was a single band of medium density DNA.

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(So this is what you need in Stage 2) A line in the middle of the Testube

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No heavy DNA existed any longer, so they were able to reject conservative replication.

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The DNA extracted and centrifuged after two rounds of replication gave two bands one medium and one light. So for the third test tube you need a band in the middle and a band above that to represent the light

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The presence of both medium and light DNA bonds confirmed semi conservative model.

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Because there is a medium band, you will have a DNA Molecule containing one light stand and one heavy strand. And, you also have a light band, so you will show a DNA molecule made up of just light DNA strands.

Hope this Helps.