Answer the following questions based on Meselson and Stahl's experiment :
a. Write the name of the chemical substance used as a source of nitrogen in the experiment by them.
b. Why did the scientists synthesize the light and the heavy DNA molecules in the organism in the experiment?
c. How did the scientists make it possible to distinguish the heavy DNA molecule from the light DNA molecule? Explain.
d. Write the conclusion the scientists arrived at, after completing the experiment.
Answers
Answer:
a. NH4Cl
b. To prove semi conservative nature of DNA replication.
c. *They grew E.coli in 15NH4Cl medium for many generations. (15N is heavy nitrogen not radioactive element)
* The result was that 15N was incorporated into newly synthesized DNA and other nitrogen containing compound as well.
* This heavy DNA molecule could be distinguished from normal DNA by centrifugation in a cesium chloride (CsCl) density gradient.
* Then they transferred the E.coli into a medium with normal 14NH4Cl and let them grow.(E.coli divides in 20 minutes)
* They took samples at definite time intervals as the cells multiplied, and extracted the DNA that remained as double-stranded helices.
* Various samples were separated independently on CsCl gradients to measure the densities of DNA.
* The DNA that was extracted from the culture one generation after the transfer from 15N to 14N medium had a hybrid or intermediate density.
* DNA extracted from the culture after another generation (after 40 min.) was composed of equal amount of this hybrid DNA and of 'light' DNA.
d. They proved that DNA replication is semi conservative in nature where in the DNA molecules have one old strand and one new strand.
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